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 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 which are easier to construct at lower frequencies - 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
to 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 1.
- The second harmonic of the local oscillator (20 MHz) is >= 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 2, on the RF Distribution page (link), 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. One might wonder why a diode-ring
mixer is used rather than something like the NE602: The simple
truth is that the '602 has far inferior signal-hanling capabilities and
is easily overloaded compared to a diode mixer and its associated
Following the diode-ring mixer is
another bandpass filter - another one modified from the 30 meter QRP
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 because of the relatively
narrow passband filter on the input that reduces the number and breadth
of signals and because this is a "non
Providing the 10 MHz local oscillator is an inexpensive (approx. $2.50 in single quantities) TCXO. This device provides justenough
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 - in an unheated, not air-conditioned building - the
temperature will very
wildly - from below freezing to as high as 140F (approx. 60C)
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.
It was later found to be necessary to add yet another
RF amplifier after the one on the downconverter's output to bring the
signal level up enough to make the RTL-SDR dongle capable of hearing
the background (thermal)
noise on the 10 and 15 meter amateur bands on the respective downconverters. This amplifier was placed after
the output level adjustment potentiometer (R10 in Figure 1)
to minimize the amount of signal that this amplifier could see.
Because of the comparative deafness of the RTL-SDR in "direct"
mode, the apparent increase in noise figure by placing the gain control
at that point is irrelevant.
Because of the limited dynamic range of these dongles (no matter whether you use an upconverter, downconverter or "direct")
having such a level control on the input is absolutely necessary
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.
- RF Distribution and filter system
- Absolutely essential to any receive system is the means by which RF
is distributed - and filtered, the means by which this is done at the
Northern Utah WebSDR being 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