fonseca.ha
Junior Member
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Posts: 28
UK
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For loop bandwidth, thinking about stability, and assuming that phase noise will be dominated by the VCO, you can start by checking what is the minimum required reference frequency. And that may not be the minimum input frequency has, in case a strange multiplication ratio is needed, the pre-divider may have to divide it down.
Assuming that you have a conventional analog charge-pump pll, looking at the open loop tranfer function, for the above case the GBW should be 10 times smaller than this minimum reference frequency.
When checking the open loop gain, one think that affects it is the feedback multiplication ratio. But normally, for the minimum reference frequency not all feedback multiplication ratios are possible, as the operating limits of the VCO have to be respected.
In your case, if for example for the minimum reference, assuming it is 3.125M, the possible feedback multiplication ratios are: 128...256. I see that for your case N only changes between 2 and 128, so it seems that with the minimum reference frequency, the maximum VCO frequency cannot be generated.
With the above you may design a loop filter.
Now we have to see what happens for the other feedback multiplication ratios,
As the feedback multiplication ratio starts to become smaller the GBW starts to increase. If you look at it, the GBW may increase, because now the minimum reference frequency is also bigger. This because we need to respect the operating limits of the VCO.
But the problem is not so much the GBW increasing, the problem is more you loosing phase margin.
The consequence is that, for smaller 'N' you may need to either make a loop filter resistor that is programable with 'N' so that it traks its variation, or a charge pump current also variable with N. The difference will be that a programable charge pump will allow a constant GBW, and a programable resistor allows to track the increase in the minimum possible reference frequency.
Hope it helps Humberto
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