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:
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.
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!
Hi! I have an icom-751a and was wondering if you could help pinpoint the unused connector on the RF board you used to tap the IF that you used. Thanks!
First remove the top and bottom covers on the radio and then remove the large shield cover with one screw on the RX filter and IF board. There is a small white plug on the edge of the circuit board near the 70 MHz Mixer area. The plug is under a grey coax up against the the edge of the shield so it hard to see. The pin towards the front is ground and the pin towards the rear is the 70 MHz tap point. I used a 100 ohm resistor in series with the center conductor on the tap point.
I see you have previously done this in an Icom 745 by adding a resistor. Do you have more details?
please can you give more details about connecting the ic751 to pc using the serial?!
The IC-751 had provision for the optional UX-14 serial interface. These are almost impossible to find. However, Piexx makes the UX-14PX which is available from Piexx and better than the original. The UX-14PX has a telephone jack connector that goes to a DB-9 RS-232 connector. Many modern PC’s don’t have DB-9 serial ports on them any more, so I have a USB to DB-9 com port adapter that accepts the RS-232 connector.
A number of the common ham programs have drivers for the IC-751. A good one to start with is FLDIGI. You have to locate the rigcat XML file for IC751 and then it should start to work for you once you set up the rig in FLDIGI. I modified the FLDIGI rigcat file (now in the distribution) so that it can use the optional S-meter read capability that the piexx board offers.
Has anyone experienced degraded receiver performance on the IC-751 when tapping into the IF section with only a resistor? Doesn’t the RTL-SDR load it down? If seen kits featuring a JFET source follower to present a high impedance to the IF filter. Is this needed for an IC-751? Thank you!
Hi Rob, If you look at the circuit at the pick-off point, you will see that from the IF stage there is already an 82 ohm resistor to ground. Then a 2 pF cap in series with the pick-off SCOP connector that I use to tap to the RTL-SDR. At the 70MHz IF, the 2pF is ~ 1.1k ohms reactance. So you can see that the IF is pretty well isolated from the SCOP connector. Rather, what is observed in the RTL-SDR reception is loading the pick-off point by the IC-751A’s downstream IF filter passband. The noise floor in the IF passband is a few dB lower than outside the pass band, presumably because pass band loading is comparable to the 82 ohm load that sits there. In short — buffer amplifier is not required.
Great explanation. Thank you, Gary!