TM 11-6625-3087-12
Weighted noise filter:
C - m e s s a g e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1004Hz
.
A c c u r a c.y. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5dB
Impulse noise counting rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 ins/count 5%
Threshold accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1dB at 1700Hz
.. .
Notch filter rejection (995 to 1025Hz) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50dB minimum
Section Ill.
TECHNICAL PRINCIPLES OF OPERATION
1 - 1 2 . GENERAL FUNCTIONAL DESCRIPTION.
(Fig. 1-2). Frequency Selective Level Meter AN/USM-490 accepts signals from 50Hz to 32.5MHz and, dependent on the
measurement mode selected, displays the desired results on the front panel. Signals are applied using one of the five
impedance ranges and can be balanced, unbalanced or bridged.
The desired value is displayed as amplitude in the MEASUREMENT/ENTRY display/analog meter and frequency in the
FREQUENCY/ENTRY display. Demodulated audio can be listened to using the front panel speaker or headphone jack.
The input signal connected to the front panel is measured, then amplified or attenuated to a usable level. This
50Hz to 32.5MHz signal is then mixed with the first local oscillator frequency of from 50MHz to 82.5MHz.
This mixing produces the first intermediate frequency of 50MHz.
A low pass filter keeps input frequencies greater than 32.5 MHz from being processed. This 50MHz intermediate
frequency is amplified and passed to the IF/audio section.
When CAL is on and a calibration cycle occurs, the input signal is disconnected and a precise calibration signal
from the calibration tracking section is mixed with the first local oscillator. This is then used to automatically
check and adjust the Level Meter for operation within specified limits. Once the calibration cycle is completed,
the input signal is reconnected and normal operation is resumed.
The 50MHz intermediate frequency from the input section is passed to the IF/audio section where it goes
through a 50MHz filter to bandpass to 10KHz. This 50MHz signal with 10 KHz wide bandpass is mixed with
the second local oscillator frequency of 49.984375MHz to produce a second intermediate frequency of
15.625KHz. This 15.625 KHz signal with a 10KHz wide bandpass is amplified and split into two separate
signals. One signal goes through a 400Hz filter. The result is one 15.625KHz, 400Hz wide bandpass signal
and one 15.625KHz, 10KHz wide bandpass signal,
Depending on the front panel bandwidth selection, one of the 15.625KHz signals is passed or filtered, and then
amplified. The desired selected bandwidth of 20Hz, 400Hz, or 3100Hz is then obtained.
The 15.625KHz signal with the desired 20Hz,400Hz or 3100Hz bandwidth is amplified and split three ways.
The first path is to the digital section where the frequency of the input signal is determined.
The second path is computed to an RMS voltage and passed to the digital section where the amplitude of
the input signal is determined.
The third path is to the impairments section for demodulating single sideband audio.
A voltage is developed for the front panel analog meter and to the rear panel METER output connector.
Audio from the impairments section is amplified and passed to the speaker. Detected audio also goes to the rear
panel AUDIO connector.
The digital section monitors most functions in the Level Meter. This includes processing data, front panel
display changes, monitors circuit card operation, handles all front panel operator entries, performs calibration,
and diagnostic tests. It also contains all the front panel displays, annunciators and switches necessary for
operation.
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