RTL-SDR.com Dongle Measurements (Part 2 — Results)

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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