[EMAIL PROTECTED] wrote:
>James Richard Tyrer <[EMAIL PROTECTED]> 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
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