[DSP-10] Particular tricky problem.

[Bob Larkin]

Hello Robert -

I'm afraid this is a problem!  And I believe there are 12.5 kHz steps in 
some areas of the States, as well.  The FM signal uses the full 10 or 12 
kHz of the crystal filter. So the center frequency  for FM is right in the 
middle. The only tuning available is the 126 MHz synthesizer and this uses 
5 kHz steps. Going to smaller steps, say 2.5 kHz, gives up PLL 
performance.  Early on in the project, I had this running with 2 KHz steps 
and phase noise and microphonics were issues at that point.  Even if you 
give up performance here, this requires changes to the PLL loop filter in 
hardware, and a fair number of software changes.

If the FM deviation is small enough, to fit in the crystal filter band, 
it  is OK to operate 2.5 kHz off frequency in receive. The FM detector is 
extremely linear, and all distortion comes from phase and amplitude 
non-linearities in the crystal filter.  But, this still does not center the 
transmitter.

I guess it is obvious to all that all these problems go away in modes like 
CW or SSB where there is tuning at the 2nd 10 to 20 kHz I-F.  Here the 
bandwidth is small enough to move around in the crystal filter pass band.

Also, good luck with the assembly and testing!

73,
Bob Larkin  W7PUA

At 12:24 AM 5/5/2005, you wrote:
>Several friends and me have some troubles with posting to this list.
>Of course once i used my second email address (f6bed at free.fr)but how many 
>times
>all things was right and the post rejected.Below is my registration to the 
>list. It's also my last trial.
>
>Anyway starting my DSP10 assembly (TAPR)i read that in FM the frequency step
>can not go down 5 khz, it's a actual drawback while the european repeaters
>are working on these frequencies  *****0, ****12.5, ****25, ****37.5, ****50,
>****62.5, ****75, ****87.5, say with a 12.5 khz steping.
>Does this problem be only software problem or some hardware should be 
>changed ?
>
>73's de Robert f6bed.
>
....snip