The front panel DEV adjustment is provided to compensate for minor variations in incoming
composite signal strength, and is not intended to provide for operation at deviations other than
the 455's specified deviation. The deviation may be accurately set using either a modulation
meter or a spectrum analyzer; choose the method you find most suitable.
A. Deviation Adjustment Using a Modulation Meter.
1.If the modulation meter has a high impedance input, connect it to the baseband signal at
any convenient point after the insertion of the subcarrier to be adjusted. If the measuring
instrument has a low impedance (i.e., 75
Ω
or 50
Ω
) it is necessary to connect the instrument to an
isolated monitor point in order to prevent the additional load presented by the low impedance
input from affecting the levels of the signals on the baseband (this level change has no
direct effect on the adjustment of subcarrier deviation, and may be ignored if there are no
active receive sites that may be adversely affected).
2.Connect an audio signal generator to both the LEFT and RIGHT channel inputs of your
BTSC stereo generator. The flat (no pre-emphasis) inputs should be used. The output of the
stereo generator should be connected to the appropriate input of the 455. Set the signal
generator to provide a 1 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.Tune the modulation meter to the frequency of the 455 Transmitter. Adjust the front
panel screwdriver adjustment DEV until the modulation meter indicates a peak deviation
reading of ±25 kHz ("narrow" mode) or ±51.37 kHz ("wide" mode).
B. Deviation Adjustment Using a Spectrum Analyzer.
1.If the spectrum analyzer has a high impedance input, connect it to the baseband signal at
any convenient point after the insertion of the subcarrier to be adjusted. If the measuring
instrument has a low impedance (i.e., 75
Ω
or 50
Ω
) it is necessary to connect the instrument to an
isolated monitor point in order to prevent the additional load presented by the low impedance
input from affecting the levels of the signals on the baseband (this level change has no
direct effect on the adjustment of subcarrier deviation, and may be ignored if there are no
active receive sites that may be adversely affected).
2.Connect an audio signal generator to both the LEFT and RIGHT channel inputs of your
BTSC stereo generator. The flat (no pre-emphasis) inputs should be used. The output of the
stereo generator should be connected to the appropriate input of the 455. Set the signal
455 Operation Manual
Page 29
6/23/98
