R820T, RTL2832U SDR USB stick – sensitivity, dynamic range

After looking around in the web, there doesn’t seem to be a whole lot of information out there on the sensitivity and dynamic range of the SDR USB devices, at least not for the type I’m using here. Even the R820T datasheet isn’t all that clear – there are various versions of the R820T, also using different clock frequencies, with 28.8 MHz, being the most popular lately.

Therefore, time for some measurements.

The setup:

(1) HPAK (formerly HP, then Agilent, now Keysight) 8662A Signal Generator as the reference source, 28.800 MHz, 500 mV level.

(2) HPAK 8642B Signal Generator as the test signal source. This has a calibrated output from -140 dBm to +20 dBm, and very clean and free of spurs, and provides up to 2.1 GHz.
Absolute amplitude accuracy is about 1 dB, linearity is considerably better. As it says on the instrument cover – 70 pounds, “two person lift”.
The 8642B is phase locked to the 8662A clock, via a common 10 MHz reference signal. So even with drift, there can’t be any frequency errors getting into the way of our precision testing.

(3) Some well-shielded test cables, RG223/U, and adapters to link to the MCX connector (use a good test cable, but not your best – most of the SMA to MCX connectors aren’t all that precise, and may damage precision SMA connectors).

(4) The modified SDR USB stick, see earlier post.
r820t rtl2832u sdr usb dut

(5) Laptop PC, running SDRSharp. 1.024 MSPS, all automatic gain and frequency adjustments disabled, I/Q correction enabled.

r820t rtl2832u sdr usb test setup

First, the sensitivity check. Tuned the SDR USB to various frequencies, and measured the input power (dBm needed to get a -40 dB reading, at max gain of the SDR USB – 49.6 dB nominal), this is about 15 dB above the noise floor, and still a signal level that is very stable and can be accurately measured. Afterwards, set gain to 0 dB, and increased RF input power until 0 dB reading was obtained – this is the maximum power that can be reasonably fed to the SDR USB (no damage will occur up to +10 dBm; and even +20 dBm doesn’t seem to do much, at least not if only applied for a short time).

Power levels for -40 dB reading at max gain, and 0 dB reading at 0 dB gain:
r820t input sensitivity and max power
Sensitivity is quite constant over a pretty large range, up to 1500 MHz, no problem. Lowest frequency the thing can handle is about 24 Mhz (doesn’t tune any lower). Note that there are some spurious signals present around 28.8 MHz, (internal) ref clock leakage, and its 2nd harmonic.
R820T usb sdr dynamic range and sensitivity

The RF input power (about -130 dBm) to get -40 dB amplitude, at max gain of the SDR USB, this is quite remarkable, and still about 15 dB above the noise floor. So the R820T exhibits very high sensitivity, no doubt.
Here is an estimation of the dynamic range – “useful” because, it is still has some margin for noise. For the full dynamic range, add about 15 dB.
r820t sdr usb dynamic range
About 93 dB (108 dB full range, from noise floor, at 49.6 dB gain, to 0 dB at 0 dB gain).

7 thoughts on “R820T, RTL2832U SDR USB stick – sensitivity, dynamic range”

      1. Is it about several hundred hertz or kilo hertz? Because bandwidth has a significant impact to sensitivity, thermal noise is proportional to bandwidth. I did some calculations about my cheap CB radio, the minimum ‘detectable’ signal strength is -118 dbm while bandwidth is 12.5 kHz, consider the antenna temperature noise is about 2 db on uhf band, then that is 10 db above thermal noise level, . Without the bandwidth of reference signal, I can’t compare the performance between my cheap CB radio and rtl-sdr.

        1. It’s as narrow as can be, from a 8642b or 8663a generator. Usually I only know their phase noise, but the phase noise may be equivalent to less than 100 Hz

          1. If the bandwidth is 100 hz, the sensitivity is really high, 10 db above thermal noise is the minimum detectable signal, especially for it’s price.

  1. Regarding the question about bandwidth, I think the issue is not the bandwidth of the reference signal (which as noted is very narrow CW), but rather the bandwidth of the receiver. I’m doing some very similar measurements right now and setting the dongle to 1.536 msps, then decimating in software by 8 to yield 192 kHz bandwidth. If I were to decimate that further, down to say 1 ksps to look at just a 1 kHz bandwidth, the noise floor would drop by 20 dB or so and the dynamic range would correspondingly increase (too lazy to do the math, but decimating by 4 gains 6 dB dynamic range). FWIW, my measurement of MDS at 192 kHz bandwidth and 50 dB gain (RTL-SDR.com v3 dongle) is -145 dBm, and overload point is -53 dBm, for a dynamic range of about 95 dB. That’s greater than expected, so despite having AGC turned off, there may be some gain control (or compression) going on in the tuner chip.

    1. Hi John,
      You are right, I did the test at 1.024 MSPS, with no further decimation (just zoomed in to check the signal amplitude). You can in fact increase the dynamic range for most SDRs, by oversampling the ADC, Analog has some good documents about it, e.g., http://www.analog.com/en/technical-articles/maximizing-the-dynamic-range-of-software-defined-radio.html
      All in all, the devices are quite useful, I have done some more modifications to use the USB sticks, including, removing the switching power supply from the circuit which introduces a lot of noise and spurious signals, and wrapped the whole thing in adhesive copper foil and metal filled JB weld to seal it from anything that is radiating into or from it.

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