Once I had the RTL-SDR radio working as a pan-adapter for my IC-751A, I quickly became interested in seeing what I could do with all of those signals that were apparent on the SDR radio’s waterfall. My first goal was to see if I could receive the complete JT65 and JT9 signal band, with both modes usually in a band region up to 5 kHz wide on the digital sections of the main ham bands. The WSJT-X program is happy to decode signals with this audio bandwidth, but the native IC-751A only has about 2.8kHz audio bandwidth on the upper side band mode. So this could be a simple enhancement of the existing receiver if it really worked. I set up the RTL-SDR and various software components as described previously. But first I wanted to know just how good is the SDR receiver really?
There is a big advantage to have the RTL-SDR on the IF of a good multi-band receiver. By the time the RF signals get to the 1st IF mixer, they have already been filtered by the low-pass band filters of the parent receiver, which effectively eliminates interference from local high power FM stations that can degrade the performance of the RTL-SDR. Hence, the RTL-SDR should be able to perform just about as well as it ever will in this situation.
Questions immediately arise when trying to set the proper receiver gain. There are many places to turn up or down gain levels. Here is a list starting at the antenna:
- The antenna tuner can dramatically increase the RF signal level when the antenna is resonant.
- The IC-751A has an RF preamp before the 1st mixer that can be deployed.
- The RTL-SDR control allows “Tuner AGC” or fixed Tuner Gain from 0 to +50 dB.
- The RTL-SDR also allows “RTL AGC.”
- The HDSDR software has AGC and Volume controls that affect the demodulated output levels.
- The sound card channels have level adjusters.
- WSJT-X has an input audio gain setting slider.
What I discovered was that the RTL-SDR receiver needed as much RF gain as I could give it to get comparable results to the IC-751A. A tuned antenna helped the RTL-SDR much more than it helped the IC-751A for clean reception. With the antenna well tuned, the IC-751A RF preamp was less important than if the antenna was not resonant. The RTL-SDR Tuner Gain needed to be all the way up, +50 dB, for best performance. “Tuner AGC” seemed to accomplish the same thing as just setting the gain at 50 dB. Once past the Tuner Gain setting, all of the later gain adjustments for the demodulated audio had little effect unless you turned things down completely. I followed the WSJT-X recommendation to set audio levels such that the quite band noise level showed about 30 dB on the WSJT-X level indicator.
The figure above shows what I manage to receive using the two receivers on the same signals. From the antenna to the 1st IF stage of the IC-751A, the signals travel the same path. After that, the RTL-SDR dongle’s tuner and 8 bit ADC generate digital signals that HDSDR converts to an audio stream, which is then sent on to one instance of WSJT-X. The other WSJT-X window is decoding signals processed by the IC-751A in the conventional way. Hence, just as I did before, we have a simple method to compare the relative receiver performance using the reported JT signal to noise reports generated by the WSJT-X software as the metric. One thing you can immediately notice is that the HDSDR bandwidth is large enough to cover both the JT65 and JT9 signal regions of the band whereas the 751A’s filters limit the bandwidth to about 2.8kHz. The other thing you might notice if you look carefully, is that some of the splatter that you see on the IC-751A in the presence of a very strong signal is not present on the SDR radio. Notice, at time 19:46, the strong signal at ~1200Hz, and the splatter 2200 – 2500 that is absent on the upper image. Goes to show that arithmetic doesn’t generate inter-modulation images, where as real mixers and amplifiers can.
So on to the noise performance data summary from thousands of JT65 and JT9 signal reports, generated by simultaneously decoding the signals on the two radios. The table below summarizes the experimental results. The first column is how much better the IC-751A is compared to the the dongle using HDSDR.
With the non-resonant antenna, especially on 40m, the RF signals are weak and need amplification to achieve the same performance as the IC-751A. However, you can see once the RTL-SDR dongle is given enough gain by tuning the antenna or using the preamp, then there is really very little difference in the quality of the decoded signals that WSJT-X provides between the software and hardware radios.
At first I found this quite surprising, bearing in mind that the RTL dongle has merely an 8-bit ADC. Realize that the range of signal levels the WSJT-X can decode, and I accept for comparison, is about 20 dB for JT65 and >40 dB for JT9 signals. Now bear in mind that with 8 bits, the RTL’s ADC will be limited to a maximum of 48 dB dynamic range. So, I guess this “just fits” more or less. Very seldom do we bump into the top end of the dynamic range of the JT signals. Rather we are almost always struggling to pull them out of the noise. So as long as there is sufficient gain at the front end, the RTL tuner can do its job. It begs the question about any advantage an SDR with a higher resolution ADC might have. Maybe someday I’ll get one and I can find out!