How come all the tuned input circuits I see in amplifiers look like
PI-networks,
Bill? This is a low-pass configuration or am I over simplfying things? I
suppose if I take a standard 3 pole low-pass pi-network with say 0.5 dB
ripple and start increasing the ripple, eventually it will look like a
bandpass
with an extra pole at DC. Energy storage would increase commensurate
with the amount of ripple as could be seen in the impulse response.
Guess I never tried relating pi-network Q to the ripple of a 3 pole low-pass
(pi configuration). There is probably an equation to be derived in there
somewhere. Probably an interesting relationship between the impulse
response and the Q also [overdamped (low-Q) vs. underdamped (hi-Q)].
Am I on the right track here?
Without doing some analysis, I still find hard to believe that a 5 pole
low-pass wouldn't provide enough energy storage to smooth out the
non-linear impedance presented by the cathode of the g-g amp. Of
course when the filter is in the exciter, I suppose you have to worry
about cable length (especially on 10 meters).
73 de Mike,
W4EF........................................................................
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>
> A low pass filter has no energy storage. The idea is that a band pass
> filter with a capacitor on the output of it connected across
> the non-linear cathode circuit (or grid in some cases). The capacitor in
> conjunction with the inductor make an AC energy storage system. The amount
> of energy stored in the system is related to the Q. A low pass filter has
> a Q of less than 1 (actually it is Q= center frequency/
> bandwidth=1/2). The reason for the capacitor being the at the output is
> because it is a short term constant voltage device. It and the
inductance,
> tries to maintain a constant sinusoidal AC voltage between the cathode and
> ground during several RF cycles. If the capacitive reactance is 1/3 of
the
> cathode's minimum instantaneous impedance it maintains a very good
> sinusoidal wave form for a single RF cycle (what is needed) and does
fairly
> good for 3 cycles. This is why high Q is not necessary. The exciter sees
> a constant impedance over each RF cycle also.
> If the output component of the filter connected to the cathode is an
> inductor (Inductor output L or T or Pi-L) it will try to maintain a
> constant sinusoidal RF current thru the RF cycle. This will make the IMD
> even worse.
>
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