The non-linear component of the tube's input impedance represents
energy at harmonics of the drive signals fundamental or a DC term. If you
consider that the resulting voltage and current waveforms present in the
non-linear circuit can be decomposed into a linear superposition
of sine waves (fourier series), then determining the "average impedance"
is just a matter of doing a linear analysis on the resulting first order
terms. The higher order terms shouldn't matter since these are represenative
of energy which is no longer at the fundamental. Unless
a significant amount of this harmonic energy appears across the source
impedance, then you can ignore it in terms of its contribution to VSWR.
If the harmonic energy present at the source impedance is significant
compared to the first order component, then the reflected power component
of the VSWR reading could be in error somewhat, depending on how you
look at it.
An interesting test would be to plot the input VSWR of the amp as a
function of drive level. If the departure from linear behavior is as
significant as everyone seems to be claiming, then you would expect
to see a change in VSWR as a function of drive level.
This is all classroom B.S. of course. The proof is in the pudding, and
you guys are the ones out there cutting metal. My first amp is still a
pipe dream pile of parts in the garage waiting for some attention.
73 de Mike, W4EF.............
From: Jon Ogden[SMTP:firstname.lastname@example.org]
Sent: Wednesday, February 03, 1999 3:46 PM
To: Michael Tope; 'AMPS'
Subject: RE: [AMPS] MFJ-259 and matching circuits
>Would it not be possible to tune the circuit for min VSWR
>using the exciters VSWR meter, as Rich suggested (real
>world operating conditions), then turn off the amp, terminate
>its input port in 50 ohms and measure the impedance of the filter
>looking back from the cathode with the MFJ-259B (cathode disconnected
>of course)? This impedance should be approximately equal to the
>complex conjugate of the large signal cathode impedance. It
>would be interesting to see if this number comes close to Jon's
>estimate of 110 ohms -j (2*pi*f*27pf)^-1.
Theoretically, yes. However, interfacing the MFJ-259 or any other
impedance measuring device to the cathode pins is a challenge and error
will be introduced there. Rather, one could set the impedance as Rich
suggests, remove the tube and put the 110 Ohm//27 pF network in place and
check the impedance. That is a little easier.
But you have your network theory correct. Again, though we (you and I)
are sort of applying a linear network analysis to a very non-linear
system. As others in the group have instructed me, a tube running in
class AB does not have a constant input impedance. Rather it varies over
the drive cycle as the tube is cycled on and off. But I will still argue
an "average" value can be used to simulate a more linear system.
If folks doubt me on the "average" bit, please define then how your AC
wall outlet can be "110 Volts" or "220 Volts." Actually, these are RMS
values if I remember correctly, but my point is that AC voltages vary
wildly over their cycle as well, yet we call them by specific values.
"A life lived in fear is a life half lived."
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