6
Fig. 4.5. Measuring Linearity by the Null-Balance Method.
3. Then obtain the amplifier noise resolution spread
by measuring the FWHM of the pulser spectrum.
The detector noise resolution spread for a given
detector bias can be determined in t he same
manner by connecting a detector to the preamplifier
input. The amplifier noise resolution spread, of
course, must be subtracted. The detector noise will
vary with detector size and bias conditions as
indic ated in Fig. 4.4 and possibly with ambient
conditions.
Ampl i fi er Linearity Measurements
The
measurement of amplifier linearity c an be quickly
and simply done by utilizing the method outlined in
Fig. 4.5. The method consists of bucking out two
voltage signals from low-impedance sources and
measuring the amplitude differential at a null point.
The following conditions of Fig. 4.5 should be
considered when linearity measurements are made.
The output impedance of the Direct Output must be
100
Ù
. The amplifier must be set in the inverting
mode of operation; i.e., for the negative input shown,
the amplifier must produce a positive Output Pulse.
The impedance seen from point A t o ac, or signal,
ground via point C should be equal t o t he
impedance seen from point A to ac, or signal,
ground via point B. The diodes D should be
germanium units with high g
m
. The diodes can be
replaced with high-frequency germanium transistors
with the base connected to the collector so that the
emitter-base functions as the diode. Transistors
suitable for this test include 2N779, 2N964, 2N976,
2N2048. The diodes serve as bipolar voltage clamps
to limit the voltage s wing at point A to the forward
voltage drop across the diodes. The diode-resistor
network should be constructed so as to minimize the
stray capacitance around this network. The network
should be physically located on t he oscilloscope
input connector for the same reason.
Initially the output of the Pulser and amplifier should
be set for 10 V. This should be measured with cars,
and consideration should be given for the output
impedance of both the Pulser and amplifier. By
observing the waveshape at point A (Fig. 4. 5), the
fine gain of the amplifier and the attenuation controls
should be adjusted until a null is obtained between
time t
1
and t
2
. At null, the sensitivity of the
oscilloscope should be set to 10 mV/cm for best
Summary of Contents for ORTEC 480
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