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Science Forum Index » Electronics Forum » PLL filter problem
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Message |
| Damien Teney |
 Posted: Sat Aug 30, 2003 12:39 pm |
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Guest
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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/schematic_from_motorola_application-note_98
0.gif).
Thank you in advance for your answers  |
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| DocBrown |
| Posted: Mon Sep 08, 2003 8:00 am |
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Guest
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Hi Damien,
Hard to say without complete text, but R1,R2,C1 and Cc all critical.
First observation: there are 2 R2 and C1 sets. They need to match in
value, due to tollerance dif. markings are not allways enough. Have
you measured them? Are you certain Both R2 are exactly equal, same for
C1? The same applies to both Cc.
Second observation: R1, there are 4 R1/2. Are all 4 R1/2 exactly
equal? Have you measured them? Are you clculating that R1/2 + R1/2 =
R1? Are you using R1/2 for these or have you made an error and used 4
R1 values?
Double check your work, use an ohm meter and capacitor meter to make
certain your parts are what they say they are. Precision components
are expensive, but 10% - 20% items won't cut it here. Unless you A)
make sure they are matched and B) plug the actual value readings back
into your equations.
Final observation: All the rules apply. Clean circuit construction,
short leads, good solder joints, good isolation of circuit sections,
minimise stray capacitance and inductance, avoid ground loops.
This is a great learning experiance for you.
Have FUN!
Doc
"Damien Teney" <damien.teney@sobelairva.net> wrote in message news:<biqr9l$dc8$1@reader08.wxs.nl>...
Quote: Hello all,
I 've built PLL in order to command the VCO of a VHF receiver. Unfortunately
it doesn't work very well: the VCO output is on average equal the same as
the frequency programmed in the PLL, but it is constantly varying a bit
around it. I guess the problem comes from the output filter, but I've
re-calculated it and it should be okay. Where could the problem come from ??
FYI, the PLL is build from a schematic of the Motorola application note 980
(http://www.mcarsweb.com/_divers/schematic_from_motorola_application-note_98
0.gif).
Thank you in advance for your answers |
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| Back to top |
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| The Captain |
| Posted: Sat Sep 13, 2003 1:24 pm |
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Guest
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"Damien Teney" <damien.teney@sobelairva.net> wrote in message news:<biqr9l$dc8$1@reader08.wxs.nl>...
Quote: 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/schematic_from_motorola_application-note_98
0.gif).
Thank you in advance for your answers
I'm scraping the bottom of the memory barrel here, but I do remember a
lecture in which we were told that a PLL needs to have an error
against which to work, in order for it lock to the wanted frequency.
This can be an extra small voltage applied to the VCO control voltage
input. I know this sounds a bit anti-intuitive, but it works.
John |
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