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

Jay Salsburg jsalsburg at bellsouth.net
Sat Jul 7 02:08:46 UTC 2012


Can I say I am impressed with what you are doing friedtj. One thing I have
wanted to do with these new Dongles with the e4000 tuners is bypass the
RTL2832 chip and piggyback a better ADC on the e4000 tuners output. Since
there is so much activity in the "Group" of squeezing the maximum
performance from the Dongle Tuners, it may be time to add better hardware.
Just to test to see if this is a good route to take, it should be possible
to connect a decent PC audio card to the output of the e4000 tuner to see if
it suffers any degradation, the AGC debacle can also be checked with this
method. If no problems arise then addition of a dual input, 14 bit, 100
Mega-sample, ADC would be the next step to take full advantage of the
bandwidth and dynamic range of the tuner that the RTL2832 chip is obviously
and purposefully limiting; forget the RTL2832 chip. Will someone publish the
e4000 tuner's datasheet and stop all the IP BS.

-----Original Message-----
From: osmocom-sdr-bounces at lists.osmocom.org
[mailto:osmocom-sdr-bounces at lists.osmocom.org] On Behalf Of friedtj
Sent: Friday, July 06, 2012 2:53 AM
To: osmocom-sdr at lists.osmocom.org
Subject: stroboscopic (aka equivalent time sampling) using EZCAP DVB dongle
?

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 ?

Thank you, Jean-Michel

--
JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 32 av. observatoire, 25044
Besancon, France





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