PLL configurations

The default PLL configuration for the XEM3005 and many of the examples in the Front Panel manual have large common factors in the dividers. For example, the default values of P and Q to generate 400 MHz from 48 MHz are P=400 and Q=48. Wouldn’t it be better if these were P=25 and Q=3? I thought that a synthesizer would have better phase noise if the dividers had the common factors removed. Similar reductions are possible for the other examples in the manual.

I doubt that there would be any noticable effect, but it seems like better practice.

Hi Bob–

Yes, there are many different ways to configure the PLL for best performance, depending on the frequency(ies) required.

Quite honestly, the large factor was originally chosen to allow 1 MHz resolution while changing only one of the numbers. That way, we could fiddle with values from (say) 90 MHz to 140 MHz by just changing a single parameter. It’s not meant to be optimal in any way.

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