OT - Generating a 20MHz clock that can be adjusted by +- 2%

LTC6900, LTC6903 from linear technology

Rene

Also, Maxim have VCO & clock chips, and Cypress have a range of programmable clock generators. Even a Cygnal uC will come close : They have an internal current injection OSC, with a 8 bit DAC, that is nominally 24MHz, but trims 16-24 MHz IIRC. You can also build one using a LC osc and a varicap :)

-jg

The question isone of adjustable range (2%) vs. inherent stability. A VXCO gives you very good stability, but limited range (as a result of the high Q value that is responsible for its stability)

Resistor-based oscillators, like the ones from Linear Technology, offer a wide range, but are much more affected by Vcc,temperature etc.

If you want both stability and wide range, you are in for complex circuitry, like DDS phase accumulators etc, Max acceptable jitter then becomes the issue... Peter Alfke

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How close does the clock have to be? Is this a one-shot job or a high volume run.

If it's just a one shot job, how about a collection of crystals. Just insert the right one. Order them now and you will have them by the time you could design and debug a fancy circuit.

Peter's "DDS phase accumulators etc" suggestion reminds me that radios commonly synthize frequencies. I assume they use a PLL locked to a crystal. There are probably some neat chips for this purpose, but I'm not familiar with them. Radios generally want a reasonably stable signal so they probably have low enough jitter.

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Nial, Got a Xilinx with a DCM spare? Try this crap idea.

Frequency in = Fin

From N= 20 to 32, use a DCM to give you

Fout = (N/(N-1))*Fin Gives you a range of multipliers from 1.052 to 1.032 in 12 steps Scale the frequencies accordingly to fit the DCM specs, i.e. use CLKFX at (say) double your 20MHz. Of course, this gets a lot harder to do on the fly, but might work for compile time. Do I win £5?

Cheers mate, Syms.

Or something with a 'Medium Q' - the pull-ability/Stability trade off of an OSC is determined by the Q, and there are other resonance solutions with intermediate Q's. Inductors can come in a range of Q's, I thing the highest are silver plated helical resonators, and SAW resonators can be made with moderate Q's...

-jg

Thanks for all the replys guys, I should have stipulated that the frequency has got to be adjustable on the fly.

This looks like my best bet, apart from....

I can get them from the LTC web site, does anyone have any experience of their delivery times?

Thanks again for all suggestions,

Nial

: Thanks for all the replys guys, I should have stipulated : that the frequency has got to be adjustable on the fly.

: > LTC6900, LTC6903 from linear technology

: This looks like my best bet, apart from....

: > > Can be sourced from distributor in a couple of days.

: I can get them from the LTC web site, does anyone have : any experience of their delivery times?

The delivery time is very short!