> Have you tried a "micro" Henry choke on the sense line?
> Something
> from 1-10 micro Henries is what is recommended. I don't
> think the resistor
> is really needed since this is a very low current application
> (high current
> would be bad for the human contact!).
No I have not. I had assumed that the sense line wrapped around the torroid
would end up looking similar to using a choke in series in the line.
Perhaps I am mistaken. Ten turns of the wire around an FT-50B-77 (half inch
torroid using the 77 material) didn't do much to solve the problem. Maybe
it's not the same as a choke actually in series with the line.
>> So far, I have found no good way to RF bypass the sensing
>>line. What I want to do is basically shunt all HF energy to ground on
that
>>line, but I can't load down the line otherwise the sensing circuitry won't
help.
>
> Perhaps you could use a capacitor to pass the 14Mhz on
> the hood to
> ground while, at the right value could look like a higher
> reactance to the
> sensing line's capacitance. Cap of 0.01 microF would give
> 1.1 ohms at 14
> Mhz & 93 ohms at 170Khz. So connect the cap to the hood
> somewhere or at a
> couple of places, then gorund the other end of the cap(s).
Tried that. The .01 uF cap prevents things from working correctly. That's
93 Ohms at 170 KHz and I think that it pulls the circuit to ground too much.
I think a much, much smaller cap will need to be used.
>
> The problem is that the sensing line is operating on a
> "Change-of-Capacitance" to change the operating state. The
> 14Mhz bypass to
> ground might desense the "sensing line" enough to kill it. I
> think that's
> what you've found already. So we're back to getting the RF
> choke right.
> This all depends alot on the free running oscillator's
> operating frequency.
> Is it spec'ed anywhere?
Exactly. No, I don't have a spec on the oscillator frequency. I think I
will finally puy my O-scope to a useful purpose and try to measure it. Then
I can try to make some useful sense out of things.
UGH! There has got to be a solution though. So close!
73,
Jon
NA9D
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