Damien Teney wrote:


In fact the problem is that the receiver is less sensitive and
products a lot of whistles, even when I ONLY connect the output of the
VCO to the PLL, and control it with the potmeter (see
http://www.mcarsweb.com/_divers/3sketch.jpg).

That shows that the problem cannot be in the output filter of the PLL.

I see....Ok, sounds like the trouble is in the VCO itself or in how the
VCO is coupled to the mixer. Do you have access to a spectrum
analyzer? It is beginning to sound like the oscillator may have a dirty
output or what used to be called 'squeegging' (sp?).
Here are some things I would try.
Use a couple of resistors to set the DC voltage on the varactor to some
moderate value. Make double-dog-darn-sure that the DC voltage feeding
the VCO and the varactor are absolutely clean (with a scope).
See if problem persists. (Obviously you will be at a fixed frequency
here). If so, I would suspect the oscillator problem described above.
Perhaps less feedback, or running it at a lower value of Vcc might help.
I think you said that the basic rx worked fine with a different LO. Is
this the case? If so, check the output of that LO with a scope or RF
probe and see if your VCO output is a similar P-P value. Maybe you are
overdriving the mixer?

My gut feeling is that the problem will turn out to be a noisy DC
control line on the varactor or dirty output from the oscillator.

Best of luck. Lets us know what you come up with. Remember.....this
kind of thing builds character and troubleshooting skills!

These are just basic troubleshooting ideas, but might lead you to
something meaningful.

I have also tried to put the PLL circuit alone in a grounded closed
metal box to shield it, but I still get these whistles in the
receiver output.

Where do you think the problem could come from ?

where did you find that PLL app note? It looks much like others I have seen
from Motorola on that series of PLL but I'm always looking for more
documentation.

Motorola application note 980
http://www.mcarsweb.com/_divers/Moto...note%20980.pdf