[DSP-10] Particular tricky problem.

[John B. Stephensen]

FM on 10 meters should always be 3 kHz audio and 3 kHz deviation for a 12
kHz bandwidth. However, VHF FM is often 3 kHz audio with 5 kHz deviation for
a 16 kHz bandwidth. 9600 bps packet is 3 kHz deviation but occupies 16 kHz
or more.

73,

John
KD6OZH

----- Original Message -----
From: "Bob Larkin" <boblark at proaxis.com>
To: <dsp-10 at mailman.qth.net>
Sent: Friday, May 06, 2005 07:51 UTC
Subject: Re: [DSP-10] Particular tricky problem.


> 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
>
>
> _______________________________________________
> DSP-10 mailing list
> DSP-10 at mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/dsp-10
>