Saturday, January 25, 2014

A-B-C VHF Narrowband FM

Corrected some problems with synthesizer PLL charge pump and fractional N settings.  Incorrect configuration led to a low phase frequency detector pulse rate which caused a spreading of the higher frequency LO.  VCO voltage would droop then shoot back with over compensation - the net result looking like a low frequency FM modulation on the LO.  With a single stage conversion the IF was ~120MHz which goes through a 32 way division masking the problem.  With a two stage conversion the higher frequency LO clearly showed the line broadening (FM wide and narrow band stations sounded like you were in a tunnel - discernible but sort of muffled ).

Below is a screen shot of NOAA/NWS VHF local weather broadcast.  The software package shows the entire 500kHz span available on the right.  Two software VFOs are used to look at seperate narrow band FM channels within the span - those are on the left.  The upper left is the NOAA NBFM, lower left is a channel tuned off to a quiet portion of the spectrum.  Audio quality is the same as previous captures for this station using A-B SDR configuration.

Antenna configuration is the same as previous work - 19" of 30AWG just hanging, no ground plane. Both B & C board amplifiers enabled, A board is unit #2 (lower noise configuration).




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