[DSP-10] Particular tricky problem.

[dufour]

Courtney, Bob, and others

Thanks for your reply, it seems that i can access to the list now.

> 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.
>
I had a look to LMX1501A/LMX1511A datasheet : we reach the phase 
detector limits ( thus, noise and spurs). It leads us to deep 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.
>
Personally i less often use repeaters, of course we can do so far, 
although as you say "it still does not center the transmitter". On other 
hand those repeaters have been improved (here, in Europe, we have only a 
2 Mhz band, 144-146) and observe strict NBFM, IMD rejection and so one.

> 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.
>
Undeniably DSP-10 brings us a must, it's the outcome (or nearly so )  
for the hardware. The "to do list" turn around soft, as an old (75) ASM 
programer i appreciate the work done ( by Bob ) but we have to plan (you 
not me, alas ! ) to lay it down and  don't forget  for, that Linux will 
win soon in the Ham Kingdom !!
73, Robert f6bed.

> Also, good luck with the assembly and testing!
>
> 73,
> Bob Larkin  W7PUA
>
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