Rex wrote:
On 11/14/2010 7:24 PM, David I. Emery wrote:
3. External reference LNBs with 10 MHz (pretty universal) going
up the cable that also carries power and brings the L band signal down.
I'm not entirely sure how many of these designs simply bandpass filter
and then limit the 10 MHz and use that directly as a PLL reference and
how many phase lock a VCXO to the 10 MHz coming in. Otherwise similar
(and often derived from designs for) the internal reference PLL types
in 2 above.
These ER types are more apt to be used for more exotic
specialized applications where very high frequency accuracy or some
degree of phase coherence with other equipment or LNBs is useful.
Obviously with the high multiplication factor, one needs a quiet
reference inside the PLL bandwidth (and that is pretty wide to ensure
reliable lock) - one suspects that issues with degradation due to
mechanically induced noise and phase shift in the cables can be a
problem.
I picked up an LNB a about 10 years back that used this external
reference method. The LO in the thing was a DRO that locked to a 10 MHz
signal coming up the cable. The LO in this one locked to 10.750 GHz.
Experimenting, I found that the DRO adjustment screw could be turned and
it would lock every 10 MHz. I found it would lock as low as 10.690. I
assume it would tune up about that far too.
I needed an LO in the 10 GHz area, so I hacked mine to use it just as a
LO, not using any of the receive chain. Here's a page where I described
the LNB and what I did with it.
http://www.xertech.net/Projects/sat_lo.html
Maybe the description there will help someone. If nothing else, it show
pictures of what one external ref type LNB looks like.
Interesting.. NORSAT makes lots of those sort of devices for VSAT (Very
Small Aperture Terminal) applications (e.g. satellite internet).. I've
seen a few of them at work being used in that kind of application.
Those would make life easier for my scheme.
They're not quite so common as the LNB for Ku-band TV (at least I've not
seen them at Best Buy/Radio Shack kinds of places), nor have I seen them
on neighbors houses or out at the curb on trash day.
But I'm starting to think the pilot tone technique might be easier... At
first glance, if one considers two alternatives:
LO locked to reference, down converting RF to baseband and digitizing
and
LO not locked, but with in-band pilot tone, down converting RF to
baseband and digitizing, then essentially mixing with the downconverted
pilot tone
should produce comparable results. In the first, you're controlling the
LO (at least inside the loop bandwidth), and in the second, you're
really measuring it, then taking out its changes.
Assuming that the pilot tone amplitude is small enough that it doesn't
cause nonlinearity problems in the LNB, and that it's in band, it should
be comparable.
But, maybe there's a SNR of the reference issue? In a PLL, you're
comparing a very strong signal to a very strong reference, so the noise
floor of both is very small, while in the pilot tone, the "reference" is
strong, but not as strong, relative to the noise flor.
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