If the VCO can be made to have good short-term stability (we could discuss
this requirement for a long time), then there are more options for how to
make the control loop. I was toying with the idea of 2 simple 8-bit D/A's,
one operating a coarse tune varactor and one running a fine tune varactor.
Of course the problem here is that the overall 16 bit tuning curve would not
be monotonic, and could have numerous slope reversals. This might be
overcome by having a 'learn' mode where the system builds something like a
frequency lookup table for subsequent open-loop random access. Once you've
pre-tuned to within approx 1 part in 65,000, the closed loop part could take
over. As to +/- a few hertz wandering affecting the PSK31 signal, I think
it's mostly a matter of how fast/slow the wander rate is. Slow, small
frequency wandering will probably be tolerated by PSK-31 software's AFC
function.
Joe
"xpyttl" wrote in message
...
Joe
I actually have (had?) a project in flight to do just that.
Unfortunately,
pressures on my time have prevented me from making much progress lately.
Perhaps as the weather cools.
My background is as a chemical process control engineer. This seems like
an
obvious application of traditional process control algorithms. Even more
to
the point, I have a history of applying simple computers to process
control
problems, and this seems like a good candidate.
I spent a LOT of time working on the basic VCO. In my case, it happens I
was looking for something in the 9 MHz neighborhood, to build a 20 meter
rig
with a 4.9 MHz IF (to take advantage of cheap crystals, of course). It
turns out to be a lot harder than it sounds to come up with a reasonable
stable VCO at that frequency.
In my case I wanted to be able to tune the frequency, but this is really
not
so different from what you want. I get the frequency by twiddling the
shaft
encoder, but once the frequency is gotten, I now plug it into the
controller, so the second part is basically what you are talking about.
I, too, had assumed when changing frequency I may need to go to a ten
times
a second sample, then change to once a second after I got there, but I'm
not
entirely sure this is what I'll do. Given that you have a VCO and a DAC,
and the VCO has a relatively fixed relationship between the control
voltage
and the frequency, I ought to be able to output the voltage for the
desired
frequency immediately, then fine tune based on the count.
Which brings up another issue ... the DAC. In my case, I wanted 70 kHz of
coverage, so I needed a DAC with a fair bit of resolution. 8 bit DACs are
cheap, but when you get into the 12-16 bit neighborhood they tend to not
be
so cheap. Also, it seems like the lion's share of serial DACs are surface
mount, and you really want serial to keep the parts count down.
My initial thinking was that you might be able to get away with an 8 bit
DAC
given that you really only want one frequency. Assuming a 4.915 IF, you
would want to build a VCO with a range of about 9.1549 to 9.1551 - seems
pretty tight, although it looks like a candidate for a VXO, which could be
a
lot simpler problem. The challenge you have is that you can't have the
frequency hopping about for more than a couple of Hz for PSK, so you
really
want about 1 Hz or better resolution on your DAC.
In fact, as I think about it, the VXO looks like a great candidate. Since
the VXO can be quite stable, you aren't going to need to be quite so quick
to respond to disturbances that want to move the frequency, so the once
per
second sample to allow 1Hz control isn't going to be such a big deal.
If you build the VXO such that, under "typical" conditions it is spot on
with the DAC output at half scale, now you can preload the reset in your
PIC
controller to deliver half scale at turn on. The total drift of a VXO
ought
to be only a few tens of Hz anyway, so the PIC shooed drive the frequency
to
be spot on within 5 or 10 seconds. It's unlikely that you will be able to
find a QSO in that amount of time, anyway. Of course, thermal
disturbances
are going to want to move the VXO a bit, but these are slow, so the PIC
should have no trouble keeping them in check.
This probably doesn't really deliver to your "building block" concept, but
it seems pretty straightforward for PSK. If you want a generic building
block, as best I can tell you will need 1) a good, stable VCO (hard); and
2)
a high-resolution DAC (expensive).
For my project, I am starting to lean toward the AD9850 as being simpler
and
cheaper, although I started out where you are now, feeling like it was
overkill for the purpose. Still, I'd like to prove the concept of using
PID
control for frequency.
..
"W3JDR" wrote in message
...
Richard,
Now you're talking my language...almost!
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