[R-390] Hartley bfo FET Prod.Det.

[Michael Murphy]

Hi Dan,

This is fun stuff. I have built them both ways as well. If you want the
least coupling and stability, (important if you are building a transmitter
VFO and want to key something upstream without chirping) I think the source
connection is the way to go.

For power, or if you want to multiply to a higher frequency, the drain is
the way to go. For raw p-p voltage, use this connection into your buffer.

If you want purity, the gate is the way to go. Actually, not the gate but
the tuned circuit in the gate.  The tuned circuit itself can be coupled to,
just like in the pre-ECO Hartley days, with a good old link on the cold side
of the coil. This gives you some impedance flexibility too. It's a BFO; to
hell with the buffer.

Mike Murphy WB2UID


----- Original Message ----- 
From: "Dan Merz" <djmerz at 3-cities.com>
To: "R-390 List" <r-390 at mailman.qth.net>
Sent: Friday, July 01, 2005 7:37 PM
Subject: [R-390] Hartley bfo FET Prod.Det.


> Hi,  I'm mostly thru the mechanical building part of the product detector
> for 390a similar to the one shown by AI2Q on this list,  which has an agc
> circuit in addition to the detector/bfo,  and just connects to the i.f.
out,
> diode load,  and agc connections on the back of the 390a.  But instead of
> using the crystal-controlled bfo oscillator  I opted to use a tapped coil
> bfo unit in a can salvaged from a  National NC 100.  In tapping the MPF102
> FET bfo signal  into the base of the 2N2222 buffer stage,  I wonder if
it's
> best to take the signal from the drain,  much like an electron-coupled
tube
> oscillator or from the source connection.  Most circuits I find for FET
> oscillators take the signal from the source,  analogous to cathode tap for
a
> tube, and presumably a lower impedance coupling.  Are there any opinions
on
> why most handbook circuits show the source connection?  I suppose either
> would work by fiddling with the various coupling impedances.  I've got a
> week of thinking about it before I'll be able to get back to the project.
> Any thoughts?  Best regards,  Dan.
>
>
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