In this post you can read about the sensitivity test of Software Defined Radios (SDR). SDRs like the Monitoring and Collecting Receiver SRM-3000 and the Search and Intercept Receiver SRS-3000. For additional content visit our resources page or subscribe to our newsletter here!
Receiver sensitivity specifications
One of the most important features of a radio is its ability to receive low level signals. In other words: its sensitivity. We have lot of different definitions for receiver sensitivity. Some excellent descriptions can be found on radio-electronics.com. For linear modulation formats, like AM, SSB, and CW, the Signal-to-Noise Ratio (SNR) or one of its variants, e.g., the signal plus noise to noise ratio ( =(S+N)/N ), are most commonly used.
In the first case, we can measure the signal level and the noise level separately. This may be done with a spectrum analyzer employing a simple sine wave test signal. In the second case, we resort to measuring the signal and the noise together. Where we can’t separate the noise from the signal. For this we utilize a wide band power meter or a Root Mean Square (RMS) voltmeter.
If the difference between the signal and the noise level is greater than 10 dB, the above defined two ratios are practically equal. When we look at the specs of different receivers, sometimes it is hard to immediately compare the performance of different models, because they are specified differently. For example, the SNR may be defined with different bandwidths in mind. More specifically, 10 dB or 12 dB SNR values represent vastly different receiver sensitivity based on whether it is defined for 500 Hz, 2.1 kHz, or 2.4 kHz bandwidths.
Practical receiver sensitivity test of the DRU-244A-based SDR
Receiver sensitivity test setup
The test setup is very simple. We need to use a calibrated test generator to feed -80 dBm and lower signal levels into the input of the receiver, while we measure the audio output level with an RMS voltmeter.
Receiver sensitivity measurement procedure
Switch on and tune the receiver to the test frequency (F) with a given bandwidth (BW). First, we disconnect the signal source and measure the output noise level. Secondly, we connect the RF signal source, and increase the signal starting from a very low level, until we have an audio output voltage with a given level. The signal level on the generator (P) shows the receiver sensitivity for a given bandwidth and the SNR level. Instead of traditional voltage meter, like the venerable HP-400, you can use a sound card-based scope and audio analyzer. Usually, it has built in SNR measurement capability.
For our last measurement, we used the Multi Instrument software by Virtual Instrument Technology.
You can download the 21 day free trail from this page: http://www.virtins.com/downloads.shtml
Or you can use other similar audio analyzer program from Daqarta where you can download a 30 days trial of the latest version: http://daqarta.com/dqdown.htm
We already have digitized samples in the SDR radio, so it is possible to skip the DAC/ADC sound card conversion, and with the Virtual Audio driver we can send the samples directly from the SDR radio software to the measurement software. We’ve used this audio driver to connect the SDR receiver to the DRM decoder in one of our recent post.
SDR receiver sensitivity test results
We have tested the DRU-244A at F = 10.1 MHz, BW = 2.1 kHz, and S+N/N = 10 dB with and without a pre-amplifier. During the tests, we used a ZX60-P103 amplifier from MiniCircuits with fixed 23 dB gain and less than 3 dB noise figure. It is specified from 50 MHz, however, it can be used down to 2 MHz.
The following pictures show the different steps of the SDR receiver sensitivity measurement for SSB, CW, AM, and FM signals.
SSB (2.1 kHz) and CW (400 Hz):
AM and FM with signal display on the SDR receiver, noise and signal out, and the generator.
Receiver sensitivity results and conclusion
As you can see from the receiver sensitivity measurement results, the sensitivity is
SSB -111 dBm at 10 dB S+N/N with 2.1 kHz bandwidth
CW -119 dBm at 10 dB S+N/N with 400 Hz bandwidth
AM -105 dBm at 10 dB S+N/N with 30% modulation
FM -108 dBm at 10 dB S+N/N with 3 kHz deviation
The sensitivity can be improved with some external low noise preamplification and additional external gain to reach -122 dBm sensitivity in SSB operation mode.
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