[time-nuts] BPSK Receiver & GPS Antenna siting

[J. Forster]

You cannot put a narrow filter before the squarer for reasons previously
cited. In a low S/N area, squaring just makes matters worse wrt dynamic
range and clipping.

-John

==========



> Hi
>
> A PLL locks to phase. If the phase switches by 180 degrees, the phase
> tracking switches signs. There's no way to track that. You either need to
> double the frequency (and thus eliminate the modulation) or demodulate the
> signal and lock to the result. If you simply put up a real narrow filter
> and hit it with continuous 180 degree phase shifts, the output will be
> nothing at all

>
> Bob
>
>
> On Sep 27, 2012, at 7:11 PM, johncroos at aol.com wrote:
>
>> Various comments -
>>
>> Hal mentioned SNR for the scheme I suggested. A PLL can be a coherent
>> demodulator of arbitrary
>> bandwidth. Thus the PLL at the output of the doubler can have a small
>> bandwidth since at that point
>> there is no PSK, it having been removed by the doubler. So given a
>> stable VCXO you can probably get down
>> to 1 Hz and thereby achieve a good SNR. There is a lot of stuff out
>> there on phase tracking receivers
>> that do exactly that. You know the frequency so the loop does not have
>> to search far and the BW can be increased
>> for acquisition and closed up for tracking.
>>
>> On writing reams of code - my point was that it is not required to used
>> the admittedly more powerful software
>> techniques to do this job - I noted that one reason to write reams of
>> code is for the fun of it, this is after all
>> a hobby.
>>
>> GPS Antenna Siting -
>>
>> Lets not make this so hard. Mine is at 6 ft elevation and is blocked to
>> an elevation angle of 20 to 30 degrees by a house
>> within 15 ft and a forest of trees. I have room and no restrictions but
>> I also have severe thunderstorms - so the house
>> plays lightning protect for the antenna. My T bolt tracks a Rb to better
>> than 1e-12 over 24  hours with no serious 10 MHz phase bumps
>> as plotted on a  strip chart recorder.
>>
>> Sooooo  -
>>
>> Put your antenna at 6 ft in back yard. Start out on a photo tripod - who
>> is gonna notice?
>> set up a t bolt at EL=5 AMU=0 Damping = 1.2 and Time Constant = 100 sec.
>> get the t bolt manual
>> get Tbolt monitor
>> get Lady Heather and read all that stuff.
>>
>> Run Lady Heather antenna survey (command SAS)  for at least two days -
>> you get a map of signal level in dBc vs elevation
>> Reset the Tbolt elevation mask to reject anything that is shown as
>> blocked using the signal
>> level map. Likewise experiment with the AMU setting to reject the weak =
>> poor signals. Mine works good
>> with AMU all the way up to 10 as fewer good satellites are better than
>> lots of weak ones.
>>
>> The satellites are in high orbits so masking those below 25 degrees is
>> OK and the AMU sets the acceptable signal
>> level - at AMU 10 my setup throws out those below about 40 dBc - the
>> strong guys go up to 50. This is a function
>> of you antenna performance so some experimentation is required.
>>
>> -73 john k6iql
>>
>>
>>
>>
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