[time-nuts] Re: Bruce's common emitter transformer feedback amplifier: oscillations with capacitive load

[timeok at timeok.it]

   Hi Matt,

   I've built a buffer amplifier just for the use you want.

   The design comes from Bruce and was optimized and engineered by me and a group of friends.

   You can see the solution at the addresses I enclose.

   http://www.timeok.it/wp-content/uploads/2015/08/Frequency-Distribution-Design-Basic-Module-v-4.1.pdf

   http://www.timeok.it/wp-content/uploads/2015/08/hp5065A-option-H10-v-2.00-2.pdf

   Compared to the one you propose, it has several improvements, such as a higher input impedance and a much higher reverse isolation that exceeds 100dB.

   I hope this info can be useful to you,

   Luciano

   Luciano P. S. Paramithiotti
   timeok at timeok.it
   www.timeok.it

   Da "Matt Huszagh via time-nuts" time-nuts at lists.febo.com
   A "Discussion of precise time and frequency measurement" time-nuts at lists.febo.com
   Cc "Matt Huszagh" huszaghmatt at gmail.com
   Data Sat, 22 Apr 2023 18:21:07 -0700
   Oggetto [time-nuts] Bruce's common emitter transformer feedback amplifier: oscillations with capacitive load
   Hi,

   I built Bruce Griffith's common emitter transformer feedback amplifier
   (http://www.ko4bb.com/~bruce/OCXOBufferAmplifiers.html). My build is
   mostly identical to the first schematic in the previous link. But, I
   added a second stage for increased reverse isolation. I placed 65R
   across the first stage output, which gave me about the right gain in
   simulations. I also added a second 2-stage buffer in parallel. I
   custom-wound the transformer on a Fair-Rite 67 toroid. I'm planning to
   use this to get buffered/isolated front and rear 10 MHz outputs from the
   internal 10811A in my HP 5065A.

   Overall, the performance seems excellent. I've attached a residual phase
   noise measurement, showing the combined 2 cascaded stages. These results
   seem only a little worse than what John Miles measured for his single
   stage (http://www.ke5fx.com/norton.htm).

   I measured the reverse isolation to be 76 dB and 77 dB and the isolation
   between outputs to be 49 dB or 59 dB, depending on which output I inject
   the signal into. This is all at 10 MHz of course.

   However, I am experiencing one issue, which is that if I add a
   capacitive load to the output (such as from a coaxial cable), I get
   large oscillations (10+Vrms) at the output. The oscillations are at
   about 20MHz for a 1m long RG400 cable (94pF/m). These are not present
   when the output is unloaded, nor when it's loaded by the typical
   50ohms. Since this only appeared when I added a capacitive load to the
   output, I figured it was due to the parallel LC between the output
   winding and coaxial capacitance. My output impedance came out a bit
   lower than I'd wanted (35ohms), so I tried inserting 10ohms in series
   between the output winding and coaxial connector. This cut the
   oscillations nearly in half (now about 6.5Vrms), but didn't kill
   it. I've also tried loading down the output with a 1k parallel resistor,
   but though this helps too, the oscillations are still pretty
   large. Going lower helps further, but I don't want to load down the
   output too much.

   Any thoughts? Has anyone else seen this behavior? I can post more
   details and images.

   Thanks,
   Matt