Using a FET for a VCR?

Hi,

Firstly I'd like to point out that I'm new to Electronics so please bare with me!

For a little background; I'm currently working on my first project which is to control a servo using some software I've written. The software provides a 0-5v DC analogue output which i intend to have as the input for a circuit which acts as a voltage controlled resistor (VCR) and integrates into a simple 555-pulse generator used to control the servo.

I've heard FETs can be used for this purpose however my searches have been a little fruitless. Would it be possible for someone to either recommend some literature which covers this in some depth or to provide the basic calculations i'll need along with an outline of the schematic? I intend to use this circuit to replace a 47k pot on the pulse generator which currently controls the position of the servo.

Of course, if anyone has any easier ways to convert a 0-5v DC analogue pulse to a 1-2ms digital pulse with a 20ms rest, I'd be more than happy to hear about it!

Many thanks,

Jamie

As you're generating the original signal in software, why not just generate the required pulse in software instead?.

That's how it's usually done.

hi Jamie,
The FET resistor configurations I have used are not linear, ie: Res versus Vgs.
Are you saying, for example, 0Vdrive is 1mS, +2.5V drive is 1.5m and 5V is 2mS within a 20mSec frame rate for your servo.?

Which MCU are you using.?

EDIT: N,
Beat me too it again...

You could also use a digital pot.