On Jun 6, 4:24 am, Scott wrote:
Searching Google, I was able to find a copy of the manual for the
frequency doubler unit (HP 11690A) which conveniently describes the
operation and has a schematic. I could make one, except there is one
part that is just labeled as a transformer (T1) with three windings.
That probably wouldn't be too hard to make, but no idea how many turns
for each winding...

The manual is atftp://bama.sbc.edu/downloads/hp/11690a/11690a_service_6.pdf

Ideas??

Scott
N0EDV



Scott wrote:
I recently picked up an HP 8640B Opt. 323 (8640B in a can) and wonder if
anyone has information on the external frequency doubler so that it can
use it's upper frequency position (512-1024 MHz). I want to get 902 MHz
out of it to test the feed for my parabolic dish's return loss (SWR).

I can build a diode doubler to generate harmonics and then put a 1/4
wavelength of shorted-end coax across its output to select the 902 MHz
harmonic, but I'd like to be able to tune all freqs between the 512 and
1024 MHz without building a lot of shorted stubs. How does HP do it
with their doubler? Bandpass filtering? Could one bandpass filter
cover that much range?? Maybe something as simple as a low pass filter
with an Fco of 1050 MHz or so, but with this, how would I keep the
fundamental freq (256-512 MHz) out of the output??

Thanks for any leads!

--
Scotthttp://corbenflyer.tripod.com/
Gotta Fly or Gonna Die
Building RV-4 (Super Slow Build Version)

Beware!! The schematic is a little misleading. The top winding is
not coupled to the other two. Consider in a perfect transformer, if
it were, the output (right) ends of the upper and lower windings would
be identical. The purpose of the top winding is to improve the
balance at the output. In the bridges we've built, we normally make
two (as nearly as practical) identical transformers, both with bifilar
windings, but the one used for the "upper" winding in the schematic
shown just shorts the two windings together, so it's just an inductor,
but it matches the inductance of the other one, used as a current-mode
balun transformer, and also matches the parasitic resistances and
capacitances. The circuit for the balun is easy to simulate (e.g. in
LTSpice) and demonstrate how the added inductor dramatically improves
the balance at low frequencies.

As others have pointed out, you really don't need much filtering; the
circuit suppresses the fundamental and odd harmonics, so if you've
built it carefully, just a LPF that cuts off at 1.2GHz or so should be
about all the help it needs, unless you want a really clean signal for
some specific application, in which case a little bandpass filter
could clean things up further.

Cheers,
Tom