generator to provide a 10.396 kHz test signal at a 100% modulation level at both inputs. The
pilot should be switched off for this procedure, and the SAP and PRO channels should also be
OFF.
3.Temporarily disconnect the audio signal generator. Tune the spectrum analyzer to the
frequency of the 455 subcarrier. Set the 455 Transmitter to the "narrow" deviation mode. The
spectrum analyzer should be set for a scan width of not greater than 20 kHz per horizontal
division, and not less than 5 kHz per horizontal division, and the bandwidth should be no
greater than 3 kHz. Verify that the desired carrier is clearly visible at the exact center of the
analyzer screen. Adjust the gain/attenuation settings of the analyzer so that the peak of the
carrier is near the top of the screen, and the vertical resolution is 10 dB per division. Reconnect
the audio signal generator. Sidebands of the carrier frequency should be visible at 10.396 kHz
intervals on the screen. The center (carrier) signal may or may not be visible, depending on
the existing adjustment of the front panel DEV control. Adjust the front panel screwdriver
adjustment DEV for a minimum level (1st Bessel null) at the center (carrier) frequency. You
have now adjusted the deviation for exactly 25kHz peak (50kHz peak-to-peak). If you intend
to use the 455 system in the "wide" deviation mode, change the transmitter to "wide", and
change the audio generator to 9.306 kHz. If necessary, slightly readjust the DEV control for a
minimum level (2nd Bessel null) at the center (carrier) frequency. The deviation is now set to
51.37 kHz peak (102.74 kHz p-p).
4-6. Use of the Overmodulation Indicator.
In normal system use, activation of the overmodulation indicator (OVERMOD alarm) provides a
direct indication that the DEViation control is adjusted too far in the clockwise direction. The
overmodulation indicator is triggered by the deviation limiter circuit which is set at
approximately ±168 kHz in the "wide" mode, and ±82 kHz in the "narrow" mode. The
deviation limiter is designed to prevent the 455 from interfering with adjacent channels in the
event of an excessive input signal, and should not be activated under normal operating conditions.
4-7. RECEIVER FRONT PANEL ADJUSTMENTS.
NOTE:Prior to setting levels be sure receiver is ON and set to proper frequency. See Section 3.
455 Operation Manual
Page 30
6/23/98
