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I had used that chip for several years, with a 2.5kHz reference frequency,
and never ran into those kinds of problems. My first iteration of that synthesizer did have some high level reference sidebands (-40dBc), but after I redid the board with a top layer ground plane, all of the problems cleared up. Are you using a ground plane on your board? Are you also using that anti-slewing filter at the op-amp input, whereby you split one of the Rs, and use a shunt C between the two? If you would like a schematic of my loop filter, give me a shout. Pete Michael A. Terrell wrote in message ... Damien Teney wrote: Hello, About the quartz, I haven't used a 10.24MHz one as in the given schematic, but a 5.12 MHz one. The R ratio is 1024, so the reference frequency is 5 KHz. The N ratio used is from 196 to 284 (with the 64/65 prescaler; VCO goes from 63 to 91 MHz). I don't think the loop is unstable, because the VCO output is quite clean (only a bit blur, see http://www.mcarsweb.com/_divers/vco.jpg). The loop filter values have been calculated with the fomulas given in the motorola application note 980. So, if you think I should use a additional RC network, could you tell me how to calculate the values :-$ ? Thank you ;-) Damien Do you have a 5 kHz notch filter in the control voltage feeding your VCO? It will reduce phase noise in the output. A simple L/C trap will work, but either the coil or cap should be adjustable for maximum rejection. -- Michael A. Terrell Central Florida |
I don't have any ground plane. I only have a veroboard, which I took the
unused tracks apart. I also don't have an "anti-slewing filter"; I first tried to do it but the output signal was very "unclean", so I removed the two capacitors. I'm now having two resistors in serial between the phase-detector and the op-amp, and it is much better. I would be interested if you could mail me the schematic of your loop filter ;-) (mail it to ) Damien I had used that chip for several years, with a 2.5kHz reference frequency, and never ran into those kinds of problems. My first iteration of that synthesizer did have some high level reference sidebands (-40dBc), but after I redid the board with a top layer ground plane, all of the problems cleared up. Are you using a ground plane on your board? Are you also using that anti-slewing filter at the op-amp input, whereby you split one of the Rs, and use a shunt C between the two? If you would like a schematic of my loop filter, give me a shout. Pete |
Hmmm... A fixed 187 KHz spur sounds as if it might be an oscillation on
the DC voltage regulator feeding the VCO. Although as has been mentioned, the op-amp in the loop filter could oscillate and cause similar symptoms. Jim Pennell N6BIU |
Hmmm... A fixed 187 KHz spur sounds as if it might be an oscillation on
the DC voltage regulator feeding the VCO. Although as has been mentioned, the op-amp in the loop filter could oscillate and cause similar symptoms. Jim Pennell N6BIU |
It's on the way. Anyway, a ground plane will help a great deal, in cleaning
up some of those problems. Pete Damien Teney wrote in message ... I don't have any ground plane. I only have a veroboard, which I took the unused tracks apart. I also don't have an "anti-slewing filter"; I first tried to do it but the output signal was very "unclean", so I removed the two capacitors. I'm now having two resistors in serial between the phase-detector and the op-amp, and it is much better. I would be interested if you could mail me the schematic of your loop filter ;-) (mail it to ) Damien I had used that chip for several years, with a 2.5kHz reference frequency, and never ran into those kinds of problems. My first iteration of that synthesizer did have some high level reference sidebands (-40dBc), but after I redid the board with a top layer ground plane, all of the problems cleared up. Are you using a ground plane on your board? Are you also using that anti-slewing filter at the op-amp input, whereby you split one of the Rs, and use a shunt C between the two? If you would like a schematic of my loop filter, give me a shout. Pete |
It's on the way. Anyway, a ground plane will help a great deal, in cleaning
up some of those problems. Pete Damien Teney wrote in message ... I don't have any ground plane. I only have a veroboard, which I took the unused tracks apart. I also don't have an "anti-slewing filter"; I first tried to do it but the output signal was very "unclean", so I removed the two capacitors. I'm now having two resistors in serial between the phase-detector and the op-amp, and it is much better. I would be interested if you could mail me the schematic of your loop filter ;-) (mail it to ) Damien I had used that chip for several years, with a 2.5kHz reference frequency, and never ran into those kinds of problems. My first iteration of that synthesizer did have some high level reference sidebands (-40dBc), but after I redid the board with a top layer ground plane, all of the problems cleared up. Are you using a ground plane on your board? Are you also using that anti-slewing filter at the op-amp input, whereby you split one of the Rs, and use a shunt C between the two? If you would like a schematic of my loop filter, give me a shout. Pete |
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