Understanding SDR receivers is not that hard. Let’s apply what we have learned to four popular radios and their HF performance.
At present, I have four SDR receivers: the ubiquitous RTL-SDR dongle, an SDRPlay RSP2, Microtelecom’s Perseus, and the Flex Radio 6300 transceiver. Let’s look at the HF performance of these radios. Three of them use direct sampling followed by digital down conversion, while the RSP2 has a quadrature sampling design.
At the low performance end, the RTL-SDR dongle has a front-end tuner covering 24-1700 MHz. followed by low IF sampling. Although there is a tracking preselector, HF coverage is effectively wideband. Recently, this dongle provides ways to bypass the tuner and attach an antenna directly to the ADC, which runs at 28.8 MHz. So, with a bit of alias filtering, you can direct sample from 0 to 28 MHz. Otherwise, you need an HF converter to translate up to 100 MHz. Since the specs for the RTL2382U quadrature detector are proprietary, performance information is scarce. But it appears to do some decimation and add to the nominal 7 bit performance.
Although I have played with the RTL-SDR on HF (and it works okay) I mainly use it as a spectrum analyzer on HF, and for stuff like ADS-B airplane tracking on microwave.
At the higher end, the Perseus and 6300 have comparable dynamic range figures which are more than sufficient for my listening environment in western Canada. The 14 bit ADC in the Perseus performs on par with the 16 bit ADC in the 6300 – which confirms that you need to check Effective Number of Bits, not just raw number. Both of these rigs have extensive processing gain achieved through oversampling.
In the middle of the quality range is the RSP2, which is effectively wideband at HF and has 30 dB less dynamic range. Because of its QSD architecture and lower sampling rate, there is less processing gain to help out and more spurious artifacts to deal with. However, for normal HF listening it sounds comparable to the Perseus when used side-by-side with SDR Console.
The Flex does all its DSP in the receiver, while the others rely on PC software. All four can operated remotely. However, the RTL, RSP2 and Perseus require a server while the Flex Radio is the server.
Since these radios range in price by more than two orders of magnitude, comparing them is probably unfair. Also, the RTL and RSP2 are optimized for DC to Daylight coverage, while the Perseus and Flex are optimized for HF only. Still, all four have comparable sensitivity on HF. Since the external HF noise at my location rarely drops below -125 dBm, these MDS levels are more than sufficient. The Noise Figures are in the typical 15-20 range, except for the RSP2 which is amazingly low.
Compared to the typical superheterodyne shortwave and premium receivers of the late 20th century, even the more modest RSP2 more than holds its own at a fraction of the price.
Understanding SDR Receivers – Additional Resources
While there is tons of information to help with understanding SDR receivers, I am going to mention a few resources that are particularly useful and, for the most part, quite readable at the intermediate level.
Andrew Barron, ZL3DW, has an excellent book An Introduction to HF Software Defined Radios. He also has a good paper on SDR Performance Testing. Adam Farson, VA7OJ has done many presentations on the subject, including A New Look at SDR Testing.
A useful overview of SDR design is provided by Pentek in their Software Defined Radio Handbook. Also worth checking out is a 1996 NTIA research paper on RF and IF Digitization in Radio Receivers. Although 20 years old, it is still quite concise and accurate at predicting the future of SDR development.