There are resonant pi networks and low pass filter pi networks......
These are resonant. If you perform a frequency plot of the output vs the
input you will
get a small signal up to the point of resonance ( the low pass part of the
frequency plot) and then a peak at resonance that produces a much larger
signal and as you increase frequency further the output continues to drop.
The magnitude of this peak compared with that at lower frequencies is
related to the Q of the network.If your pi network in your tube output
transceiver was simply a low pass filter you could not peak the output
power when tuning it.
At 08:24 AM 5/5/2003 -0700, Michael Tope wrote:
>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........................................................................
>...............
>
> >
> > 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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