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

[Richard (Rick) Karlquist]

On 4/22/2023 6:21 PM, Matt Huszagh via time-nuts wrote:

> 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

> 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
>
> Matt
> 

Your 67 matl has rather low permeability and high Q.  IMHO, not a good
choice for a 10 MHz transformer.  I am wondering if it is contributing
to your oscillation.

Rick N6RK