I have built the PPL frequency to voltage converter using the ideas on AN-210 (from National.com), but I need a quick, stable converter that will operate between 50-250hz, and I cannot get these circuits to run down that slow. Does anyone have an idea how I can get a f/v converter down to 50hz? The input signal changes from 50-250hz over about 10seconds
( ignition pulse from 2 stroke motor) and I am sampling the voltage at 50 times per second.

I'm posting a block diagram below. I haven't built this, or even simulated it, but it should work. The principle is:
1. Create a voltage that is proportional to period (1/F).
2. Pass that voltage through a reciprocal (1/V) circuit.

It should be very fast, with no ripple. It does depend on a clean input, i.e., no spurious pulses and low jitter, because, in contrast to most F-V converters, it has no filtering.
I believe the AD633 datasheet has info on how to make an analog divider. If you need more help, I will try as time allows.

I've tried the LM2917.. it won't cope with a changing input frequency, and has high ripple. The ppl circuit copes well, but even though the app note suggests increasing capacitor size, it slows the settling time down too much to sample quickly.....