[ARC5] HF Converter - feeding mixer with a square wave?

[David Stinson]

Hello, Leslie.  I hope you and yours are well.
An interesting circuit.  I'm less "tech-savy" than you,
but if I may, would suggest an easy way to test the circuit.

A garden-variety receiver, with an RF amplifier which has
cathode bias.  Disable the LO and Mixer tubes,
so the RF amp is isolated. For instance- in an ARC-5
receiver, disconnect the ground at the 12K8 cathode
(lift R-4, 620 Ohms, from ground), which will turn-off
both LO and Mixer, leaving the 12SK7 RF Amp isolated.

Open the RF amp cathode connection to ground or,
in the case of the ARC-5 where RF cathode is part of the
variable gain-control buss (junction of C-5 and R-1,
the "low end" which is electrically closest to ground
and is grounded when the gain control is at max),
so we can insert our test parts between there
and ground.

Insert your FET and resistor between the "low" end of
the cathode resistor and ground. Use a LO signal
which would result in the receiver's IF freq.
Scope the dormant Mixer's grid for your results.

The FET in the diagram is in a "Class-C-like" condition,
which means the first fraction of the driving waveform
will have to reach a certain voltage before it will
turn the FET on.  Even with a sine-wave input, the
output is going to be some pulsed fraction of the
input waveform, with sharp "square-like"
transitions dependent on the switching speed and
input capacitance of the FET.   If you can carefully
forward-bias the FET to give it some "idling current"
so it will be closer to Class AB or similar, it should
better introduce the Sine Wave test signal.

Now as I say- I don't know everything and have
forgotten more than I know- sometimes
even how to tie my shoes.  If I'm
incorrect in any of this, I'll depend on the Smart
People here who have so often "pulled my fat
out of the fire," as have you on more than one
occasion, Leslie, so don't "sell yourself short."
Our community has some sharp, helpful folks in it.

Looking forward with interest to your results.

GL OM ES 73 DE Dave AB5S

On 3/16/2022 10:35 PM, Leslie Smith wrote:
>
> ARC5 at mailman.qth.net
> HF Converter - feeding mixer with a square wave?
>
> Hi Wayne:  You might think (and correctly) that my question suggests I 
> don't know much on this topic.  On another web-page David Newkirk 
> published a circuit showing a 2N7000 FET in the cathode circuit of a 
> 6SH7 mixer.  (I hope the attached circuit comes thru and is visible.)
>

-- 
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus