The 8640jr is a signal source
designed by Wes Hayward, W7ZOI. Mine covers 3.46 to 32.9 MHz in 3 ranges. Like its namesake, the Hewlett-Packard
HP-8640 signal generator, it uses a high frequency oscillator range and divides it down for lower
ranges. In my case, the 13 - 33 MHz range is digitally divided by 2 to get the
middle frequency range, and then divided by 2 again for the low range. Switched low pass filters are used
with each of the outputs. A separate high impedance output is provided for a frequency counter.
My first attempt, with a multi-section AM-FM receiver
capacitor did not work out well; the minimum capacitance was too high. That was one of a few that I bought on
closeout for about $2 each from Amateur Electronic Supply a few years ago. I removed it and ordered a 365 uF
capacitor from the AES that had low minimum C; they still carry it for less than $15 as of January 2019.
The actual range is 14 - 384 pF. I had to relocate it due to its mounting hole configuration.
That required filling in front panel holes with JB Weld and drilling new holes. It worked out fine, covering the
3.5 to 33 MHz overall. One trick employed in Wes' design was to have a coarse tuning range with the main cap,
and a fine tuning range with a paralleled lower capacitance capacitor. I set up the front panel with the Fine
cap set to the middle of its range to give a plus-or-minus fine adjustment. The output sine wave is generally good
except at the bottom of 80 meters. It looks to be a slightly distorted triangle wave, possibly due to the divider.
Here is a link to PDF of the W7ZOI article for downloading.
"8640jr by W7ZOI"
Click on the photos for closeups of the rear panel,
the oscillator section and the divider section.
Rear panel
Frequency divider and LPFs
Oscillator
