[ARC5] ARC-5 receiver mixer mod attached as PDF

[Arden Allen]

> http://www.w7ekb.com/glowbugs/rx/Pullen/A%20Like-New%20Mixer%20Circuit.pdf

My two bits on this article:

It's a cathode coupled analog multiplier, or more specifically, a 
transconductance modulated mixer, imperfect as it is.  Some additional 
oscillator-mixer isolation is accomplished by the low impedance cathode 
circuit that acts to swamp capacitive coupling between the grid and 
oscillator tanks.

There are some misconceptions stated in the article, particularly the 
reference to the number of IF stages following the mixer affecting noise 
figure.  Wherever the primary noise originates from, usually the mixer, it's 
not reduced by adding IF stages.

In the "theory minded" description triode B is a grounded grid amplifier, in 
effect, for the RF signal and a common cathode amplifier for the oscillator 
signal.  Better isolation and conversion gain could be accomplished by 
buffering the oscillator signal with a cathode follower (see last paragraph 
on page 5).

Obviously, the 12AT7, having the highest transconductance of the bunch, has 
the highest conversion gain according to Table 1.  A frame grid dual triode 
like the 6DJ8 may even be too hot and be unstable without some tricky 
neutralization.

This I find a bit hilarious:  ".....A common problem with many conventional 
mixers is cross modulation, in which two carriers become 'intertwined' and 
an unwanted signal rides in on the one you want. ..."  It should be stated 
that overdriving a mixer, good or mediocre, that causes grid circuit 
rectification (detection) will cause two RF signals of different frequencies 
to amplitude modulate each other, or as Fourier would explain, causing 
difference frequencies in the audio realm to modulate the signal envelope of 
the tuned to frequency.

The advent of true mixer tubes was back in the days just before octal tubes 
made their appearance.  As for pentodes, the 6AC7 was a real hottie for 
radar IF's and even though short cutoff with poor overload resistance it was 
an outstanding 1st RF amp and as well making a good mixer.

Too bad there was no explanation for the need to have different plate 
voltages for the two triodes.  I think it has more to do with the ratio of 
cathode currents to get the best mixing efficiency - probably empirically 
derived.

"Exotic mathematics" is what helps researchers and designers to develop 
newer and better devices.  But for the happenstance designer or experimenter 
like myself trial by error emulates the process.

Arden Allen, KB6NAX

Heaven goes by favor. If it went by merit,
you would stay out and your dog would go in.
-Mark Twain