[time-nuts] ADEV
Rex
rexa at sonic.net
Mon Nov 15 05:39:18 UTC 2010
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.
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