[ARC5] BC-221 Testing Results

[Leslie Smith]

Hi John & group,
My comments below (about wave-forms at different frequencies) are based
on conjecture, without supporting theory.
Briefly, I built an interesting oscillator, based on the principle of
negative resistance.
This circuit requires a two terminal tank to be connected to the active
"negative resistance".
The negative resistance consists of a JFET and a PNP transistor, with 2
resistors and by-pass caps.

One side of the tank is grounded.  This makes a simple circuit that will
oscillate over the frequency range from sub-sonic (say 5 or 10Hz) to HF.
 Generally the output waveform is sinusoidal (as seen on a CRO), and so
relative "clean".
I never put it on a spectrum analyser, but I think a good-looking sin
wave will be pure on the analyser, because that's what the theory says,
and I trust the theory.

In general terms the neg resistance circuit will oscillate when the
reactances of the tank components are as low as 70 to 100 ohms, and much
higher (say 1000 ohms, reactive).  However, at the lower sub-audible
frequencies the waveform became quite distorted.
This (distortion) occurred when the reactance of the tank "L" (in this
case I used a power transformer) was high.
I put this down to large energy storage in the tank "L" (large "L",
large reactance) and the same energy amount of energy was then shifted
to a small "C".

I see a corresponding situation in your high band vs low band
observation in the BC-224
It's conjecture, but interesting conjecture, on my part.

BTW, I found your report VERY informative.
I love this mailing list, for this reason.
There are so many interesting observations, and they are instructive.


  Les
  vk2bcu at operamail.com


On Sat, Nov 24, 2012, at 11:29, J. Forster wrote:
> I recently got a new toy, a BC-221-AK  with the A-133 AC supply and have
> been testing it with modern test gear. Actually, it is suprrisingly good.
> 
> Both bands work and put out a couple of volts of RF when measured on a
> 'scope with a 10X probe connected to the antenna leads.
> 
>  The lower band is  an asymmetric pulse waveform, about 25% low and 75%
> high. The waveform is not atall clean, but looks like it was drawn by a
> drunk with palsey. The upper band is not a bad sine wave.
> 
> When checked in the frequency domain, using the FFT on the 'scope, the
> lower band has a lovely comb, extending to well over 3 MHz; the upper
> band
> is also a comb, but with fewer lines, extending to over 20 MHz.
> 
> If I put the unit in CHECK mode and zero beat against the internal
> crystal
> at 3000, 6000, 12000 kHz, I get 2.99999 Hz on the counter. IMO that's
> pretty good after nearly 70 years.
> 
> Best,
> 
> -John
> 
> ==============
> 
> 
> 
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