PLL Programming - Analog Devices

That shouldn't be such a problem. There is a divisor dividing the reference down to the channel distance aka comparator frequency and then there is a divider dividing the output frequency down to the channel distance. The higher the divisors, the higher the phase noise.

Rene

Yes, calculating the divisor is no problem at all. My issue, however, is that I need to select the optimal configuration for the divisor given that:

N = (P*B) + A

I need to automate the calculation of P, B and A in software based on a set of desired output frequencies to obtain the correct value for N. I got something working yesterday that seems to do the trick, however.

If you have a choice, you'd normally want to minimize the N factor, subject to the N >= P*P-1 constraint inherent in the dual-modulus divider logic. The more multiplication the PLL chip has to do, the more it will amplify the reference's noise (and its own inherent noise). Your step size -- hence your comparison frequency, in a simple single- loop PLL -- is 1.3375 MHz. That's a perfectly-reasonable comparison frequency for these chips, so N should simply be your output frequency divided by 1.3375 MHz.

P is the factor you'll actually have to choose. You didn't mention your minimum output frequency, but let's say it's 1337.50 MHz. If N is 1000, you can't use a prescaler value of 64, because 64*63 is 4032, much larger than 1000. The next available P modulus is 32, which works (32*

You *could* choose N to be some integral fraction of your step size, say (output frequency / 1.3375 MHz) * 10. If your comparison frequency were much higher than a few MHz, that might be worth considering... but I'd say not in this case.

You can simulate your loop with ADIsimPLL (free download from Analog), and take the results to the bank. It's good stuff.

-- jm

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Sorry, I missed some parentheses: should be N >= P*(P-1).

-- jm

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I've done this. I wrote an app to generate all possible combinations between two frequencies of interest. It puts the output values into a delimited text file so you can import them into Excel and sort them based on frequency or comparison frequency or what ever you want. It's written for a Pocket PC.

Thomas