[ARC5] BC-221 Testing Results

[J. Forster]

> 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.

Generally correct, but it's not so easy to judge how 'good looking' a sine
wave is. It can have a fairly high harmonic content and still look pretty
good.

> 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.

What probably happens is the negaqtive resistance circuit goes non-linear.
It is only possible to make a negative resistance circuit over a limited
range.

> 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.

I only remarked that the comb on the high band has fewer spikes than on
the low band, and this is accounted for by the high band being closer to a
sine.

Further, I speculated that the asymmetry in the low band waveform might
account for the nice envelope of the comb, but I've not found the theory
to confirm that as yet. A symmetrical square wave has predominently odd
harmonics.

Best,

-John

===========
>
> 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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