[ARC5] What the heck is a "1/2 harmonic Q-5er?

[Tom Lee]

Hi Hue,

Subharmonic generation is indeed much rarer than harmonic generation, so 
your intuition is spot on, and "1/2 harmonic Q-5er" is a poorly chosen name.

However, there is one uncommon (but not uncommon enough) case of 
subharmonic generation. It doesn't show up in vacuum tube PAs (it could, 
but doesn't), but does appear in transistor-based PAs. Thanks to 
strongly bias-dependent input capacitance, pumping at some frequency f 
can cause a negative resistance to appear at f/2 (this is how a child's 
swing works, by the way). If there's a tank around with a resonance 
around f/2 (not all that uncommon from narrowband matching networks), 
you can set it singing.

-- Cheers,
Tom

-- 
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 5/10/2020 18:05, Hubert Miller wrote:
> NO, dammit !
> I was going to take another look at my emails before going out for a walk, and saw this.
> Gentlemen, and ladies too maybe, I was so energized by this subject I moved 3 heavy, I mean heavy,
> boxes to reach the ER box. At risk of my  back, which I pained working on my van last week.
>
> So YES dammit it sure does depend on the IF + oscillator setting. The ER No. 304 article, page states,
> you set the BC-453 to 415 kHz; repeat: 415 kHz. Then the osc is running at 415 + 85 = 500. This gives
> you 1000 kHz for your 915 kHz input from the BC-348.
> I don't want to reread the whole article right now, but one real interesting thing is, it states that the
> audio output jack on the 348 has enough IF on it that you can pick off the 915 kHz right there.
>
> How did this misinterpretation of the article get started ?
>
> Article also discusses using 5th harmononic of BC-435 with dial set at 498 kHz to work with the BC-455
> receiver. Article's main point is expanded with a lot of interesting historic material.
>
> There is NO way I know to get "1/2 harmonic" except by a special transformer at low frequencies or an
> oscillator locked to a higher frequency.
> -Hue Miller
>
>> Well, first of all the BC-453's IF doesn't even matter. It is completely unimportant for our present purposes.
> You simply tune the BC-453 to the IF/2  (1/2 harmonic) of the BC-348, or 915/2 = 457.5 kc.
>
> If Jeep really does have his BC-453 tuned to 407 kc, then his BC-453 calibration is off by 50 kc. I find that absolutely impossible to believe.
>
> But I'm betting either he didn't mark his dial with the "5", or his photo isn't clear enough to show the "5". Furthermore, if his dial settinig IS exactly 457, then his BC-453 dial is off by
> 0.5 kc, which is much more reasonable for a BC-453 in good condition.
>
> MY BC-453s MIGHT be off as much as 1 dial-marking width....maybe. Calibration in a properly restored and aligned BC-453 is unusually good.
>
> Ken W7EKB
>
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