There are a few possible sources of problems
(1) Power supply for the PLL and VCO - noise on these will appear in the
output. use separate supplies with ?zeners rather than IC regulators
(2) Is the amplitude from the buffer enough to drive the prescaler properly?
(3) Is the loop stable - how did you arrive at your calculations for the
loop values?
(4) The VCO tank seems to be heavily loaded - it has an output directly
to the Rx and a large (68p) capacitor to the varicap which is low Q.
This would make it noisier. Why not take the output to the Rx from the
buffer? (is that how it was originally?)
Also could you use a smaller cap to the varicap? (say 5p or less)

Richard


Damien Teney wrote:
Hello everybody,
I have build a VHF receiver for the airband, published in the magazine
Elektor a few years ago, and I've managed to build a PLL to improve it. The
PLL is build on another PCB, that is connected to the main circuit, and that
replaces the original potmeter, in order to choose the frequency. The PLL is
able to lock on the right frequency, but the receiver is a lot more noisy,
and it seems really less sensitive with the PLL. See these sketches,
I think it is better than a long explaination.

http://www.mcarsweb.com/_divers/1sketch.jpg
http://www.mcarsweb.com/_divers/2sketch.jpg
http://www.mcarsweb.com/_divers/3sketch.jpg

I have also included the schematics of the VCO (with the buffering
transistor, T4), and the basic schematic of the PLL (taken from a Motorola
application note). I don't know where the problem could come from. I would
say from the bad buffering of the VCO output, but I don't see how it could
be improved.

http://www.mcarsweb.com/_divers/4VCO.jpg
http://www.mcarsweb.com/_divers/5PLL.jpg

I'm really getting mad with this problem ! So thanks to all of you who will
answer ;-)