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"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 |
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