RF Distribution and filtering
RF Distribution and Filtering:
Block diagram of the various RF signal paths at the Northern Utah WebSDR.
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:
- 630 meters
- 160 meters
- 80/75 meters *
- 60 meters
- 40 meters *
- 30 meters
- 20 meters *
- 17 meters
- 15 meters *
- 12 meters
- 10 meters *
* - Multiple outputs connections needed if narrow-band "Softrock" type receivers sound cards are used.
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.
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: While 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) must precede the at least some of 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!
Signal distribution strategies:
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 from cascaded splitters 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 - such as the "Diplexer" type splitting.
To accomplish this several modules were built, depicted in Figure 1:
- 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. Having this signal branch "future
proofs" the RF distribution system of the WebSDR should a
high-performance 0-30 MHz direct-sampling receive system capable of
supporting many users at once become available.
- The "Splitter/AM BCB Reject/Amplifier module". See figure 2, top, for a
schematic diagram. (For an article on
this device, see reference #1, 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
directly to the input of the "narrowband" branch of the RF system.
This input is afforded a degree of lightning protection with the
use of two gas discharge tubes in parallel.
- 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 just outside
this range (e.g. the 630 meter band and the 160 Meter and higher bands) as well as the HF spectrum in general.
- Included in the design of the AM broadcast band reject filter is a "bypass" adjustment that
amount of ultimate rejection of the filter to be reduced so
that the signals across 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 2, upper-middle, for a
- This module gets its input from the "split" HF output of
previous module, but it could have been 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 and significantly attenuates local oscillator
bleedthrough (an issue
with "QSD" type mixers found on "SoftRock" receivers) while also
providing 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 60 meter 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 2, 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 GALI74+ MMI. This amplifier is
useful at this point in the signal path because the filters themselves incur 1-3dB of loss and
this factor shouldn't be callously disregarded as one approaches the
top end of the HF spectrum where noise figure can start to be an issue.
Also depicted in Figure 1 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 at 20 meters and 15 dB at 10 meters. 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 at 10 meters 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 you are 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 2
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 source 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 through 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!
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"
Pages about other receive gear at the Northern Utah WebSDR:
- Softrock Receivers
- This page describes the "High Performance" receivers that use
"Softrock" direct-conversion receivers and sound cards. These
receivers cover limited bandwidth (up to about 192 kHz) but have excellent weak and strong signal handling properties.
- RTL-SDR Dongle-based receivers
- Described here are the "not high performance" receivers using the
ubilquitous RTL-SDR dongles. These receivers cover up to 2 MHz of
bandwidth, but their limited A/D bit depth (only 8 bits)
means that they can suffer from too much and/or too little signal input
- often depending on band conditions. Included on this page is
information about how to make the most of these as well as helping to
manage when multiple RTL-SDR dongles are used on a Linux-based system.
Go to the main "RX Equipment page.
- RF Downconverter for RTL-SDR receivers
- While there are RTL-SDR dongles that contain built-in upconverters to
allow reception across the entire HF spectrum, this may not be the best
way to do it. When receiving frequencies at or above the Nyquist
frequencie(s) on HF, one can downconvert to lower frequencies and get
good results, all described on this page.
- 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