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Thread: Difference between LM565 and 4046?
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face BY : David VanHorn email (remove spam text)



At 02:10 PM 4/18/01 -0400, Tim Hamel wrote:
>David VanHorn wrote:
>
> > Close enough..
> > They are both PLL chips that are workable in that range.
> > Your first goal is to get the pll to lock on to a tone at the subcarrier
> > freq, then to keep it locked when the tone is modulated. This has to do
> > with the loop filter.
> >
>
>Eek! I hope my understanding isn't flawed. I thought decoding SCA was a
>matter of high-pass filtering the signal at the discriminator in an FM radio,
>running it into the PLL (which is tuned for 67kHz or 92kHz). At the PLL, the
>error voltage is sent back to the phase-comparator; this is where I'd tap the
>audio. To avoid confusion, here is the circuit of interest:
>http://www.fineware-swl.com/images/SCA.gif
>
>Is my understanding correct, or do I need to do more studying?

That's correct.
The PLL may or may not need that prefiltering. That's a function of it's
lock-in range.

Limit cases: A completely stable oscillator, would generate a large error
signal, but getting it in tune has all the conventional problems.

A very easy to slew oscillator would track the subcarrier modulation, but
generate a very low error signal.

The National Semi website has a bunch of tools for PLL design, most if not
all are free.

You'll want to take your signal from the discriminator output, as this
signal is almost always filtered out of the audio path, since it's not
audio.  Nothing like trying to receive a signal that isn't there! :)
The four stages of filtering can probably be reduced to one, or none, with
adequate PLL design.
Just don't give the VCO enough range to follow the audio, or even other
subcarriers.
Each SC is only modulated by a small amount, so that's all the range you need.

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