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There are a few possible sources of problems
(1) Power supply for the PLL and VCO - noise on these will appear in the output. use separate supplies with ?zeners rather than IC regulators (2) Is the amplitude from the buffer enough to drive the prescaler properly? (3) Is the loop stable - how did you arrive at your calculations for the loop values? (4) The VCO tank seems to be heavily loaded - it has an output directly to the Rx and a large (68p) capacitor to the varicap which is low Q. This would make it noisier. Why not take the output to the Rx from the buffer? (is that how it was originally?) Also could you use a smaller cap to the varicap? (say 5p or less) Richard Damien Teney wrote: Hello everybody, I have build a VHF receiver for the airband, published in the magazine Elektor a few years ago, and I've managed to build a PLL to improve it. The PLL is build on another PCB, that is connected to the main circuit, and that replaces the original potmeter, in order to choose the frequency. The PLL is able to lock on the right frequency, but the receiver is a lot more noisy, and it seems really less sensitive with the PLL. See these sketches, I think it is better than a long explaination. http://www.mcarsweb.com/_divers/1sketch.jpg http://www.mcarsweb.com/_divers/2sketch.jpg http://www.mcarsweb.com/_divers/3sketch.jpg I have also included the schematics of the VCO (with the buffering transistor, T4), and the basic schematic of the PLL (taken from a Motorola application note). I don't know where the problem could come from. I would say from the bad buffering of the VCO output, but I don't see how it could be improved. http://www.mcarsweb.com/_divers/4VCO.jpg http://www.mcarsweb.com/_divers/5PLL.jpg I'm really getting mad with this problem ! So thanks to all of you who will answer ;-) |
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