In Part 1 I described some background about SDR dynamic range as an introduction to the measurements I did on a RTL-SDR.com v3 USB “dongle” SDR. Now for the results.
I wrote a Gnu Radio program to help measure the dongle. It tunes the dongle, sets its front-end gain, shows a spectrum display, and also shows the power in a 500 Hz bandwidth around the tuned frequency. That power reading is what I used for these measurements. The flowgraph is available at my Github repository.
With the dongle set to receive a 192 kHz bandwidth, and using an HP 8642A signal generator and 10 dB attenuator, I obtained these results at 144.2 MHz:
| Gain | Noise | MDS | Clipping | Dynamic Range |
| 0.0 | -77 dBFS | -95 dBm | -15 dBm | 80 dB |
| 12.5 | -77 dBFS | -112 dBm | -33 dBm | 79 dB |
| 22.9 | -77 dBFS | -122 dBm | -44 dBm | 78 dB |
| 32.8 | -77 dBFS | -131 dBm | -52 dBm | 79 dB |
| 42.1 | -74 dBFS | -135 dBm | -60 dBm | 75 dB |
| 49.6 | -71 dBFS | -138 dBm | -67 dBm | 71 dB |
This shows more dynamic range than theory suggests. I’m not sure why. It does show clearly the impact of increasing the front-end gain: as the gain increases, the minimum discernible signal (“MDS”) gets lower, but so does the clipping level. All the gain does is move the operating range of the radio; it does not change it (much). But at gains >40, we do see the noise floor start to go up, and the dynamic range come down. That indicates that internal noise starts to become a factor, and suggests keeping the gain below that point.
By the way, I also did some spot tests at other frequencies. While there is a dB or two of difference, the results generally hold, at least through 1 GHz.
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