"budgie" wrote in message
...
On Wed, 18 Feb 2004 09:17:11 GMT, (Mike W)
wrote:

I have a need to produce an accurate 4Mhz 50% dutycycle TTL squarewave
to use as a timebase.

I have a 10Mhz IQD frequency standard of suitable accuracy. How can I
divide this to produce the 50% duty cycle 4Mhz signal?. Is it even
possible with logic alone?. I can see how to mix with either 6Mhz or
14Mhz, but this then detracts from the required accuracy.

Several approaches spring to mind. You stated "accurate" - they all
provide
that, but jitter is introduced in all of them:

1. VCO at 4MHz, divide by 4 and lock to Fref = 1MHz from your 10 Meg
source
divided by ten.

2. VCO at a multiple of 10M - say 40 MHz - locked to your 10M ref and
divided down (by in this case 10) to give 4 MHz output.


I would suggest using a 4MHz crystal oscillator as your VCO. Small varicap
to alter the VXO frequency (or use an ordinary diode or LED as the varicap,
see http://www.hanssummers.com/radio/varicap/varicap.htm). The inherent
stability of the VXO will allow you to use a very slow PLL, which will
result in minimal jitter.

Hans G0UPL
http://www.HansSummers.com