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

Sylvain Munaut 246tnt at gmail.com
Sun Jul 8 15:29:02 UTC 2012


> 1) Yes, if all you need is to be able to detect a signal on an analog stream its Nyquist Frequency will do, (2 times the frequency being measured) a 20 megasample ADC would be adequate for the E4000 tuner's I/Q output.

This is IQ signal you don't need twice the bandwidth, you only need
once. So a 8 Msps IQ would be adequate to represent a 8 MHz bandwidth.

> [snip ....]
>You need at least 10 times that bandwidth to accurately reconstruct Phase Modulated Signals.

No you don't. You want a concrete example: Most GSM phone are
demodulating a phase modulated signal by sampling at exactly one
sample per symbol (so they're even sampling _lower_ than the actual
signal bandwidth)

And although oversampling is useful in certain case to analyze signal,
it's essentially useless if the signal has _already_ been filtered
like it is in the E4000 case. If the IQ output has only 8 MHz of
useful content at the output, you can oversample all you want, it
won't bring back what the E4000 discarded ... Those antialiasing
filter you mention, they're in the E4000 !


2) I didn't say bigger / better SDR would be useless ... I'm just
saying if you're spending hundreds / thousands on better ADC and
better PC interface you might as well ditch the E4000 and use a better
RF front end as well. The spectral output of the E4000 is far from
being "clean", there is distortions / LO leakage / random spurs /
28.8M clock leakage / ... etc ... Oh and Elonics declared bankrupcy so
the E4000 supply is fading ...


Cheers,

    Sylvain




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