How RTL-SDR samples signals

David Basden davidb-sdr at rcpt.to
Tue Aug 20 03:17:34 UTC 2013


The RTL2382U has an ADC onboard; The dongles don't use a seperate
ADC. The main other component is the tuner.

The RTL2382U uses a sigma-delta ADC, so it samples at a much higher
rate than it needs, but only at 1 bit. It then trades the high
sample rate for higher dynamic range. (Think reverse PWM, although
that is a huge oversimplification).  


http://www.maximintegrated.com/app-notes/index.mvp/id/1870

The crystal is to drive the clock of the RTL2832U. It's almost certainly
running a single 1 bit sample per clock.

If you were to get the samples at the 28.8Msps rate, they would be 1 bit 
samples, which you would still have to filter and downsample for most
uses.  I'm not aware of any way to get the samples out at that rate.

David

On Mon, Aug 19, 2013 at 11:44:51PM -0300, Lucas Ingles wrote:
> Hello to all,
> 
> Please, can someone help me to understand how RTL2832U samples signals?
> I know it has two ADCs, one for the I (in-phase) component and other ADC
> for the Q (quadrature) component.
> 
> In http://superkuh.com/rtlsdr.html the author states the signal is sampled
> initially at 28.8 Msps and then re sampled to present whatever sample rate
> is desired.
> 
> But
> http://www.realtek.com.tw/products/productsView.aspx?Langid=1&PFid=35&Level=4&Conn=3&ProdID=257
> says
> that 28.8MHz is the frequency of the crystal, not the ADC.
> 
> I am confused with that, can someone help me?
> If RTL2832U is able to sample at 28.8Msps, why don't use the maximum sample
> rate? Maybe USB limitations?
> 
> Also, all ADCs used in RTL-SDR dongles are inside RTL2832U? Or we have
> external ADCs?
> 
> Thanks very much in advance,
> Lucas Ingles




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