"Ian White, G3SEK" wrote in message ...
Tom Bruhns wrote:
"Ian White, G3SEK" wrote in message
...

....
I could have a closer look at the schematic (and in the HP
Journal article about the 8640B) if anyone has a serious need to know
some more details.


Are those old HPJs on the web anywhere?

For a while they were, then there was only an index, and now I'm
unable to find anything. Just yesterday, I boxed up a _complete_ set
of HPJs to move to our new site, so if there is a specific article you
need, please ask and I'll see what I can do (after the move in a
couple weeks). I have the HPJ that covers the 8640 at home, though,
and could turn part or all of it into a PDF file. Unfortunately, the
PDF would be large because it's a set of images, not text-based, from
my scanner. I also have the service manual on the 8640M, which is a
little different from either the A or B models.

It may have been in this group a while ago that mention was made of
the way the 8640 generates its output bands: an oscillator from about
250 to 500MHz, with digital divide-by-2 stages followed by LC filters
to attenuate the harmonics. Someone said, I believe, that there is
only one filter per octave. That is not the case with my 8640M, which
on the higher bands has two filters per octave, one for the low half
and one for the high half of the band, which are somehow automatically
switched. I didn't look into just how the switch point was
determined.

There's a mechanical switch on the slide-rule tuning mechanism.

Presumably it's a bit different in mine, since there is no slide-rule
tuning mechanism. I'll have to have a look.

But the true glory of the 8640 series is the way it keeps the FM
deviation constant when more dividers are switched in. Who else but HP
would have used a differential gearbox?

In the 1970's when the 8640 was designed, we did, and people like
Collins did, but now we don't. There are very few pots or trimcaps in
our circuits these days, too, for the same reason--and there are a lot
of processors of various sorts, and lots of DACs and the like. I used
to think the 8640 phase noise was pretty good, but it doesn't hold a
candle to what we do these days. My fun these days is in finding ways
to make meaningful measurements 120dB below full scale with data from
a 12-bit or 14-bit ADC. :-)

Cheers,
Tom