RTL-SDR sample bit depth
John Ackermann N8UR
jra at febo.com
Sun Mar 4 15:15:05 UTC 2018
As a follow up to this, I've been doing testing with an RTL-SDR.com v3
dongle and Gnu Radio flowgraph. I set sample rate to 1.536 msps and the
flowgraph decimates by 8 to yield 192 ksps, which is displayed on an FFT
without further processing. Gain control is set to manual, and I step
the RF gain between 0 and 50 dB for testing. I use a precision signal
generator (HP 8642A calibrated with an HP power meter) as a signal
source and test at 144 MHz.
For an approximation of the minimum discernable signal (MDS) I adjust
the signal generator amplitude until I see a noticeable signal that is
consistently just above the noise. To find the overload point, I
increase the amplitude until I see the first spur appear -- it's a very
sudden transition, with a 1 dB amplitude increase triggering spurs many
dB above the noise.
Based on the assumptions in my earlier message, I would expect to see a
dynamic range of about 59 dB (~50 dB from 8 bits at 1.536 Msps, plus 9
dB processing gain by the decimate-by-8).
However, I'm seeing closer to 100 dB dynamic range -- for example, with
the RF gain set to 20 dB, the MDS is -124 dBm and the overload level is
25 dBm. This tracks for various settings of the RF gain, though there
seems to be a few dB of compression with gains above 30 dB.
I'm trying to understand this discrepancy, which could be the result of:
1. Some AGC operation or gain compression in the R820T2 tuner chip;
2. My assumptions about the internal sample rate bit depth or
decimation being wrong; or
3. My math being wrong (for example, is there a log10 vs. log20 error
in my dB calculations, or is the dB scaling in the FFT showing voltage
rather than power?).
Any thoughts would be appreciated.
Thanks,
John
----
On 03/02/2018 09:46 AM, John Ackermann N8UR wrote:
> Hi --
>
> I'm trying to understand the sampling and decimation structure of the
> RTL-SDR dongle, to work out the effective number of bits after decimation.
>
> From Google and looking at the librtlsdr code (which is beyond my
> programming depth), I think I've figured out the following. I'd much
> appreciate verification/correction/amplification.
>
> 1. Actual ADC in the RTL-2832U is a single-bit sigma-delta running at
> some very high rate.
>
> 2. This is converted to 28.8 msps at 8 bit depth.
>
> 3. 28.8 msps is downsampled to the rate requested by the user and sent
> over the USB bus as 8 bit unsigned IQ pairs.
>
> Based on that, I *think*:
>
> a. Any processing gain in the downsample from 28.8 msps/8 bits within
> the chip is lost because the wire samples are limited to 8 bits. The
> output is 8 bits dynamic range regardless of the sample rate set.
>
> b. THEREFORE... for best dynamic range one wants to set the RTL-2832U
> to the highest sample rate that avoids lost samples, and do further
> decimation in the host processor, where the added bits aren't lost on
> the wire.
>
> I'd appreciate any verification or correction of that analysis.
>
> Thanks,
> John
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