Oh, sorry, I forgot to hit the "replace LaTeX by rendered formulas"; let
me re-do that, before I comment on your comments :)
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.
Cheers,
Marcus
On 12.01.2016 18:43, Jason Matusiak wrote:
> Thanks for the quick response Marcus!!!!
>
> Since my Latex isn't very good (as in pretty much non-existent). Let me
> see if I can rewrite what you recommended in my dumbed down language and
> see if I am close.:
>
> *I have two dongles, dongle 1 will be my modified dongle, dongle 2 will
> be my un-modified dongle.
>
> *Put a a known reference tone into each of the dongles where Ftune =
> Fref - Foffset
> ** Foffset should be roughly a third of the sample rate
> **An example at a sample rate of 1.024Msps would be a reference tone at
> 98MHz, and then tune the dongles to 97.659MHz
>
> *I'll now see a baseband signal for both dongles whose offsets won't be
> exactly the same.
>
> *Multiple the resulting signals found above against each other (offset,1
> * offset,2)
> *Pass that through a LPF with a cutoff of Fsample/4, or 256khZ in this
> case
> **This will give the difference between the frequencies at frequency at
> Foffset,1 +/- Foffset,2
>
> *perform a QT freq sync or a quad demod into a QT time sink to compare.
>
> Is that close? I think I am missing something in there, and I have a
> feeling that it has to do with the multiplication step as that makes the
> least amount of sense to me. Any way to enlighten me on what I am
> missing above? Thanks!
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