[time-nuts] Re: Bruce's common emitter transformer feedback amplifier: oscillations with capacitive load
Bob Camp
kb8tq at n1k.org
Sun Apr 23 13:42:34 UTC 2023
Hi
A schematic would be a big help here â¦..
Flying blind, best guess is that your output stage has an issue. If the two output bufferers
are identical to the input stage, thatâs probably the issue. A different configuration for the
buffers would be the most obvious fix.
Bob
> On Apr 22, 2023, at 9:21 PM, Matt Huszagh via time-nuts <time-nuts at lists.febo.com> wrote:
>
> 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
>
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