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Thread: [EE] Phase locking DRO's
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picon face BY : Robert Rolf email (remove spam text)



Alan B Pearce wrote:
>
> >or have phase matched down
> >conversion and then mix the signal to get a correlation (and thereby
> >pull your extraterrestrial signal out of the noise). PLL LNB's are
> >just too expensive for an amateur group to afford, so I'm hoping to
> >be able to hack some cheap LNBs to do the job and get us started.
>
> Just a suggestion - I have no experience with doing this - but have a look at
> the chips sets used in GPS receivers, especially the correlator control chips
> that have up to 12 correlators in them to lock onto the satellites. The two
> chipsets I looked at are normally supplied with no code in the micro contained
> in the correlator chip. If you get real friendly with a rep, they may donate you
> a sample chip.

Unfortunately these chipsets are very application specific.
Grossly simplified, they
look for a specific set of spread spectrum digital 'chips' {the pseudo
random spreading sequence unique to each satellite) at a very specific
rate. When they find a matching 'chip' they synchronize and
then provide a 50Hz recovered data frame rate which is subsequently
processed into pseudorange and ultimately location.

The Philips chipsets ARE available with microcode, for a price,
but the signals that are being extracted are so different from what
we look at that this approach would not work. Thanks for the suggestion
though.

Radio astronomy 'correlators' compare the signals from multiple
dishes and provide an output that is higher if the signal is correlated,
and lower is it is not (just random). In it's simplest form a
correlator is just an RF multiplier. Noise * noise= Noise, average
value=0. Signal * Signal = Signal ^2, with some average value.
[ever wonder why they're called 'square law' detectors?]

With noise+signal, the noise averages out to zero, but the -correlated-
noise (i.e. the same signal seen by two different dishes) is non zero.
This is what allows us (and NASA) to see a signal that is buried in
noise. In order for the multiplication to produce a result, the phase
relationship of the two signals must match, so the LO MUST be phase
locked (or you have to do the multiplication at frequency, and at 4Ghz
that's a bit difficult (not to mention the losses in bringing the two
signals together at a central correlator).

This description is a gross simplification of the process since one has
to compensate for many other phase shifts, but it all starts with a
phase locked LO in the downconvertor.

So? How do I phase lock a DRO? Pumping 5150Mhz around to the widely
separated dished just doesn't seem to be a viable alternative.

Robert.Rolf-at-ualberta.ca

<3927826F.12A4826C@UAlberta.ca> 7bit

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