equipment used at the Northern Utah WebSDR:
There are two general types of receivers being used at the Northern
type receivers: These receiver use "QSD"
(Quadrature Sampling Detector)
mixers to convert the RF signal directly to quadrature audio where the
two channels are processed after being digitized by a sound card to
produce a receive bandwidth approximately equal to the sample rate of
the card. It is receivers of this type that are the "Higher
Performance" receivers due to their combination of high dynamic range
mixers, band-pass filtering and 16 bit A/D converters.
Dongles: These are low-performance devices with
built-in tuners and
converters that can digitize up to about 2 MHz of RF spectrum.
Because their dynamic range is limited by their 8 bit A/D
converter, they can either be sensitive, or capable of handling strong
signals - but not both - at least now without a bit of help.
These receivers are the "High Performance" receivers used at the Northern
Utah WebSDR and are based on the so-called "QSD" (Quadrature Sampling Detector)
mixers (sometimes known
as the "Tayloe" detector) 1
that use analog MUX switches to provide the "mixing" action.
mixer topology is well-used in commercial amateur gear - notably more
recent Elecraft gear - and it has the advantage of being both simple
and capable high-performance as well as being easy to interface with
conventional hardware - such as a standard audio-frequency sound card -
for final digitization. Essentially a direct-conversion
the RF energy that is present +/- the center (local oscillator)
frequency is digitized up to the Nyquist limit (half the sample rate).
The receiver itself has two
conversion channels that are identical aside from the fact that the
local oscillators are 90 degrees out of phase with each other ("I" in-phase and "Q" -
quadrature) which, with a bit of math (addition and subtraction and phase-shifting) allow the entire spectrum above and below the center
frequency to be represented in software.
Three SoftRock Ensemble II receivers in service.
Click on the image for a larger
There are two "SoftRock" receiver types in use on this WebSDR system:
- The "Softrock Ensemble II" sold by fivedash.com (link).
This receiver has a built-in local oscillator based on the
Silicon Labs Si570 (the
same chip used as the local oscillator in the Elecraft KX-3)
and can be tuned anywhere from about 1.5 MHz to 30 MHz via a USB
connection. This receiver can be purchased either as a kit (at around $65) or
as an assembled board (about
For our purposes, the two kits function identically in their role in
converting RF to audio as the mixer and audio amplification circuitry
of the two are pretty much the same. The "Softrock Ensemble
kit has the obvious advantage of having a built in, tunable local
oscillator - but it is a much more expensive and complicated kit.
The Si570 synthesizer, while convenient to use, has the
disadvantage that it is not particularly stable with frequency -
indeed, the Elecraft KX3 uses a temperature sensor and a computer lookup
table to maintain frequency stability, but the Ensemble II lacks this
its absolute frequency stability can be affected by changes in ambient
temperature. On lower bands like 160 meters this is not much
an issue, but on a higher band like 15, 12 or 10 meters this can amount
to several 10s of Hz change. Because we are using an "outboard"
synthesizer for the "Softrock II Lite" kits, we were able to
substitute a TCXO (Temperature-Controlled Crystal Oscillator) to obtain excellent frequency stability - more on this later.
- The "Softrock II Lite" also sold by fivedash.com (link).
This is a simplified version of the Softrock Ensemble II in
it comes with an assortment of quartz crystals and extra capacitors so
that the builder can choose a 160, 80, 40, 30 or 20 meter frequency and
the front-end filtering is a bit simpler/less robust:
band-specific filtering is done prior to the receiver so this isn't
important. It is
available only as a kit ($21).
Initially, we used three Softrock Ensemble II receivers for 160, 80
meter phone and 75 meter phone coverage, but a later upgrade included
the construction of a module with three Softrock II Lite receivers to
cover the entire 80/75 meter band, freeing those two units for coverage
of other bands as described below..
Both the "Ensemble" and the "Lite" receivers feed the RF directly into
the mixer (after
filtering, of course) which is then passed to low-noise
audio amplifiers - but this also means that these receivers are a bit
"deaf". For low bands like 160 and 75 meters - where both
noise and signal levels are quite high on a "full-sized" antenna like a
dipole this isn't too much of a problem, but on higher bands where the
intrinsic noise is lower - and the losses of the receivers' circuitry
is higher - it is increasingly important that some sort of RF
amplification be used: This will be discussed later.
modules using the Softrock II Lite receivers:
As noted above, the "Softrock II Lite" kit uses quartz
crystals and the frequency selection is rather limited, so if we wish
to have flexibility in our frequency coverage we'll need to use
something else to provide our local oscillator. The device
chosen for this role is the inexpensive ($18) "Progrock" kit sold
by QRP Labs (link).
As the name implies, this functions as a sort of programmable
crystal (e.g. "rock")
and these devices, based on the Silicon Labs Si5351a (the same chip used to provide
the local oscillator of the Elecraft KX-2)
cover from about 8 kHz to well over 150 MHz - and may be coaxed even
lower/higher than that if one pushes it beyond its official specs!
What's more, these devices can output up to three
programmable frequencies at once (with
some limitations) which means that a single unit can
provide the local oscillators for up to three different receivers.
Inside the dual 40 meter receiver module.
In the center is power supply filtering (on the left)
and a passive 2-way signal splitter (on the right) with
the pair of Softrock II Lite receiver modules on each side. In the center
at the top is the ProgRock synthesizer, configured for two outputs. The
receivers have been slightly modified to accept an external frequency source in lieu of the original quartz crystals.
Click on the image for a larger
As delivered, these devices come with inexpensive quartz crystals that
are prone to drift with temperature, but for less than $3 they can be
retrofitted with a TCXO (Temperature Controlled Crystal Oscillator) that "nails" down the frequency with
part-per-million stability, giving them better frequency accuracy and
stability than many commercial HF rigs. The ProgRock kits are
typically programmed using DIP switches and a pushbutton for frequency
entry using binary-coded decimal (BCD)
but newer versions may be programmed via an asynchronous serial port.
For our purposes, only the DIP-switch programming is used as
there is no reason to be able to change the center frequency of a receiver
shows the dual 40
meter receiver module. On either side is a Softrock II Lite
module built for 40 meters with the ProgRock synthesizer at the top,
two outputs used to feed the receivers which have been slightly
modified (the addition
of a single capacitor, the removal of a different capacitor)
to accept an external input in lieu of the original quartz crystal.
The ProgRock itself has been modified to use a TCXO as its
frequency reference to provide a stability of approximately 1ppm over a
wide temperature range which translates to an on-frequency stability of about +/- 7
Hz. It's worth noting that the local oscillator frequencies
fed into the receiver modules operate at four times
the center frequency of the receiver because there
digital divide-by-four circuitry to provide the
quadrature local oscillator feeds for the QSD mixer.
In the lower-center, constructed "Manhattan" style - is a power supply
filter on the left and a 2-way passive RF signal splitter on
right, the latter sending equal amounts of received RF to each of the
two receivers. As noted above, additional RF amplification is
used to bring the signal levels up a bit and this will be discussed in
greater detail later.
issues with spurious signals:
As mentioned above, the ProgRock is capable of producing up to three independent
frequency outputs at once, all from the same (tiny!) Si5351a
chip - but there is basis for some concern if one does this.
Using just a single
output, the signal is actually quite "clean", spectrally - far cleaner
in non-harmonic content than that typically obtained using a typical DDS
synthesizer module - which is why
this same chip is the basis for many commercial and kit radios these
days. If more than one output is used, the spectral purity of
Si5351a chip suffers - but how much?
In the case of the dual receiver - where two outputs are used - if a
single, strong CW signal were injected into the receiver (say, -20dBm - a "50 over"
a number of low-level spurious responses could be seen in the
receiver's waterfall, the worst being on the order of 70dB (or more) down.
What this means is that if a "20 over" signal were present on
input (about -53dBm)
of S-1 or lower would result - but this would be at or below the noise
floor on nearly any HF frequency, anyway. Whether or not you
might think that this was poor performance it's worth pointing out that
many HF receivers do have
similar spurious signal performance numbers, these "deficiencies" going
unnoticed by the casual user - particularly on a busy band.
Top: Close-up of the 2N5109 RF
amplifier - one of
contained within a single amplifier module. A later
these amplifiers included the addition of a 2dB resistive pad on the
output to assure unconditional stability
with reactive sources/loads.
Schematic diagram of the amplifier module
Click on an image for a larger
As noted previously, these "SoftRock" receivers - with no active
devices in the signal path prior to the mixer - tend to be a bit
"deaf". In theory, their audio outputs are pretty low-noise
microvolt-level RF signals appearing nicely at the output, but the
problem comes about when interfacing these same low-level audio signal
to sound cards. A typical good-quality sound card (such as those in the Asus Xonar
series) is able to "see" weak signals like this - but there are
two other issues that tend to show up:
- Many sound cards tend to get a bit "noisier" as the audio
frequency increases. This is manifest by the low frequencies
being pretty quiet, but the noise floor at the higher frequencies
By boosting the RF signal a bit the two noise sources can be submerged
by RF noise coming in from the antenna - but there is a delicate balance: Too much RF gain and the
performance of the receiver will suffer, and too little gain, weak signals are lost in the noise. "Barefoot" (e.g. without any amplification)
these SoftRock receivers will start to saturate/clip at about -12 to
-17dBm (a signal level
of about "60 over")
which means that one could "safely" add another 10-15dB gain without much worry about a few very strong signals, or the cumulative
RF power of many signals on a band, causing overload. For
example, if the receiver were to start to overload at -25dBm (about "50 over")
it would take about 100 "20 over" signals on the band (not including overall noise)
to attain this much signal power: While not impossible, this is unlikely to happen - even during contests.
- There is often noise around the "zero Hz" area.
these SoftRock receivers are direct-conversion, RF frequencies near the
local oscillator get translated to low audio frequencies with there
being the so-called "Zero Hertz" hole for RF frequencies precisely on the
local-oscillator frequency caused by AC (capacitive)
coupling. Typically this hole is only a few 10s of Hz wide,
if an AM signal's carrier landed precisely in that hole, it would turn
into a carrier-suppressed double-sideband signal. Also near
tends to be 1/F noise from the circuitry itself along with some
low-level AC mains-related hum - an inevitable result of the power
supply within the computer itself and slight circulating currents (e.g. ground loop)
between the receiver, connecting cables and in the sound card itself.
To be "safe", one must keep in mind the following for any RF
amplification that is to used:
RF amplification must be of high dynamic range and "clean".
a lot of signals on the input - particularly strong, shortwave
broadcast band signals that may be far removed from receiver's
passband, but still within the RF filter's passband, the amplifier
should not cause distortion.
- The RF amplification must be
pretty "quiet". The amplifier itself must have a reasonably
figure" - that is, it shouldn't contribute much of its own noise to the
signals that pass through it.
A useful article on this topic (among
many) is one written by Gary, WB9JPS 5
where he discusses various requirements of signal amplifiers,
including gain and noise figure. (An article that comes to similar conclusions is one written by AB4OJ - see reference 8.) Gary's conclusion - which is
unique to this paper - indicates that conservative system
noise figure requirements of HF receive systems are modest and along
the lines of:
- Where practical, RF amplifiers should be placed downstream
of band-specific RF filtering. This will prevent the amplifier - and
receivers following it - from "seeing" more signal energy than
necessary, particularly from those signals that are distant in terms of frequency from the receivers'
The amplifiers discussed in the article by WB9JPS reminded me of a November, 1984 article
in Ham Radio
magazine by Joe Reisert, W1JR 6
, where various topologies of amplifiers using the
venerable 2N5109 transistor are discussed- a device designed specifically for
broadband, low noise, linear operation and is, more importantly, still
readily available! While both the W1JR and the WB9JPS
describe amplifiers with better signal-handling performance and lower
noise than the common-emitter configuration that I used (see Figure 3 in the WB9JPS
article, which references a design by W7ZOI) the
performance of the amplifier is quite good and more than adequate for
the task at hand.
- 1.8 MHz: 45dB NF
- 3.5 MHz: 37dB NF
- 14 MHz: 24dB NF
- 21 MHz: 20dB NF
- 28 MHz: 15dB NF
- 50 MHz: 9dB NF
- 144 MHz: 2dB NF
A set of three of these amplifiers were constructed and housed in a
1590D die-case enclosure using the circuit depicted in Figure 3 on this page. Between each amplifier is a
double-sided, glass-epoxy circuit board material and each amplifier was
built on its own, private board using "Manhattan" ("dead bug")
techniques using "Me Squares" sold by QRPME (link)
that were (literally!) glued down using high-quality cyanoacrylate
adhesive (e.g. "super"
The usable frequency range of these amplifiers is on the order of 50kHz
through 200 MHz, but they are flat to better than 1dB over the range of
1.5-30 MHz. In testing these amplifiers they maintained very
output (e.g. negligible
intermodulation distortion) at power levels over +20dBm (100 milliwatts).
Even though there are three amplifiers in the same enclosure,
isolation between them was around 100dB at 10 MHz degrading to around
80dB at 30 MHz.
The dual 20 meter receiver. This is very similar to
receiver except that there is an RF amplifier for each receiver to
isolation of the LO bleedthrough between the two.
The schematic diagram of the dual receiver module.
module could be used on any HF amateur band -
it is only the receivers and the programming of the LO that
dictate the frequency of operation.
Click on the image for a larger
The idea behind three amplifiers in one
enclosure was that they could be used as general purpose gain blocks:
If extra gain was needed somewhere, these would be available
provide it - and upon installation of the WebSDR, one section was used with the 160
meter "Softrock Ensemble II" while another section was used for the dual
40 meter receiver module depicted in Figure 2: This added
gain (13-15dB) was
about right to allow the receiver to "see" the noise floor during
daylight hours, but not so much that receiver system performance was
compromised even when there were a lot of "big" signals during contests.
When the WebSDR system was first brought online the three Softrock
Ensemble II receivers were used to cover a portion of 160 meters and
most of the phone portions of the 80/75 meter band. At this
additional equipment was in the works that would be used to provide
coverage of allof
80/75 meters in three chunks and the entire 20 meter band in two -
all of these using SoftRock Lite II modules. As new modules were
constructed using the Softrock Lite II receivers,
the Softrock Ensemble II, being capable of being tuned anywhere,
became available for general use - such as providing coverage for "new"
bands or being used as a spare receiver in case of some sort of
20 meter coverage:
By observation, it was known that amplification would be required for
the 20 meter SoftRock receivers so it was designed from the beginning
to include it - but there was a bit of a twist: One issue
related to any receiver using a QSD mixer is that it can have a
"significant" amount of local oscillator energy appearing on the
antenna port, and on the 20 meter modules this signal level was on
the order of -33dBm, or about "40 over" S-9. To the receiver
itself, this amount of signal is irrelevant as it cancels out and doesn't appear as a strong "zero Hz" component, but on
the dual receiver modules the local oscillator for one receiver
appeared in the other and this "big signal" could have potentially
degraded performance - mostly in the form of a strong, off-frequency
signal that could mix in various ways with low-level local oscillator
spurious signals and the myriad of signals that might appear on a
A degree of isolation (15-20dB) between the receivers was provided by
the passive 2-way splitter, but it was decided that each, individual
receiver would sport its own, private RF amplifier, adding another 20dB
or so of isolation, the end result being that one receiver would "see"
a signal of -60dBm (about
or less from the local oscillator of the other. Because these amplifiers are relatively
and the parts cheap - the construction of the added circuitry was not
much of a burden.
Figure 4 shows the completed receiver module. It is nearly
to the dual 40 meter receiver module in that it uses a single ProgRock
synthesizer to to provide the local oscillator signals for both
receivers. If you look closely, you can see that there are
transistor amplifiers on the right-hand side of the copper-clad board
in the bottom-center, following the 2-way splitter and each one feeding
Top: The "triple" 80/75 meter
are three receivers, the physical layout is different from the
dual receiver modules and like the 20 meter receiver, each
receiver has its own, private RF amplifier - both for gain
and LO isolation.
The schematic diagram of the triple
receiver module. Like the dual receiver, it's only the
itself and the programming of the synthesizer that determine
the HF band on which it operates.
Click on an image for a larger
In testing on the workbench, the "MDS" (Minimum Discernible Signal)
of each 20 meter receiver was better than -127dBm (e.g. 0.1 microvolts in a 50 ohm
indicating that they were as sensitive as they needed to be:
words, this receiver was more than capable of hearing its fair share of
ionospheric noise when connected to an HF antenna and as such, more
sensitivity would not improve the ability to "hear" weak signals!
80 and 75
It is somewhat inconvenient that most amateur bands are sized in
multiples of 100
kHz but audio sound cards have sample rates of 96 or 192
kHz. In the case of the U.S. 40 meter band we would need two
sound cards to fully-cover the 300 kHz-wide band. A somewhat
similar situation exists on the U.S. 80/75 meter band, which covers
from 3.5 to 4.0
MHz where we would need three
receivers to cover the entire band. It is convenient that the
output three simultaneous outputs, so another receiver assembly was
constructed using three
Softrock Lite II modules.
The picture in Figure 5
the layout. In the upper-left corner is the ProgRock
and like its counterparts, it, too is equipped with a 1ppm TCXO.
Below it are the three, identical Softrock Lite II modules
the right of those, one for each receiver, are three RF amplifiers.
In the upper-right corner is a three-way splitter that
the signals to the receivers equally and provides a bit of additional
LO isolation between the receivers and on the other side of the divider
is the same type of power supply filtering found in the other receiver
also shows the diagram of
the triple receiver module, the circuitry being representative of that
in the other two modules. If you look carefully you will
that the RF amplifiers in the receiver modules are slightly different
than that depicted in the WB9JPS article - and is, in fact, a direct
"quote" of one of the amplifiers discussed in the November 1984 W1JR
article. The main difference is that these amplifiers lack
output balun/transformer which somewhat reduces the large-signal
performance, but because these amplifiers are placed "downstream"
bandpass filtering that is specific to an amateur band, they
not be "seeing" much of the HF spectrum and will be dealing with fewer
The "other" receivers - the ones that are not
considered to be "high performance" - use the so-called RTL-SDR
dongles. These USB dongles are ubiquitous and versatile:
They can cover (more
from a few hundred kHz to over 1.3 GHz using various on-device signal
paths - but all of these signal paths have in common one important
limitation - The A/D converter is only 8 bits.
limitations, they are attractive because they are cheap -
from $4 for the "bottom end" and cheapest devices (which are far noisier than they
to over $50 for units with frequency converters and a few other bells
and whistles - including band-pass filters. The devices that
are using are just $20 and are the RTL-SDR dongles sold by "RTL-SDR
Blog": These units have thoughtfully-designed circuit boards
minimize extraneous, spurious responses and include 1ppm TCXOs for
decent frequency stability as well as providing separate signal
branches for "direct" and "quadrature" signal paths - but more on that
Ideally, the maximum range represented by an 8 bit A/D converter is
around 48dB - and this is approximately
what can be expected from these devices, but as with most things in the
real world, the actual answer to the question "what is the dynamic
range" is more complicated. In reality, noise considerations
the device reduce the number of usable A/D bits and thus the dynamic
range, this noise coming from the device itself and other devices in
the signal path. When used "on air" their effective dynamic
can often "seem" to be greater than the 40-50dB that one might expect,
and this can be due to several factors:
- In reality, one can tolerate a surprising amount of
signals causing the A/D converter to hit "full scale")
before signals across the spectrum are badly degraded. A
small/moderate amount of clipping - particularly if it is due to noise
and multiple signals - tends to degrade the signal/noise ratio overall
but on a shortwave band that is, by its nature, noisy anyway,
this added noise may go unnoticed most of the time.
- These devices are typically sampling at 28.8 Msps (million samples/second)which
effectively "oversamples" narrower signals in the passband which is
then reduce later in the signal path to a sampling rate of between 1
and 2 Msps, depending on the configuration.
theory, oversampling can yield effects somewhat similar to increased
A/D bit depth and this tends to improve the performance of the weaker
Even with all of these effects, their useful range is quite limited
which means that if there are both very strong and weak signals being
digitized by the dongle's A/D converter, you are faced with a choice:
Decrease the gain to prevent the strong signals from badly
overloading it or increase the gain to allow reception of the weaker
signals, but suffer the effects when strong signals appear.
one uses these dongles it is imperative
that one avoid slapping it on an antenna, but include in the signal
path a band-pass filter that limits the signals getting into it to
those frequencies around the range of interest. In the case
WebSDR we do this because we set up a receiver to cover a specific
range and it never needs to be tuned anywhere else.
- When listening to "off air" signals there is often a
amount of noise being digitized as well. As it turns out A/D
performance can be improved somewhat if noise is deliberately
inserted in the analog input as it causes a bit of spectral spreading,
somewhat suppressing the effects of quantization noise. In
words, it's almost as if the noise will "bias" the A/D converter a bit,
statistically giving slightly better weak-signal performance.
The two signal
paths within the dongles:
The RTL-SDR blog dongles that we are using have two entirely separate signal
- The R820T
path. These dongles were originally designed to
receive off-air TV using a DVB format that is used in most places of
the world other than
the U.S. and this chip converts frequencies from a much higher
down to a lower frequency for the RTL2832 chip. It is this chip
is that may be coaxed to tune from around 20-30 MHz to over 1.3 GHz and
built-in filtering and amplification. This signal path is
typically referred to as the "I/Q" branch.
Some RTL-SDR dongles include a frequency up-converter that takes the HF
frequency range and presents it to the dongle in the 125-155 MHz range,
but the RTL-SDR dongles that we are using do not have this feature, so
we are using the "direct" branch which has certain frequency
limitations due to the 28.8 MHz sample rate. As noted
the Nyquist frequency - the maximum frequency where we can faithfully
digitize the input signal - is 14.4 MHz and this means that we can use
it to directly "receive" bands up to 30 meters: 20 meters is
problematic because the top end of this band - 14.35 MHz - is only 50
kHz away from the 14.4 MHz Nyquist frequency and making a practical filter to
remove the images at 14.45 MHz and above that would appear in the 20 meter
band is very difficult to do!
- The RTL2832
The output of the R820 feeds into this chip which does the 8-bit A/D
sampling at 28.8 MHz and can "tune" from a few hundred kHz to its
Nyquist frequency, or 14.4 MHz - or even higher frequencies via
undersampling techniques. Inside this chip is additional DSP
circuity, some of which is intended to help in the reception of
digital TV signals, is unused in the reception of signals in the
manner that we are interested. One piece of hardware that we are
using is a digital "tuner" that takes this the digitized output from
A/D and "converts" it to a lower frequency and sample rate which is
then made available via the USB port. As it turns out there
some sort of diagnostic mode built into the chip that bypasses the
aforementioned TV-related processing to allow the "raw", frequency-converted A/D samples to be
presented to the computer. Because these devices typically
operate using USB 2.0, the maximum usable sample rate for most USB ports/computers is
around 2 Msps, yielding a maximum contiguous receive bandwidth of about
2 MHz. The sensitivity on this branch is typically low, but
improved in the case of the RTL-SDR blog devices by an on-board
amplifier which helps a bit, but even more amplification is typically
required as noted below. This signal path is typically
to as the "Direct" branch - typically using the "Q" input.
At lower frequencies, such as the AM broadcast band, we can receive
signals directly - but the problem of dynamics rears its head again:
If one is located near an AM broadcast band transmitter - or
a metro area where there are several of these transmitters - the
signals from these AM stations can vary over 60dB, from the weak
"nearby" stations to the very strongest - a range entirely outside the
capability of the dongle itself unless certain heroic measures are taken. (For an article on
this, see reference #7, below.)
This article describes a means of attenuating signals within
AM broadcast band - with additional "notching" of the strongest signals
- while preserving sensitivity on the adjacent 160 meter band,
generally keeping all signals within the usable dynamic range of the
At higher frequencies, things are a bit easier to manage in that one
simply constructs a band-pass filter for the frequency range of
interest. Using a filter design program like Elsie (link)
which has a free (limited)"student"
version, one can input the desired center frequency and
bandwidth to yield simple - but adequate - designs that can be realized
using standard-value capacitors and easily-wound toroidal inductors.
Alternatively, you can get band-pass filter kits from QRP Labs (or simply "borrow" their published designs) that can be easily adapted to nearly any HF frequency range.
In this case it is useful to precede the RTL-SDR dongle with
bit of excess gain and provide an attenuator (typically post-filter)
that can be adjusted to find the "sweet spot" where the probability of
overload from strong shortwave broadcast stations is minimized and weak signals
be heard. This method is used for both 60-49 Meter and 31-30
Meter coverage on the Northern Utah WebSDR with reasonable
Using "direct" mode, these dongles effectively have a "hole" which
excludes direct coverage of 20 and 10 meters owing to the aforementioned
Nyquist and filtering limitations - and as is the case with lower frequencies which means that if we wish to
cover the 20 and 10 meters without being plagued with images, a
frequency converter (with appropriate filtering) is required. If a more expensive dongle with a built-in
converter were chosen, one would want the type with TCXOs to minimize
frequency drift, which can be greatly magnified because these
converters - and the tuner itself - operate in the 100-200 MHz range.
It is quite practical to build a frequency down
converter to yield comparable results as described later in this page. For example, if one
to mix 10 meter signals with a 20 MHz oscillator the result would be a
conversion of the 28.0-29.7 MHz range down to 8.0-9.7 MHz: By
down-converting, frequency drift of the various oscillators is
dramatically reduced as all of the frequencies involved are about an
order of magnitude lower than they would be with a 100+ MHz up-converter.
Block diagram of the HF splitter/AM BCB filter/AMP and the "Low" and
"High" splitter modules.
Click on the image for a larger
To achieve our goal, we decided from the outset that we should make the
receive system capable of receiving on every HF band, but to do this
we'd need a lot of outputs, as in:
One way that we could have done this would have been to use
conventional transformer-type splitters to divide the signal and the
simplest way - to divide it by 16 - would have yielded about 20dB
of insertion loss - and the above doesn't take into account that for
many of the HF bands (those marked with an asterisk) we'd need several
antenna connections to feed enough receivers to cover many of the bands
if we use "high performance" receivers that can provide only up
to 192 kHz of coverage.
- 630 meters
- 160 meters
- 80/75 meters *
- 60 meters
- 40 meters *
- 30 meters
- 20 meters *
- 17 meters
- 15 meters *
- 12 meters
- 10 meters *
With this method there are two other problems with which one must contend:
- For good performance, there would need to be a band-pass filter
specifically designed for the amateur band in question: This is
particularly true for RTL-SDR receivers, but it is also true for the
SoftRock kits described above as their band-pass filters aren't
"strong" enough to avoid some spurious responses (e.g. the 75 meter receiver will respond on its 3rd harmonic to strong signals in the 25 meter shortwave broadcast band).
While it is certainly possible to make this scheme work, there's another method: Use a "diplexer" type splitter.
- As noted previously, the system noise figure will need to be
lower as the frequency goes up which means that appropriate
amplification (low noise, high dynamic range)
will need to precede the splitters in the signal path of the higher
bands. Without filtering, these amplifiers must be able to handle
everything that might be thrown at it!
A "diplexer" type splitter minimizes the insertion loss by
selectively "picking" various bands from a common bus. By having
a filter that pulls only narrow ranges of frequencies of individual
amateur bands - but leaves the other frequencies alone - we can put
several of these same filters on the same bus and instead of 15-20dB of
insertion loss, we can easily keep the loss down to single digits of
dB. The idea is simple - but we decided early on that this wasn't
going to be our only approach.
The receive signal path (from
the antenna) was designed from the outset to be both
versatile and high-performance with the following goals in mind:
- Provide both a "Narrowband" and "Wideband" receiver signal
- The "Narrowband" branch includes filters designed to pass
a single amateur band.
To accomplish this several modules were built, depicted in Figure 6:
- The "Wideband" branch is for receivers that may cover
outside the amateur bands, including RTL-SDR dongles and
direct-sampling receivers (such
as the KiwiSDR) that can cover from 0-30 MHz.
Because the requirements of these receivers are fundamentally
different than for the "narrowband" receivers it is easiest to have two
entirely separate signal paths.
- The "Splitter/AM BCB Reject/Amplifier module". See figure 7, top, for a
schematic diagram. (For an article on
this device, see reference #7, below.)
- This module has a passive ferrite splitter that is
pass signals from 10 kHz to over 30 MHz from the antenna with one port going
directly to the input of the "narrowband" branch of the RF system.
- The other port of the splitter goes to an AM broadcast
reject filter that is designed to attenuate signals between 540 and
1725 kHz by at least 30dB while minimally affecting signals outside
this range (e.g. the 630 meter band and the 160 Meter and higher bands.)
- Included in the design AM broadcast band reject filter is a "bypass" adjustment that
amount of ultimate rejection of the filter to be reduced so
that the signals on the AM band may be controlled in amplitude, allowing weaker signals to be received.
- The design of the AM broadcast band reject filter also includes up to 7 tunable notch filters
selectively reduce the signal strength of individual AM broadcast band
- By carefully adjusting the amount of "bypass" and setting
notches, the difference between the strongest local signals and the
weaker signals is greatly reduced. By reducing the level of
AM broadcast band signals, front-end overload of connected receivers -
such as RTL-SDR dongles or direct-sampling receivers - can be avoided.
By "flattening" the signal level range between the strongest
weaker signals it is possible to reduce the amount of band-stop
attenuation to permit the reception of even weak AM broadcast band
signals through the band-stop filter while preventing overload of
receivers with poor dynamic range (such
as the RTL-SDR dongles with only 8 bits of A/D sampling).
- It is this system that allows even the lowly RTL-SDR
receive nighttime AM broadcast band without being overloaded by the
strong daytime signals while still having microvolt-level sensitivity
outside this band.
- High dynamic range amplifiers are included in this module
allow down-stream splitting of the wideband signal and to accommodate
losses of additional filtering, the intrinsic insensitivity of some
types of receivers (e.g.
RTL-SDR units operating in the "Direct" mode)
and the need of variable attenuation for RTL-SDR receivers to allow the
signal levels to be set to optimize for the limited dynamic range of
The schematic of the "Splitter/AM BCB Reject/Amplifier"
The schematic diagram of the "Low HF Splitter" module.
The diagram of the "High HF Splitter" module.
The diagram of the splitter/low-pass/BPF module for RTL-SDR
Click on an image for a larger
- The "Low HF Splitter Multi-Coupler Module". See figure 7, upper-middle, for a
- This module gets its input from the "split" HF output of
previous module, but it could be connected directly to the receive
antenna. A gas-discharge tube is included on the input of
this device to provide a degree of lightning protection.
- Each band output is a result of the combination of high
low-pass filters. Because of the design, there is no need to
incur the loss of individual splitters as the various bands' filters
are simply paralleled on the same signal bus. Because the
are minimized, there is no need for any amplification prior to each
output filter port.
- Because each "band" output is effectively band-pass
this limits the range of signals that each individual band's receiver
will see, improving the overall signal handling capability, reduces
possible image response, significantly attenuates local oscillator
bleedthrough (an issue
with "QSD" type mixers found on "SoftRock" receivers) and
provides a degree of lightning protection.
- The most popular amateur bands are covered - namely 160,
40 meters. An additional output is low-pass filtered at 500
to allow the possible future inclusion of the 630 and 2200 meter bands.
The only band that is not covered is 60 meters, but this is
covered using a dedicated module on the "wideband" module as shown.
- This module includes a high-pass filtered output for the "high" HF bands (30 meters and above) as
- The "High HF Splitter Multi-coupler module". See figure 7, lower-middle, for a
- There is a high-pass filtered
output from the "Low HF
Multi-Coupler Module" that passes signals above approximately 9.5 MHz.
This output is passed to a high dynamic range amplifier based
a 2N5109 transistor and this circuit is capable of more than +20dBm
of "clean" RF and has a 3dB intercept point greater than +27dBm and
about 13dB gain with a reasonable noise figure. This amplifier is
strongly recommended because the filters themselves 1-3dB of loss and
this factor shouldn't be callously disregarded as one approaches the
top end of the HF spectrum.
Also depicted in Figure 6 is another module, connected to the output of
the Splitter/BCB filter module, that feeds two RTL-SDR dongles.
As required for best performance, these devices should have
inputs filtered to pass only
the frequency range of interest and the diagram shows this being done:
A 3 MHz low-pass to accommodate the receiver that tunes 630
through 160 meters (including
the AM broadcast band)
and a 4.5-7 MHz band-pass filter for the receiver that tunes the 60
Meter SWBC and amateur frequencies and the 49 meter SWBC bands.
This module also has adjustable attenuators that are set to
"sweet spot" - that is, just enough attenuation to prevent serious
overload by strong signals and not so much attenuation that weak
signals cannot be heard.
- The output of the amplifier is passed to this "High HF"
where there are individual series-input band-pass filters for each of
the amateur bands covering 30 through 10 meters. These method
"splitting" the signals is less lossy than transformer-type splitters
and it provides a degree of isolation between the various receiver
modules as well as providing additional band-pass filtering for each of
the individual receivers - particularly important with
receivers that use
a QSD where a significant amount of local oscillator energy can find it
way out of the antenna port.
At first glance it might seem that placing a splitter at the input of
the system and losing 3dB "off the top" would be a bad idea, but this
ignores a fundamental truth about HF signal reception: As noted above, the HF
frequency range is very
noisy, which means that we can tolerate quite a bit of loss (and incur a rather high system
noise figure) in front of our receivers without
actually degrading overall system sensitivity. This simple
can be demonstrated by connecting a highly-sensitive receiver to a
full-size receive antenna and experimenting with a step attenuator and
noting the amount of attenuation required to quash the atmospheric
noise. Typically this value, on an antenna devoid of man made
noise under normal "quiet", HF conditions, implies that an acceptable system
noise figure ranges from about 45dB at 160 meters, decreasing to 24 dB
and 15 dB and 20 dB and 10 meters, respectively 5.
What this means is that even if we end up with 6 dB of added
in our HF signal path through splitters and filters, it is still
possible to recover the natural noise floor on HF without
requiring any sort of exotic, low-noise amplification.
Band-pass filter/attenuator modules for the RTL-SDR dongles:
If you've been reading along you'll already know that it is imperative
that RTL-SDR dongles used on HF (or anywhere else) MUST have
filtering of some sort on their RF input: It's not just the
signals in the frequency range of interest that are "seen" by the A/D
converter when operating in "Direct" mode, but all signals at all frequencies.
In order to maximize what (little) signal
handling capability these devices have, it is required that effective
filtering be used.
As mentioned previously, one must also provide a means of adjusting the
RF single levels being applied to the input of an RTL-SDR dongle,
trying to find the "sweet spot" where there is enough attenuation to
prevent overload by strong signals yet there is enough overall system
gain to receive weak signals. This balancing act can be quite
tricky - particularly when one considers the number of signals and that the
strength of those signals vary dramatically between day and night.
At the Northern Utah WebSDR, we are "fortunate" in that there are
no strong shortwave broadcast stations "nearby" that beam their
signal in our direction - but in Europe and eastern North America, the
story can be quite different, with multi-hundred kW stations being
beamed in your direction and only one "hop" away!
The diagram of the filter module is shown in the bottom of Figure 7
and enough information is provided for several options. A two-way
splitter is depicted on the diagram to allow the feeding of two
separate RTL-SDR dongles and their filters while off to the side, a
3-way splitter is shown. If a 4-way splitter were required, one
would cascade a pair of 2-way splitters after a single 2-way splitter
(for a total of 3 splitters) - but as noted on the diagram, each set of splitters
would incur a loss of about 3.5dB. If no splitting is required,
these would simply be left off.
The upper portion of this diagram also depicts a filter suitable for use on the AM
and 160 meter bands. The left-hand portion is a 500 kHz high-pass
filter that removes potentially strong LF signals and noise while the right-hand
portion cuts off signals above approximately 2.5 MHz. On the
output of the filter is a very simple attenuator that is used to adjust
the signal levels being fed to the RTL-SDR. Using a single
potentiometer, this attenuator is not a "constant impedance" device, but it does
provide an "approximate" load for the filters to preserve their general
characteristics. In reality, the RTL-SDR really doesn't care
about its input impedance, and at HF frequencies with fairly short
cables, it's not all that important, either!
Also depicted in the diagram is a band-pass filter along with the same
attenuator seen in the low-pass portion. The design of this
band-pass filter is one that is "borrowed" from the QRP Labs web site,
from their "Band Pass" filter products (a link to that page is here).
In the assembly manual - which may be found on that web page - you
will find a technical description of the filters (along with some
representative band-pass plots) that provide enough information for you
to build your own filters. If you wish, you may buy these modules in
kit form (and I can recommend that any of the kits sold by QRP Labs are worth getting!).
If you plan to cover a frequency range that isn't shown - such as
a shortwave broadcast band - these filters can be tuned/modified from
the nearest amateur band.
As noted previously, these RTL-SDR modules are somewhat "deaf" so it is
likely that some sort of RF amplifier will be required - particularly
to provide the bit of "excess" signal that one would need to be able to
adjust levels downward again: Any of the 2N5109-based amplifier modules
described earlier in this page will fit the bill nicely.
Finally, remember that RTL-SDR dongles in the "direct" mode aren't
really all that well-suited for covering the 20 or 10 meter bands owing
to the Nyquist limitations - and reception on frequencies between these
bands (e.g. 17, 15 and 12 meters)
will suffer a bit owing to decreased sensitivity and the increased
tendency for spurious signals to appear. On 20 and 10 meters one
would be better off using a dongle that includes an "up converter" - or
build a simple "down converter": In any case you will always want to use a band-pass filter in front of the RTL-SDR dongle's receive system to maximize its performance!
A downconverter for RTL-SDRs:
It is quite common for HF coverage via an RTL-SDR dongle to be achieved through the use of an upconverter
- a device that takes the HF spectrum and shifts it up by 100,
125 or even 200 MHz. The reason for doing this is easy to
understand: Continuous coverage across the HF spectrum is
afforded, which is very convenient - but one must still take care when
- If you do this, you will still need rather narrow band-pass filtering at the antenna
to limit the number of signals that the device will "see". This
reiterates the fact that if want to use an RTL-SDR dongle to cover the
entire HF spectrum all at once, you are asking a bit much!
- At higher frequencies, temperature-related drift becomes more of
an issue. Even if one uses temperature-controlled crystal
oscillators for both the RTL and upconverter, a drift of 1ppm can
account for several hundred Hertz of drift - potentially worsened by
the fact that there are two
of these oscillators, each doing their own drifting. If the
RTL-SDR is located in an environment that is temperature-stable, this
may not be much of an issue.
Another method of providing HF coverage is with the use of a frequency downconverter. This is fundamentally different from the upconverter in that instead of shifting the entire HF range upwards by some amount, one takes the narrow slice of interest and converts it down to a lower frequency. Doing this solves/minimizes several problems, such as:
- You do not get away from the fundamental dynamic range limitation imposed by the 8-bit A/D converter.
- At the lower frequencies involved, frequency drift is also decreased.
If the RTL-SDR can receive HF in its "Direct" mode, anyway, why would
you use a downcoverter? As mentioned previously, the A/D sample
rate of the RTL2832 chip is 28.8 MHz, which means that both 20 and 10
meter coverage via this mode has the problem of being too close to the
Nyquist limits: In the case of 20 meters, half of the sample rate
(14.4 MHz) is just
above the top of the band at 14.35 MHz and images cannot be easily
filtered. In the case of 10 meters the 28.8 MHz sample rate lands
right in the middle of 10 meters which means that even if you could
build an effective filter to suppress images, you'd only be able to
effectively cover a small-ish portion of the band. At the various
bands in-between 20 meters (17, 15 and 12 meters)
coverage is possible, but good filtering is still required - and the
undersampling means that the sensitivity will be a bit worse than it
would be at lower frequencies, presuming that the low-pass filtering in
the dongle on the "Direct" branch weren't an issue. In short, in
"direct" mode the RTL-SDR dongle works best from around 1 MHz up to
around 12 MHz: Above this, it becomes increasingly difficult to
construct anti-aliasing filters that are both simple and effective.
- This method necessitates the use of decent band-pass filters to
avoid image responses - and you'll need band-pass filters, anyway!
A converter has the obvious disadvantage that it isn't as convenient:
You probably can't just go out and buy a downconverter like this
- and the design considerations require that one pick the frequencies
of the local oscillator and down-converted frequency range a bit
carefully with respect to the intended coverage. For example, the
local oscillator's harmonic(s) should not land in or very close
either the input frequency (the one to be down-converted) as this will result in a very strong signal that could result in intermod products (e.g. birdies.)
The other issue - the down-converted frequency output - should be
carefully chosen so that its harmonics are several MHz away from the
input frequency coverage as this, too, would result in "birdies" or
other undesired responses.
A downcoverter for 15 meters:
As a matter of convenience, I chose a 10 MHz local oscillator frequency
because inexpensive TCXO devices are readily available, and it
adequately meets the design criteria:
A diagram of the as-built downconverter may be seen in Figure 8.
- The second harmonic of the local oscillator (20 MHz) is at >= 1 MHz away from the input frequency range of 21.00-21.45
- The harmonics of the down-converted output frequency range should
land outside the frequency range of interest. The output
frequency range with a 10 MHz local oscillator is 11.00-11.45 MHz which
means that the harmonics would be in the range of 22.00-22.90 MHz.
Top: Inside the 15 meter downconverter for RTL-SDR devices, using the "Direct" branch input
Schematic diagram of the converter
Click on an image for a larger
As with the diagram depicted in the bottom frame of Figure 7, above, the first input stage uses a bandpass filter "borrowed" from the QRP Labs web site,
from their "Band Pass" filter products (a link to that page is here).
In the assembly manual - which may be found on that web page - you
will find a technical description of the filters (along with some
representative band-pass plots) that provide enough information for you
to build your own filters. If you wish, you may buy these modules in
kit form (and I can recommend that any of the kits sold by QRP Labs are worth getting!). This filter does the job of both attenuating the receive image that would otherwise be present at the sum of the local oscillator and the 15 meter band (e.g. 31.00-31.45 MHz) as well as reducing the overall amount of energy impinging on the mixer from signals outside the 15 meter frequency range.
Following this bandpass filter is an amplifier based on the 2N5109
transistor - chosen in this application due to its superior performance
over a standard 2N3904 transistor: The latter will work, but both
the gain and noise figure will be somewhat higher. This amplifier
overcomes the losses related to U1, a diode-ring mixer which has an
intrinsic loss of about 7 dB. Following the diode-ring mixer is
another bandpass filter - another one modified from the 30 meter QRP Labs
design by reducing the inductance of the "larger" windings by removing
a turn or two: This filter will be a bit narrower than the input
and further-reduce off-frequency signals - and it also eliminates the
image response from the output. Ideally, a "diplexer" would be
included on the output of a mixer to assure that it was terminated at
the image frequency, but this was omitted in this "non
Providing the 10 MHz local oscillator is an inexpensive (approx. $2.50 in single quantities) TCXO. This device provides just
enough drive for the diode ring mixer and it will be stable to within a
few Hertz at 10 MHz, assuring frequency stability that is around an
order of magnitude better than what would be experienced if one were to
do "upconverting" to >100 MHz as is commonly done. In our application the temperature will very
wildly - from near-freezing to as high as 100F (approx. 38F)
so this degree of stability is important. The down-side of this
devices is that it is very tiny - about 2.5x3.5mm - so it is "super
glued" to the board bottom-up and tiny (30 AWG)
wires are soldered to the surrounding connections. This device
requires a 3.3 volt supply so a standard 5 volt regulator was used with
a normal "dim" red LED in series to provide a 1.6-1.8 volt drop.
Following the output bandpass filter is another amplifier - this time,
using the generic 2N3904 - which boosts the signal a bit more to
overcome the intrinsic (relative) insensitivity of the RTL-SDR dongle:
At this lower frequency, the gain will be higher and the noise
figure - set by previous stages - is less important, allowing the use of a general-purpose device. Hanging on
the output of this amplifier is a series L/C network tuned to the 10
MHz local oscillator frequency to reduce its energy by 15-20dB:
This signal is otherwise a bit strong and as we know, it's a good
idea to keep the extraneous signals entering an RTL-SDR dongle to a minimum!
The final stage is a simple potentiometer-type attenuator - nothing
more complicated being required as constant impedance is not very
important for the input of an RTL dongle, particularly if a short cable
is used. As we
already know, when dealing with RTL-SDRs on HF, it's best to start out
with a bit of extra
signal - and then tweak the levels downwards as necessary to find the
"sweet spot" between being able to hear weak signals and prevent
overload from strong signals. In the case of 15 meters, at this
time of the sunspot cycle when band openings are a bit rare and signals
are on the weak side it's probably best to adjust the overall system
gain to just be able to hear the background ionospheric noise - and no more!
When tested using the "SDR Sharp" program, the combined sensitivity (with R10 set for maximum signal)
of the downconverter and RTL-SDR dongle running in "direct" mode was on
the order of -130dBm in an SSB bandwidth - less than 0.1 microvolts,
and more than enough sensitivity to "hear" anything that would fall on any
decent HF antenna in an RF-quiet environment when the band was dead. When installed on site, R10
was adjusted so that, with a "dead", quiet band
the background ionospheric noise was be just registered on the S-meter and A/D converter by several dB to maximize its signal handling capability.
A downconverter for 10 meters:
A similar downconverter was constructed
to provide full 10 meter coverage, also using an RTL-SDR dongle.
This converter is nearly identical to the 15 meter converter,
except as follows:
- A 16.384 MHz TCXO was used for the local oscillator. This
is a standard "microprocessor" frequency and is both inexpensive and
readily available. The 16.384 MHz LO results in 28.0-29.7 MHz
being converted to the range 11.616-13.316.
- The input bandpass filter - a QRP Labs design - covers 28.0-29.7 MHz.
- The output bandpass filter - modified from a QRP Labs design - covers from about 11.5-13.5 MHz.
The RTL-SDR dongle is configured to run at 2.048 MSPS with the center
frequency being 28.676 MHz, providing coverage from 27.652-29.700 MHz.
Because of the RTL-SDR's internal 28.8 MHz clock, there is a
moderately strong carrier at this frequency that cannot be avoided.
The 630 meter receiver:
The 630 meter band (472-479 kHz) is the newest "MF" band to U.S.
Amateurs - the other MF band being 160 meters. Like 160 meters,
it is mostly a "winter time" band when noise - a significant portion of
which is lightning static - is lower and the nights are longer and
deeper - both of which are beneficial to reception at these
frequencies. Like 160 meters, there is a significant challenge
Full-sized antennas are out of the question which means that
overall transmit efficiency is quite poor meaning that signals are
generally weak. Because of the comparatively weak signals, the
high noise levels and the fact that the band is only 7 kHz wide, voice
modes are rarely used with most operation on CW, WSPR, JT-9 and similar
The receiver is a modified "Softrock Lite II" - the same receiver used
for many other bands as described above. Designed primarily for
operation on HF, it was necessary to slightly modify the receiver,
Diagram of the 630 (and 2200) meter dual receiver system detailing the
modifications to the Softrock Lite II receiver modules. Even
though the lower receiver is marked as being intended for 2200 meter
use, the designed frequency coverage of the input filter is for the
range of about 125 kHz to 215 kHz, easily including the 1750 meter
Click on the image for a larger version.
- Change coupling capacitor C12 to a larger value: I used 330pF, which was supplied in the kit, anyway.
- Design a new input band-pass filter. The original filters
for the Softrock Lite II use just two sections: Using the Elsie
program I designed a filter that has three sections and a significantly
sharper - and appropriately wide - band-pass.
In the end, the completed receiver looked very similar to that in Figure 4, above - with a few key differences:
- A high-dynamic range RF amplifier precedes the receiver.
While the signal levels are pretty high on 630 meters, the output
from the TCI 530 HF antenna drops as the frequency decreases requiring
that a bit of extra amplification (around 15dB) be brought to bear.
- The extra filter elements on the receiver. These extra components were mounted directly to the receiver board.
As can be seen in the block diagram of Figure 6 and the schematic in Figure 7,
the "Low-HF" signal splitter has a port labeled "<=500 kHz" that was
designed to accommodate precisely this type of receiver. At the
time that the splitter module was constructed it was known that the
antenna did work at least
somewhat at 630 meters, but the practical low end usable frequency was
unknown - but this has since been determined as mentioned below.
- There is room for another Softrock Lite II receiver, to be
installed later, which is earmarked for 2200-1750 meter coverage.
While this receiver will have its own RF input and audio outputs,
it will share the power feed and ProgRock synthesizer with the 630
meter receiver, using the second of its three outputs.
What about the 2200-1750 meter receiver that was mentioned?
There are tentative plans to add a receiver that will include the 2200 meter amateur band (135.7-137.8 kHz)
and the so-called "1750 Meter" band (see FCC Part 15 §217)
that covers from 160 to
190 kHz - the two bands being comfortably covered using a receiver with
96 kHz of
bandwidth. Unfortunately, the HF antenna at this site works
miserably below about 250 kHz which means that another antenna -
perhaps an E-field whip or
an H-field loop - will be required to cover this frequency range.
Preliminary tests indicate that this antenna cannot be located at
the building housing the receiver gear owing to its proximity to the
above-ground power line that feeds the site and low-level leakage from
the various power supplies, but it may be possible to
place it on a tower some distance away, feeding it via coaxial cable.
When this might be done is unknown as it is not very close to the
top of a very large "to do" list!
Gerald (July 2002), "A
Software Defined Radio for the Masses, Part 1" (PDF), QEX,
American Radio Relay
(Sep–Oct 2002), "A
Software Defined Radio for the Masses, Part 2" (PDF), QEX,
American Radio Relay
(Nov–Dec 2002), "A
Software Defined Radio for the Masses, Part 3" (PDF), QEX,
American Radio Relay
(Mar–Apr 2003), "A
Software Defined Radio for the Masses, Part 4" (PDF), QEX,
American Radio Relay
- Johnson, Gary, "Measurements
on a Multiband R2Pro Low-Noise Amplifier System, Part 2" (PDF)
Joe, (November, 1984), "High Dynamic Range Receivers, Ham Radio.
translation of part of this article from a Dutch web site may be found here.
Clint, (March, 2018), "Managing
HF signal dynamics on an RTL-SDR receiver"
- Farson, Adam, "Antenna and Receiver Noise Figure"
- For general information about this WebSDR system -
including contact info - go to the about
- For the latest news about this system and current issues,
visit the latest news
- For more information about this server you may contact
Clint, KA7OEI using his callsign at arrl dot net.
Back to the Northern Utah WebSDR
- For more information about the WebSDR project in general -
including information about other WebSDR servers worldwide and
additional technical information - go to http://www.websdr.org