[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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