"Michael Black" wrote in message
...
"Michael A. Terrell" ) writes:
Damien Teney wrote:

Hello al
I 've built PLL in order to command the VCO of a VHF receiver.
Unfortunately
it doesn't work very well: the VCO output is on average equal the same
as
the frequency programmed in the PLL, but it is constantly varying a bit
around it. I guess the problem comes from the output filter, but I've
re-calculated it and it should be okay. Where could the problem come
from ??

FYI, the PLL is build from a schematic of the Motorola application note
980

(http://www.mcarsweb.com/_divers/sche...cation-note_98
0.gif).

Thank you in advance for your answers ;-)

How clean is the power supply? Any noise, ripple or hum will appear
in the output of the VCO.

And of course, one reason three terminal regulators were invented
was so small sections could have their own regulators.

They are cheap enough that it's hardly worth not having a separate
regulator for the VCO. And maybe even a separate regulator for
the phase detector (and any amplifier) section.

Michael VE2BVW

In addition to proper loop filter parameters, separate supply regulation,
etc., a VCO needs to be placed and usually shielded to keep outside
noise from perturbing the control voltage ... you could be getting stray
pickup of RF, digital noise, etc. onto the control voltage ...

Carl - wk3c