In my previous post I was wrong about why the secondary of T3 is non-resonant.?
Double tuned RF transformers are quite common in radio gear.
I think on T3 for the Rose-80 with a secondary of only two turns of wire,
it simply has too little inductance to inhibit much of anything at 3.5mhz.
Let's take it to an extreme, bring that frequency down from 3.5mhz to just 60hz.
Those two turns now look like a very short piece of wire,
and the only impedance presented to our 60hz signal is that of the 470pf cap at C44.
The impedance of the cap is far larger than any contribution from the inductor,
to where the inductor has virtually no effect
Jerry
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On Thu, Dec 12, 2019 at 07:15 PM, Jerry Gaffke wrote:
Getting to your primary puzzle, I doubt the right side is trying to be resonant.
The datasheet says the right side only has 2 turns, whereas the left side has 14 turns,
So the inductance is far less, your inductance meter speaks the truth.
I'm guessing the cap at C44 would work about as well if it were 0.1uF instead of 470pf.
Would be an interesting experiment to tack a cap in parallel, see if performance changes.
It does sort of look like a series resonant circuit, so we might think it would not conduct
unless at the resonant frequency.? However, the output of that transformer is a voltage source
with our 3.5mhz sine wave, the 470pf cap at C44 does not see an inductance to interact with.
