[time-nuts] BPSK Receiver & GPS Antenna siting

Bob Camp lists at rtty.us
Thu Sep 27 23:34:03 UTC 2012


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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