Specifications

11
D. OPERATING INSTRUCTIONS AND APPLICATIONS
1. General
The principles of operation of the internal component blocks are described later in the
manual. Here, it will be assumed that the operator is familiar with the general problems
of spectrum analysis and the explanations will be confined to the controls of this
instrument.
The Model 140 is turned on by a front panel push switch with a pilot light. There is a
little warm-up time required. An internal check of operation may be made by the
following procedure.
A patch cord is supplied that connects
"Noise Output"
with the
"Microphone"
input.
Push in the noise level control to give a low level output. This will apply the internally
generated pink noise to the amplifiers that precede the analyzer section. Adjust the
"Noise Level"
control and the
"dB-SPL"
control until a reading is obtained near the
middle of the
SPL
display. The
"Display Range"
switch may be set at either 10dB or
20dB range. Finally, adjust the
"Display"
control (which has a range of 20dB in gain) so
that the 1/3 octave display is near center on the
LED
display raster. Pink noise should
give a horizontal graph within plus-minus one light. Although the output smoothing time
is long enough to give a smooth appearance to the display and reduce random level
variations, allowance must be made for the statistics of noise on the output. There will be
a small amount of fluctuation with time in the lower channels.
2. Operation From the
"Microphone"
Input
The input marked
"Microphone"
is for the dynamic microphone with an internal
impedance of 200ohms. The analyzer has a transformer input and an input impedance of
about 2 kilohms. The instrument is adjusted at the factory for a microphone with a
sensitivity of 0.13 millivolts per micro-bar. Calibration may be checked by inserting a
test signal from a sine wave generator into the
"Microphone"
input, at any frequency
from 400Hz to 1000Hz. The test signal should be attenuated through a 60dB divider or
attenuator to simulate a source impedance of 150 to 200 ohms. With 0.13 volts rms
across the voltage divider for an input voltage of 0.13 millivolts, the broadband
SPL
column should read 74QB-SPL.