Tom Bruhns wrote:
"Ian White, G3SEK" wrote in message
...
Is there also an issue about harmonic content? ISTR there is no
significant harmonic filtering after that amp, so any replacement needs
to be operating well below its quoted maximum power output (which will
be at 1dB compression, where significant harmonic output is guaranteed).
Depending on the detailed schematic, might the input and output match
also be important? Are any filters or attenuators relying on this amp
for their wideband 50R source load or source? If there is no fixed
attenuator pad at the output, then the variable attenuator is relying on
the accuracy of the amp's output impedance (at least at high output
levels, ie low attenuation).
This might be a case for using push-pull, to reduce the even harmonics,
if you can find suitable wideband transformers. Four GALIs of some kind
in push-pull parallel might do the job nicely... but if you want to get
it as right as HP did in the original design, it ain't trivial.
I shouldn't have said that - I'm now too scared to switch on my 8640A!
All good points, Ian. Indeed there's no filtering after the
amplifier, so if you want to keep the harmonic content low, you will
want a decent amplifier there. Also, the output power should be at
least +23dBm if you want to have the same performance as the original.
However, I believe the output is leveled (or at least monitored), so
the output impedance is irrelevant, except perhaps on the highest
range where the source impedance may depend on the amplifier output
impedance.
Good point.
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?
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.
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?
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek