VCO output signal

[albert]

Hi everyone,

I'm using NE564(PLL, Philips) to implement a FSK mod/demod. The VCO of
NE564 is used as a FSK modulator, by using an external variabel
capacitor different channels can be tuned. I've tested the modulator
on a breadboard. However, the output of the VCO is not a sine wave as
I expected, even under the situation that there is no input signal, in
other words, the output waveform is not a sinus under the free-running
condition. The output is a distorted wave. My questions are:

The output waveform of a free-running frequency should be a sine wave,
is it right?
How to get rid of distortions at the output?

Very stange is that the output waveform is also not a sine wave when
the circuit example described in the datasheet is tested. What 's
wrong? Hope somebody can help me. Thanks a lot.

Regards,

Albert

 

[Michael Black]

albert (wo918@hotmail.com) writes:

Hi everyone,

I'm using NE564(PLL, Philips) to implement a FSK mod/demod. The VCO of
NE564 is used as a FSK modulator, by using an external variabel
capacitor different channels can be tuned. I've tested the modulator
on a breadboard. However, the output of the VCO is not a sine wave as
I expected, even under the situation that there is no input signal, in
other words, the output waveform is not a sinus under the free-running
condition. The output is a distorted wave. My questions are:

The output waveform of a free-running frequency should be a sine wave,
is it right?
How to get rid of distortions at the output?

Very stange is that the output waveform is also not a sine wave when
the circuit example described in the datasheet is tested. What 's
wrong? Hope somebody can help me. Thanks a lot.

Regards,

Albert

It's the type of oscillator that defines it's output wavefomr. If
you use an oscillator that has a sine-wave output, you get a sine-wave
output. If you use one that puts out a square-wave, then that's
what you get.

I'm not famliar with the 564, but all the PLLs with a built in
VCO that I can think of have a sqare wave from the VCO. It makes
for a simple oscillator circuit, and maybe more important, it can be done
with an RC network. Putting out a sinewave is going to require
a fancier circuit (either something that converts that square-wave to
a sine, or a sine-wave oscillator in the first place), and won't likely
to be done with a simple RC network. These all in one PLLs are designed
to be easy to use, cheap to make, and cover as many applications as possible.
So they go with a square wave oscillator. Some will address the issue
of the wrong wave-form by allowing for an external VCO.

Michael