[time-nuts] RF Isolation Amplifier ...

[Ulf Kylenfall]

The BFT66 if that is the type ( a bit difficult to read ) is a really "wild one" on VHF. I have used it and unless surrounded by proper decoupling networks in a well designed circuit it can and will oscillate.
I would have used a spectrum analyzer to check for any unwantedoscillations well above this circuits operating frequency.
And also checked with a scalar network analyzer for possibleinstabilities in the circuits gav vs frequency.
Also the generation of unwanted overtones can be one causeof adding the capacitor where it is.
One other peculiarity with the BFT66 is that it does not use themore or less standardized E-B-C pin configuration.
One design that I came across that used the BFT66 was very difficultto make stable. After some time I gave up on it and replacedthe BFT66 with BFY90.
Best Reagards
Ulf KylenfallSM6GXV