Re: VCO control with AVR tiny/mega?


ChrisQuayle wrote:
Andy wrote:
ChrisQuayle wrote:


You've really only described half the problem here. For example, lock
up time, phase noise, jitter limits, varicap / inductor or chip style
vco, accuracy, what sort of phase detector etc...

Chris


Lock up time: 0.05s
Phase noise and jitter of the VCO - I don't have the figures, I am using
the oscillator of the SA612 in a Lx(C+varicap) Colpitts configuration.
More details on the oscillator: the oscillator is tuned with a varicap
(through a small network of capacities, to scale the tuning range to
200kHz), currently driven by a manually-controlled 10-turn pot.

With the AVR, I would hope to replace the analog pot (+ multiturn dial)
by something much cheaper. The AVR would also take care of the slow
freqency drift of the VCO, adjusting the driving voltage by steps small
enough to bring the freqency back to the target one +- 500Hz (or better??).


-- Andy

With the exception of the lockup time, the requirements sound pretty
loose - not even phase locked loop territory, more like an afc /
frequency locked loop system. My guess is that if you are prepared to
sacrifice initial lockup time, a timer set up to generate pwm, with a
long time constant filter on the output, could handle the control bit.
The remaining problem is measuring the frequency of the oscillator,
which at 15Mhz might not be possible using a pulse counting timer
alone.(haven't used avr) You'll probably need to prescale the vco to
bring it within range. The last thing to consider is the vco / varicap

There's an interesting frequency counter using a PIC [ :) ] that is
reported to work up to 50MHz.
http://ironbark.bendigo.latrobe.edu.au/%7Erice/newfm/newfm.html
--Rocky

.

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