[HomeBrew] Any PLL experts out there?

[Steven Weber]

ian,

I assume your talking about FM modulation and not AM modulation.

The problem is most likely an unstable loop. Designing a stable loop filter, 
especially if your tuning steps are small - less then 10 kHz and the tuning 
range is fairly wide - is not an easy task.

Analog devices has a free simulation software package called PLLsim which 
will calculate the required filter values. It is of course designed to be 
used with thier PLL's such as the ADF 4001 (usable at HF frequencies), but 
it would likely work okay for other PLL chips since mostly you need to know 
the reference frequency, the /N ratio and the VCO tuning rate to figure out 
the proper filter values.

Steve KD1JV

>
> A problem that I keep running into with various PLL projects is what I
> think is hum
> modulation of the output frequency. Would appreciate corresponding with
> anyone
> who understands shielding, grounding, etc, for PLLs or can offer me
> suggestions
> for what might be happening.