RTL-SDR Pan-adapter using HDSDR with the IC-751A

There is little doubt that the RTL-SDR tuner dongle radio receivers are the hottest new thing for the amateur radio experimenter.  Re-purposing the $10 tuner dongle to be an extraordinarily wide-band software defined radio is the subject of countless internet articles and videos.  Here I am adding my experiences to the crowd.  What I will describe are the steps required to use one of these devices as a full-band pan-adapter for a conventional receiver.  Once that is working, then we can look more closely at the actual RTL-SDR receiver performance, compared with the native performance of the parent receiver.  I’ll discuss the performance of RTL-SDR receiver in another segment.  For now, let us get this up and running.

First a diagram of what we are about to do:

Block diagram of hardware and software components.

My RTL-SDR is the common RTL2832U  chip with R820T tuner.  The device can tune from about 30 to 1700 MHz.  To be useful for the HF ham bands, some up-conversion is required.  Many HF receivers employ a first IF stage, mixing upward to a 45 to 75 MHz intermediate frequency.  This is perfect for the RTL-SDR tuner.   Tapping into the IF after the first mixer is easy on the IC-751A since there is an unused connector on the RF board that is designed for scope monitoring of signals at this location.  On my old IC-745 I had to solder in a resistor at the right spot to get the signal.  Usually a little study of the receiver schematic will make clear where you need to tap into the 1st IF signal, and whether the radio design will make this easy or not.  It is important to tap in right after the mixer and before the 1st IF band-pass filter, since otherwise the pass band that you can tune with the RTL-SDR as a pan-adapter will be severely curtailed by the IF filter.  Ideally, use a short piece of 75 Ω coax to match to the RTL-SDR tuner input impedance (but 50 Ω is okay too).

Now we can bring up the RTL-SDR in the HDSDR software.  If you are doing this for the first time, some good instructions are located here.  Of the various SDR radio programs that are floating around the internet, HDSDR seems to be the best for interfacing the RTL-SDR with another radio and then being able to co-ordinate the tuning of the two systems.  When you first bring the dongle up in HDSDR you should be able to see HF band signals when you tune HDSDR to the IF frequency.  This makes it confusing to remember where you are tuned, so HDSDR has the ability to shift the tuning display by the IF frequency offset (RF front-end frequency options & Calibration tab).  I suggest tuning the receiver to a well-known strong AM station (WWV at 5.0 or 10.0 MHz is a good choice) and then playing with the IF-offset until you have HDSDR tuned correctly.  Including a global offset of 10kHz is a good idea so that the IF zero beat is not in the middle to the HDSDR’s audio output band.  Additionally, it is important to make sure that the side bands are not mirrored, and the corrections for tuning USB, LSB, CW, etc. match that of the parent receiver.  Tuning JT65 signals can help clarify this to make sure they are not mirrored (and un-decodable!). For whatever reason, I needed to check the “Swap I and Q Channel for RX Input” to achieve the correct result.  With my radio recently adjusted to be on-frequency, I found the settings shown in the figure above to work well.

HDSDR will be the master of all, so we set up the HDSDR Omni-Rig interface to talk to the IC-751A via the serial RS-232 port.  At this point, it should be possible to tune with either HDSDR or with the receiver directly and have them follow each other.  As a pan-adapter, you are ready to go.  You can open the band in HDSDR and see signals across the band.  A simple click and you will tune to that signal on the 751A.  For phone operation, you are done – have fun.

Note:  There is/was a bug in the OmniRig IC-751.ini file that resulted in flaky behavior when switching modes. If you have any problems with the present OmniRig distribution, send me a note and I’ll get you the correct file.

RTL-SDR tuned to the middle of the SSB section of the 20 m ham band. Many upper side band conversations are visible in the waterfall.

With everything working, the pan-adapter begins to provide useful information.  The example snapshot above shows a smattering of signals on the USB phone section of the 20 meter band where the HDSDR is tuned.  It appears that the IF pass band of the 751A drops the signal level on the RF waterfall a little, so you see less background in the center of the waterfall where the parent receiver is tuned.  With the pan-adapter synchronizations working, clicking on a signal will immediately return the receiver so you can copy the signal.  In fact, you can hear the audio on either the parent receiver, or on the computer speaker from the HDSDR demodulated audio output.

For digital modes, we need to run other programs to decode the digital signals.  WSJT-X for JT65 and JT9 signals and FLDIGI for a multitude of digital mode transmissions are the programs I use most often.  Besides running those programs concurrently with HDSDR, we need to plumb in the audio and control signals to these applications.   For most straightforward operation, the digital mode program will be connected to the audio channels of the tranceiver just as you would normally for the stand-alone configuration, but the rig control must now come from HDSDR. To do this you need a Virtual Serial Port pair.  There are a couple of possible programs that are available to do this for you.  I’m using VSPM from Steve Nance, K5FR.  You will have to write to him with your call sign to get a copy for you to use.  Reading between the lines, it looks like Steve’s program, updated and expanded, is being sold as Eltima’s Virtual Serial Port Driver, which I’m sure also works, but is not free.  A second free program, Eterlogic VSPE, also might work for you.  VSPE has some nice additional features, like the ability to have port “splitters” as well as port pairs, but I also experienced some BSOD (Blue Screen Of Death) errors when using this program on my old Windows Vista laptop.  Whatever you use, once you install a port pair you can now connect the HDSDR’s “CAT to HDSDR” port to the rig control communication port in WSJT-X or FLDIGI.  HDSDR talks in “Kenwood TS-50S” language, so the digital rig interface should be set up accordingly.

So far, the RTL-SDR dongle is just showing you a wide view of the ham band and the parent transceiver is operating in the normal way.  But there is more that we can do with the RTL-SDR dongle than just look at the big picture.  If we pipe the output of HDSDR’s demodulated audio into a digital mode program, we can use the RTL-SDR dongle as the real receiver rather than just the panoramic view generator.  To do this we need another little program called Virtual Audio Cable.  This program is not free, but is inexpensive for what it does and how well it works.  Once installed, the program can generate pairs of sound-device ports, virtual audio cables, that can be used to pipe audio streams between applications.

We can connect the “RX Output” from HDSDR into a virtual cable, then connect the other end of the virtual cable to, for example, the WSJT-X sound card input so that we could decode JT65 and JT9 signals directly from the RTL-SDR dongle.  Once you discover the flexibility of Virtual Audio Cables and Virtual Serial Ports you will come up with many interesting configurations of software components that can be strung together in interesting ways.

I found that the RTL-SDR dongle can be used as a serious receiver of digital-mode signals.  However it lacks any transmit capability so you must return to the parent transceiver for that function.  Beware that the frequency calibration “going around the loop” between the RTL-SDR receiver and the conventional transmitter must be carefully maintained.  I would fine tune with the USB offset number in the RF calibration tab.  The wide audio bandwidth that the RTL-SDR can generate is not present in the conventional transmitter.  Just because you can decode a JT9 signal up at 5 kHz on the audio of the RTL-SDR does not mean that you can point the transmitter there and have it work!  You can use some of the same principles, using VSPs and VACs to connect other SDR radio programs together with a slew of audio processing and decoding programs.   The possibilities are almost endless — you better go get one of these and start playing with it!

Update:  See Using the RTL-SDR Pan Adapter for some example of how useful this device really is!