[Elecraft] Tank Circuit?

[Ron D'Eau Claire]

A classic "tank circuit" is a coil across a capacitor. At its resonant
frequency (the frequency at which the inductive reactance of the coil is
equal to the capacitive reactance of the capacitor) it exhibits a very
high impedance across the coil-capacitor pair and a very low impedance
to any load placed in series with the coil and capacitor. 

They are not used in the Elecraft rig outputs, but many hams often refer
to the "output tank" when referring to the output circuit of a
transmitter. They were very common a few decades ago. Almost any old
tube rig that shows a knob for "Plate Tuning" will have a "tank circuit"
in the output. Look at the schematic of the final stage and you will
almost always see some arrangement of a coil with a capacitor across it
connected to the plate circuit. In the 1950's they gave way to a
variation called the "pi-network" that uses a coil and two capacitors
and is especially well-suited for use with the coaxial cable
transmission lines that had become very popular since World War II. 

Tank circuits are not used much today because: 1) They require tuning to
work across a whole amateur band and 2) they do not have sufficient
selectivity to meet the FCC rules for spectral purity of the transmitted
signal of most transmitters and 3) their intrinsic high impedance makes
them somewhat difficult to use with the very LOW impedance of most solid
state power amplifiers. 

Rigs like the Elecraft rigs use a complex output filter network that is
at once broad band enough not to require tuning across an amateur band
and selective enough to reject the harmonics as required by the FCC. 

The problem with the output filters used today is that they are
difficult to make "tunable". They are designed to work ONLY into a 50
ohm non-reactive load. So we end up using an ATU in most cases because
our antennas cannot provide a termination for the rig that is low enough
in reactance and shows the right impedance. In short, we have removed
the 'tunable tank circuit' from the output of our rigs and put it into
the ATU instead. And then, in the case of  tuners like the Elecraft
units, we hooked a ton of logic circuits to it to automatically adjust
it as needed so we don't have to twiddle with any knobs. 

Ron AC7AC
K2 # 1289

Hi,

Someone who is technical, give a quick definition or a schematic of the
what the 'tank circuit' looks like and the purpose of it.

I have both the KAT100 and KPA100 PDF manuals on hand.  So just tell me
a page number to look at.

Daniel
kg4dni