piclist 1998\11\09\115814a >
Thread: VHF PLL controlled with PIC
picon face BY : Peter L. Peres email (remove spam text)

On Mon, 9 Nov 1998, Dave VanHorn wrote:

> Tricky bits: We were controlling the power to the VCO and PLL, and I had
> to add a delay for them to power up, since there was a filter cap that
> had to get charged.

How did you mute the output during this time ? I used a PIC pin to quench
the synthesizer's oscillator directly during this time through a low Cr
diode.  (the synth could be programmed statically). I also turned PA power
on after the lock flag went on, and off again before changing frequency.

> When you want wideband modulation, and wide deviation, where does your
> loop filter go? (Or should I FM the reference frequency instead, which
> is a whole 'nother can of worms?)

Without being an authority, I'd say leave the reference alone forever ;) I
don't like the idea of modulation running through such a complex device as
the PLL phase comparator output driver (which often contains un-documented
proprietary slope control features)  with more non-linearity points than I
want to know about. I am saying this, while I own and use several TDA7000
series based FM receivers which work on similar principles, hovewer they
were designed for this from scratch, and have no problems whatsoever.

For FM operation, the books say to use a slow PLL loop, at 30Hz or
thereabout. A reasonable loop filter at 30Hz using a single op-amp is very
easy to have. It uses only 2 external components ;) (hint: integrator, and
steal a 1/2Vdd reference from somewhere). Been there.



In reply to: <36470B4D.65F50AFB@cedar.net>
See also: www.piclist.com/techref/microchip/math/filter.htm?key=pll
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