How RTL-SDR samples signals

David Basden davidb-sdr at rcpt.to
Tue Aug 20 04:24:21 UTC 2013


Hmmm. You're right. My bad; I confused it with (I suspect) the E4000
VCO, and it really doesn't make sense given the wider channel bandwidth
of a DVB signal.

At 8 bits, 225 Msps isn't going to fit down USB2 though :(

On Mon, Aug 19, 2013 at 08:47:20PM -0700, jdow wrote:
> That doesn't necessarily square with "7-bit ADC for RF signals level
> measurement". It's unclear to me where it would get the signal with
> enough levels that a 7-bit ADC would get used. You'd have to decimate
> down to 225 Msps or so in order to get 7 bits of data.
> 
> {^_^}   Joanne/W6MKU
> 
> On 2013/08/19 20:17, David Basden wrote:
> >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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