>> 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.
>>
>Yes, quite right on the output PI network, Bill. However, if you plug the
>values for Q=1 into a circuit simulator (I just ran them), the frequency
>response of a tuned input PI looks a lot like a classic chebyshev
>low-pass filter with a ripple of about 0.2dB (values taken from
>Orr's Radio Handbook, K8RA 3CX800A7 amp, 14 MHz tuned
>input). For Q=2 (taken from my Drake L-7 schematic), the frequency
>response is starting to look more like a resonant PI (2 dB ripple).
>Not sure why Pittenger used Q=1 (seems like I hear people generally
>recommending Q=2). But if a 3 pole Q=1 resonant PI network
>provides enough energy storage to be useful as a tuned input, but
>only has 0.2dB frequency response ripple, then why wouldn't a 5
>pole low-pass with the same 0.2dB ripple provide some useful
>storage? Perhaps I need to break out the Spice simulator and run
>some half cosine pulses thru said 0.2 ripple low-pass to find out.
>
>BTW, just for the record I am defining Q=1 to be Xc1=Xc2=50 ohms.
>Not sure if that is a correct definition when the Q gets this low.
>
** This simplified method of determining Q is the one used in Eimac's
'Care and Feeding', however, the actual Q is seemingly a bit higher.
Theory and analysis have their place but the goal is that the SWR must be
low enough to prevent the transceiver's PA-protect circuit from
throttling back. With some radios, this means a SWR of under 1.2 to 1.
Seemingly the fastest way to get there is to start with Xc1= 15 - 25
ohms, and make the other two reactances adjustable
cheers, Mike,
>
>
>
- R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K,
www.vcnet.com/measures.
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