Testing the RTL-SDR V3 Direct Sampling Mode for use in a 2-FSK RPiTX Modem

Over on his blog, Rowetel has been testing our RTL-SDR Blog V3 in order to possibly use as a cheap FSK receiver for his RPiTX 2-FSK modem project. His post details some measurements that he's done in order to determine the lower HF band performance of the RTL-SDR V3 running in direct sampling mode, and it's viability for use in his 2-FSK modem system.

In the first test he uses RPiTX to generate a 2-FSK signal, which is then received and decoded by a RTL-SDR V3 connected to an attenuator and laptop. The Bit Error Rate (BER) is then measured while the attenuation is increased until the decoder fails. With this test he found a MDS somewhere between -115 dBm and -125 dBm, and a maximum input power of -30 dBm before clipping.

In another test he measures the RTL-SDR's ability to withstand a blocking CW signal. The results show that even with a 65 dB stronger signal just 7 kHz away, the 2-FSK modem system was able to continue working.

Finally he concludes:

So I figure for the lower HF bands this receivers performance is OK – the ADC quantisation noise isn’t likely to impact performance and the strong signal performance is good enough. An overload of -30dBm (S9+40dB) is also acceptable given the use case is remote communications where there is unlikely to be any nearby transmitters in the input filter passband.

Test Setup
Test Setup
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