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 ;-)

what frequency steps are you using???and what values do you have for the
loop filter??
"Damien Teney" wrote in message
...
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 ;-)

what frequency steps are you using???and what values do you have for the
loop filter??
"Damien Teney" wrote in message
...
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 ;-)

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 ;-)

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


Michael A. Terrell
Central Florida

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 ;-)

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


Michael A. Terrell
Central Florida

We need some more information.

--------------------

What is the tuning sensitivity of the VCO in MHz/volt ?

What are the values for the divide by N ?

What is your desired reference frequency ?

===================

Even if you have calculated the loop filter correctly, there are many
other ways to have a synthesizer full of undesired spurs....

For example, how well filtered is the DC voltage feeding the VCO ?

Many VCOs will respond quite nicely to any sort of noise on the DC voltage
and that'll show up as undesired noise on the VCO output.

---------

Another question is how well buffered is the VCO output to the MC12017
pulse swallowing prescaler ??

Quite a few of the pulse swallowing prescaler chips slightly change their
input impedance when going from one divide ratio to another one, and this
may easily result in spurs on the VCO output signal.

So, all in all, designing a quiet synthesizer can take some time and
careful circuitry. Of course, one of the early things to solve is the loop
filter components, and the first three questions will help in doing that
calculation.

I do not recall the website ofhand, but there is one that will calculate
the loop filter given the proper information.

Maybe one of the main manufacturer's webpages ??

I forget.....


Jim Pennell
N6BIU

We need some more information.

--------------------

What is the tuning sensitivity of the VCO in MHz/volt ?

What are the values for the divide by N ?

What is your desired reference frequency ?

===================

Even if you have calculated the loop filter correctly, there are many
other ways to have a synthesizer full of undesired spurs....

For example, how well filtered is the DC voltage feeding the VCO ?

Many VCOs will respond quite nicely to any sort of noise on the DC voltage
and that'll show up as undesired noise on the VCO output.

---------

Another question is how well buffered is the VCO output to the MC12017
pulse swallowing prescaler ??

Quite a few of the pulse swallowing prescaler chips slightly change their
input impedance when going from one divide ratio to another one, and this
may easily result in spurs on the VCO output signal.

So, all in all, designing a quiet synthesizer can take some time and
careful circuitry. Of course, one of the early things to solve is the loop
filter components, and the first three questions will help in doing that
calculation.

I do not recall the website ofhand, but there is one that will calculate
the loop filter given the proper information.

Maybe one of the main manufacturer's webpages ??

I forget.....


Jim Pennell
N6BIU

"Michael A. Terrell" ) writes:
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 ;-)

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

"Michael A. Terrell" ) writes:
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 ;-)

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

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