Damien Teney wrote:

Hello all,
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 ;-)

Some other points. Do you have a filter to remove the reference
frequency from the loop. A notch filter will clean out a lot of noise.
I had to straighten out and re-certify the design of a commercial
synthesizer after the purchasing department bought different capacitors
used in the VCO circuit, and in fixing that, I found several other
problems.

Does your power supply use a linear or a switching regulator? To
achieve a very low phase noise we had to use a pair of low dropout
linear regulators inside the shielded case.

How do you bring signals in and out of the synthesizer case?
feed-thru capacitors and coaxial connectors are a must. The Microdyne
synthesizer I mentioned used a DB 25 to connect the power and logic
inputs, and had a two rows of feed-thru capacitors behind the connector
to filter all signals, including DC. Also, make a spare top cover with
small holes to reach test points. Screw it down to the case, and make
any adjustments. Working without a cover can change the frequency, and
in some cases, take the VCO to the edge of its range where you will get
a lot more noise.

A big part of building a clean VCO is electronics, but a lot is
mechanical, too. A solid case, good shielding, and board layout. Mount
the PC board to the case with short spacers, preferably permanently
attached to the case. For homebrew work you can use brass spacers and
solder the them to the case after they are screwed in from the back. It
will make a big difference.

A clean ground system in the RF circuits is critical. We had to
replace a couple cases where a PEM standoff didn't make good contact
after it was pressed in, but moved the three circuit boards to a new
case, and they passed on the first try.
--


Michael A. Terrell
Central Florida