Re: [Discuss-gnuradio] Testing crystal accuracy

[Marcus Müller]

Capture the same reference tone at $f_\text{ref}$ with both dongles,
tuned to $f_\text{tune} = f_\text{ref}-f_\text{offset}$, choose
$f_\text{offset}\approx \frac{f_\text{sample}}3$.
You would see a baseband tone at $f_{\text{offset},1}$ and
$f_{\text{offset},2}$, respectively, and since no two oscillators are
identical, $f_{\text{offset},1}\ne f_{\text{offset},2}$. W.l.o.g., let
$f_{\text{offset},1}$ be the tone as seen by your "improved" dongle.

Multiply both signals; what should happen is intermodulation; use a
low-pass filter with a cutoff of ca $\frac{f_\text{sample}}4$, so that
you just get the difference frequency at $f_{\text{offset},2}\pm
f_{\text{offset},2}$. Qt frequency sink, or quadrature demod->Qt time sink.

Best regards,
Marcus


On 12.01.2016 16:07, Jason Matusiak wrote:
> I have a handful of the NooElec blue USB DVB-T receivers that we
> occasionally use at work.  I managed to get a hold of a TCXO to replace
> the crystal on the board for an attempt at better accuracy (with less
> drift).
>
> The dongle seems to be functioning fine (so I know that the oscillator
> is functioning OK), but I can't seem to figure out a way to quantify how
> much of an improvement I can realize from the swap (if at all).
>
> Is there something simple I am missing that I can do in a GRC script to
> test the difference between a modified and unmodified dongle to compare
> the benefits of the TCXO?
>
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