ESCO Technologies ETS Lindgren 3301B, User Manual

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30 | Theory of Operation
Finally, the interaction of the rod with the ground must be understood. The
theoretical understanding of the rod assumes that it operates in reference to an
infinite ground at 0 Volt potential. The closer the test situation is to this scenario
the more accurate will be the readings. A small vestigial ground is provided with
the unit in the form of a counterpoise. Care should be taken to reference this
counterpoise to true ground.
The procedures for this are carefully explained in various standards. If the
counterpoise is not well grounded, an impedance may build up through the
cabling and other instrumentation. Reading differences of as much as 20 dB can
be found with ungrounded counterpoises. This will be true for all rod antennas
regardless of their design.
Another counterpoise problem is the potential for positive feedback. Good
amplifier design calls for the output to be 180 degrees out of phase with the
output signal so that the shield current is in phase with the input. If the shield and
counterpoise are tied through the body of the unit, then the counterpoise will
receive some of this current. If the counterpoise does not have an extremely low
impedance to ground, then this return current can drive a potential on the
counterpoise in phase with the input resulting in errant readings. The
Model 3301B has an internal common mode choke to minimize the impact of an
inadequate test setup. However, proper test configuration will always be critical
to ensure measurement accuracy.
Also of concern is the contribution of edge effects from the vestigial ground
plane. Most theory is developed on the assumption of one infinite ground plane.
How using a small vestigial ground plane or counterpoise affects the theory has
not been well studied to date. Even more complicated is the effect of this vestigial
ground plane when the unit is used in a screen room with the ground planes on
all sides. In practice, the best results are obtained by tightly typing the
counterpoise to the screen room through an ultra low impedance ground strap.
This at least assures that at the lower frequencies all of these surfaces are at an
equal potential.
9.2 The Pre-Amplifier
The Model 3301B pre-amplifier is designed in two stages. The first stage
provides impedance transformation and current gain. The second stage provides
voltage gain and impedance mating on a 50-ohm output.
The first stage of the pre-amplifier contains an n-channel JFET and an NPN
bipolar silicon transistor. Through the use of extremely precise layout techniques,
careful parts selection and feedback to virtually eliminate the Miller effect, the
total input capacitance is kept extremely low. This low capacitance in turn allows
for an extremely high input impedance.