stroboscopic (aka equivalent time sampling) using EZCAP DVB dongle ?

Stefan Sydow st at metafly.info
Fri Jul 6 08:19:04 UTC 2012


The rtl-sdr (and all other software) has only a bandwidth of 3.2MHz
maximum available. Only the hardware demodulator inside the realtek chip
can process full 8MHz. If you consider the filter setting of the rtl-sdr
you effective bandwidth is about 2 MHz. It is hard coded in the tuner
code but reasonable for most settings, as it avoids aliasing.

Am 06.07.2012 09:52, schrieb friedtj:
> I am trying to implement equivalent time sampling using an EZCAP dongle configured using
> gnuradio-companion. Since I am not completely clear about the tasks of the E4000 wrt RTL2832,
> it could be that I am missing a significant information, so here is my experimental setup aimed
> at developing a monostatic pulse mode RADAR :
> * I am using a radiofrequency acoustic delay line to generate 4 echos delayed 1 to 2 microseconds
> after an incoming excitation pulse is generated by a frequency synthesizer. The excitation pulse
> is 125 ns long, and repetition rate is 4 microseconds. The carrier frequency at 860 MHz is chopped
> by a fast duplexer, one side being connected to the frequency synthesizer and the other sizer to the
> EZCAP dongle
> * the EZCAP is configured with an LO at 860 MHz, 2 MS/s output, and I plot |I+jQ| after keeping
> only 1 every 8 sample (2 MS/s=500 ns delay between samples, and 1 every 8 samples means a spacing
> of 4 us, close to the emitted pulse repetition rate)
> * equivalent time sampling is obtained by slightly tuning the emitted pule repetition rate off the
> 4 us: I have checked that I can indeed obtain time stretching by tuning the pulse repetition rate
> +/- 50 ns away from the 4 us dela, with time stretching factors of up to 10^5 (ie the 4 us pulses
> are recorded as 400 ms traces (thus including 10^5 points or an equivalent sampling rate of 25 GS/s).
> 
> Now of course a stretching factor of 10^5 is overkill and the RF front end definitely does not have such
> a huge bandwidth, this is just to demonstrate the concept. With a more reasonable stretching factor of 10 (ie
> sampling every 4.4 us with a 4 us emission pulse repetition rate), I can expect to convert a 2 MS/s sampling rate 
> to an equivalent time sampling of 20 MS/s, which would already improve my monostatic RADAR resolution by a factor 10,
> and more or less fit my targetted resolution.
> 
> So now the questions:
> * the 125 ns pulse I generate would span 8 MHz. From my reading of the E4k description, this is within the
> IF bandwidth available. However, although I can observe the emitted pulse (0 dBm), I cannot see the echos
> (-35 to -40 dBm), whatever LNA gain I use (from 0 to 40 dB). Is the default configuration of the E4k an IF of
> 8 MHz, or lower. What parameter should I give the rtlsdr gnuradio source block to make sure I have the right bandwidth ?
> * since I am not clear about task distribution between the E4k and the RTL2832, is there another limitation that
> will prevent a 8 MHz wide (bandwidth) signal to be recorded through the 2 MS/s I/Q output recorded ?

The e4k does the filtering but has an analog output. The RTL2832 is in
effect an USB ADC.




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