Service manual

9
Fig. 7. A Clamp Circuit that can be used to Prevent
Overloading the Oscilloscope Input.
Fig. 8. Pole-Zero Adjustment Using a Square W to
the Preamplifier.
USING SQUARE WAVE THROUGH
PREAMPLIFIER TEST INPUT
A more precise pole-zero adjustment in the 575A
can be obtained by using a square wave signal as
the input to the preamplifier. Many oscilloscopes
include a calibration output on the front panel, and
this is a good source of square wave signals at a
frequency of ~1 kHZ. The amplifier differentiates
the signal from the preamplifier so that it generates
output signals of alternate polarities on the leading
and trailing edges of the square wave input signal,
and these can be compared as shown in Fig. 8 to
achieve excellent pole-zero cancellation. Use the
following procedure:
a. Remove all radioactive sources from the vicinity
of the detector. Set up the system as for normal
operation, including detector bias.
b. Set the 575A controls as for normal operation;
this includes gain, shaping, and input polarity.
c. Connect the source of 1-kHz square waves
through an attenuator to the Test input of the
preamplifier. Adjust the attenuator so that the
575A output amplitude is ~9 V.
d. Observe the Unipolar output of the 575A with an
oscilloscope. Adjust the PZ ADJ control for
proper response according to Fig. 8. Use the
clamp circuit in Fig. 7. to prevent overloading
the oscilloscope input.
4.6. OPERATION WITH SEMICONDUCTOR
DETECTORS
CALIBRATION OF TEST PULSER An ORTEC
419 Precision Pulse Generator (or equivalent) is
easily calibrated so that the maximum pulse height
dial reading (1000 divisions) is equivalent to 10-
MeV loss in a silicon radiation detector. The
procedure is as follows:
a. Connect the detector to be used to the
spectrometer system; that is, preamplifier, main
amplifier, and biased amplifier.
b. Allow excitation from a source of known energy
(e.g., alpha particles) to fall on the detector.
c. Adjust the amplifier gain and the bias level of
the biased amplifier to give a suitable output
pulse.