[Open-graphics] Clock generation

[James Richard Tyrer]

mykrowatt at comcast.net wrote:
 >James Richard Tyrer <tyrerj at acm.org> wrote:
>> The Analog chip would be OK and it is inexpensive, but we would 
>> also require a VCO.  The VCO would have to be 2x since we need to 
>> run it through a flipflop to get 50% duty cycle  So we are looking 
>> for 50MHz to 660MHz (plus some extra for tolerances) that would be 
>> stable.  IIRC, thee are issues making a stable VCO with that high a
>>  infrequency and that wide a range.  An RC VCO would need to be
>> ECL; One with an inductor wouldn't have enough range -- you would
>> need a 1, 2, 4 & 8 prescaler.

> I've built varactor-tuned VCOs in both Hartley and Colpitts 
> configurations. They're not too difficult.  There's a tradeoff 
> between tuning range and phase noise, of course.  After picking the 
> quietest available reference voltage source and paying careful 
> attention to layout so that no stray currents flow through the local 
> ground network, one option would be to switch fixed capacitors in and
>  out with FET switches or relays, to reduce the varactor tuning range
>  and hence the effect of the random noise coming from the voltage 
> reference.  Another option is just to build several VCOs covering 
> different frequency ranges, and switch the power to them.
> 
A 2:1 frequency range seems to be the practical limit.  You can get
chips with everything but the tank (coil and 2 varactors).

It is easier to scale the output with a divide by 2^N rather than trying
to modify the tank if you can get a little over 2:1.

Or with a 2x fmax (660MHz) you can use a divide by N scaler if you can 
get a fmin of 440MHz (both + or - for tolerances). That is 3:2 which is 
more reasonable.

Alternately, you can use a higher frequency with a narrower range and 
then a divide by N scaler.  Doing this, it would probably be possible to 
use Maxim chips that have the varactors built in:

http://para.maxim-ic.com/results.mvp?q=rf_vco_buffers&an_1=Family&av_1=VCOs%2FVCO%20Buffers

-- 
JRT