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From: rasmusms on 28 Mar 2006 00:51
I would like to use one PLL for both FSK modulator and demodulator
because of price. I probably gonna switch between a tvxco and FSK input
to "sig in" on a 4046. I gonna modulate the FSK on after the Loop
filter. My consern is about the loop filter. When I am using txvco as
center carier frequency I would like a small BW for loop filter (to
stay looked on carrier frequency), but when pll is used as demodulator
the loop filter should inclued the two FSK frequency?. Or am I wrong?
I would like to use the txvco as center carieer because of large
temperatur swing (-30 - 80) degrees celsius.

From: Ben Jackson on 28 Mar 2006 03:03
On 2006-03-28, rasmusms <rasmusms(a)gmail.com> wrote:
> I would like to use one PLL for both FSK modulator and demodulator
> because of price. I probably gonna switch between a tvxco and FSK input
> to "sig in" on a 4046. I gonna modulate the FSK on after the Loop
> filter. My consern is about the loop filter.

Variable bandwidth loop filters are also used to get faster lock time
(and better frequency agility) while still having good stability once
lock is achived. If you google around you'll find some appnotes with
techniques for accomplishing that.

You might want to consider other PLLs like the MC145170. It has serial
programming of the divider/counter values which saves on external
parts, but it does lack an integrated VCO.

--
Ben Jackson
<ben(a)ben.com>
http://www.ben.com/
From: rasmusms on 28 Mar 2006 04:58
My center carrier is about 1-3Mhz

Do you know who makes the MC145170? I can't find it on
fairchild/motorola homepage anymore.
It might be a alternativ.

Do you have a example on an appnote?

From: Andrew Holme on 28 Mar 2006 06:55

rasmusms wrote:
> I would like to use one PLL for both FSK modulator and demodulator
> because of price. I probably gonna switch between a tvxco and FSK input
> to "sig in" on a 4046. I gonna modulate the FSK on after the Loop
> filter. My consern is about the loop filter. When I am using txvco as
> center carier frequency I would like a small BW for loop filter (to
> stay looked on carrier frequency), but when pll is used as demodulator
> the loop filter should inclued the two FSK frequency?. Or am I wrong?
> I would like to use the txvco as center carieer because of large
> temperatur swing (-30 - 80) degrees celsius.

For a demodulator, the loop bandwidth must be appreciably wider than
the data rate. The actual mark and space frequencies themselves are
irrelevent; it is how fast you switch between them that matters. For a
modulator, the loop gain must roll-off well below the switching speed,
or else the loop will fight against the modulation. Injecting after
the loop filter doesn't avoid this. You may be able to control the
bandwidth by varying the loop gain. You will have to do the PLL
stability analysis.

From: Andrew Holme on 28 Mar 2006 06:59

Andrew Holme wrote:
> rasmusms wrote:
> > I would like to use one PLL for both FSK modulator and demodulator
> > because of price. I probably gonna switch between a tvxco and FSK input
> > to "sig in" on a 4046. I gonna modulate the FSK on after the Loop
> > filter. My consern is about the loop filter. When I am using txvco as
> > center carier frequency I would like a small BW for loop filter (to
> > stay looked on carrier frequency), but when pll is used as demodulator
> > the loop filter should inclued the two FSK frequency?. Or am I wrong?
> > I would like to use the txvco as center carieer because of large
> > temperatur swing (-30 - 80) degrees celsius.
>
> For a demodulator, the loop bandwidth must be appreciably wider than
> the data rate. The actual mark and space frequencies themselves are
> irrelevent; it is how fast you switch between them that matters. For a
> modulator, the loop gain must roll-off well below the switching speed,
> or else the loop will fight against the modulation. Injecting after
> the loop filter doesn't avoid this. You may be able to control the
> bandwidth by varying the loop gain. You will have to do the PLL
> stability analysis.

Another thought: if you leave the modulator idling (at mark or space)
for too long, it will return to the carrier centre frequency.

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