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Old 7th Aug 2018, 8:41 pm   #53
G0HZU_JMR
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Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
Default Re: JRC NRD515 HF Coms Receiver, 1982, FM on VFO?

Hope this isn't too alarmist, but here's a doctored version of your plot to show what it would look like if the loop was about to go unstable at its natural frequency. Just like a regenerative receiver that was adjusted to the verge of oscillation to get both gain and selectivity, the PLL noise response peaks up to a narrow/noisy gain spike at the natural frequency of the system if the loop heads towards instability and the version below is doctored to show how a PLL looks when right on the limit of going unstable.

Yours is still some way off this but it doesn't take much in terms of a change in division ratio or a change in VCO tuning gain to rapidly hit this problem. So I'm not sure if your latest loop response is how it should be. Sometimes the combination of the VCO noise response and the choice of loop bandwidth can give a false impression of an under damped system but yours does look to me to be a bit under damped.
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Regards, Jeremy G0HZU

Last edited by G0HZU_JMR; 7th Aug 2018 at 8:55 pm.
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