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

[Neal Pollack]

Luciano:

I would like to try the amplifier for my 10MHz output.
Where can I find details for the toroid transformer,
how many windings, and what gauge wire?

Thanks,

Neal
N6YFM


On Sun, Apr 23, 2023 at 6:32 AM Luciano via time-nuts <
time-nuts at lists.febo.com> wrote:

>
>    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
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