[ARC5] ARC-5 receiver mixer mod attached as PDF
Kenneth G. Gordon
kgordon2006 at frontier.com
Tue Jan 3 19:44:28 EST 2017
I agree with your statements vis-a-vis the article and the author referenced below, but most,
if not all, of your questions were answered directly by Keats himself in his own article.
Link here:
http://www.w7ekb.com/glowbugs/rx/Pullen/PullenMixer.pdf
Ken W7EKB
On 3 Jan 2017 at 15:48, Arden Allen wrote:
> > 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
>
>
>
>
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