[ARC5] BC-221-AKs

[Bill Fuqua]

    Technically, it would not be a square wave but a rectangular waveform 
if it did not have a 50% duty factor.
The shorter the pulses the more even in amplitude the harmonics would be.
A square wave (50% duty factor) would have only fundamental and odd harmonics
deceasing in amplitude as the harmonic number. The third harmonic would be 
1/3 the
amplitude of the fundamental, the 5th would have 1/5, etc, in voltage 
level. This is the downfall of
some of the newer solid state crystal calibrators since they use a divide 
by 2^n
IC. So if you have a 100kHz solid state calibrator of this sort by the time 
you get to
20.1 MHz the harmonic would be 1/201 of the fundamental, 46db below the 
fundamental.
And at 100.1 MHz 60db down. Naturally at 20MHz (even harmonic) there would 
be little or nothing.
The only reason there is something at the even harmonics is that due to 
some slight delays in
the ICs they don't produce a perfect 50% duty factor.
     Just about all RF LC and crystal Vacuum Tube oscillators are class C. 
And by having
a high cutoff bias the plate current would be fairly short pulses compared to
a 50% duty factor.  The oscillators usually are self biased via grid 
rectification often
referred to as "grid leak bias".
     If you want a nearly harmonicless waveform you pick your signal off 
the LC directly,
if you want harmonics you take it from the plate.
     Looking at a schematic of a BC-221M, it appears that the LC oscillator 
is a electron coupled
Hartley oscillator. The signal going to the antenna terminal and to the 
input of the
mixer tube is directly off the plate with no LC in the plate circuit.
     73
Bill wa4lav

At 08:00 PM 11/25/2012 -0800, J. Forster wrote:
>I heard back from my signal processing expert friend, and it is indeed the
>asymmetry of the square wave on the low band that produces the nice comb
>with lines at both the odd and even frequencies.
>
>Best,
>
>-John
>
>============
>
>
>
> >
> > Square wave would give high odd harmonics.
> > Jay
> >
> >
> >
> > -----Original Message-----
> > From: Kenneth G. Gordon <kgordon2006 at frontier.com>
> > To: Arc5 <Arc5 at mailman.qth.net>
> > Sent: Sun, Nov 25, 2012 12:48 am
> > Subject: [ARC5] BC-221-AKs
> >
> >
> > By the way, I would have thought that we did NOT want a pure sine-wave
> > output
> > rom these
> > n the interests of maximum harmonic output. Am I wrong?
> > I would have thought that the output should be as close to a square-wave
> > as
> > ossible.
> > Ken W7EKB
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