Operating instructions

OPERATING MANUAL 2945A
46882-311D 3-57
Look and Listen
B1417
!
!
RF
DET.
LOCAL
OSCILL-
ATOR
RECEIVER
CIRCUITS
AUDIO
AMP.
SWEEP
GEN.
OSCILLO-
SCOPE
LOG.
AMP.
AM/FM
DEMOD
20 dB
ATTEN.
I/O
SWITCH-
ING
Fig. 3-16 Spectrum Analyzer setup, Look and Listen function
The Look and Listen facility allows the instrument to be used for ‘OFF-AIR’ monitoring of
transmissions while examining the area of radio spectrum on which the transmission being
monitored is centred. The width of RF spectrum scanned in the Look and Listen mode is
adjustable in steps from a minimum of 100 kHz to 1 MHz.
The RF signal from a suitable antenna is coupled to the Service Monitor through either the
antenna BNC connector or the ‘N type’ connector on the front panel. The signal is passed
through the spectrum analyzer circuits to be displayed as a conventional spectrum analyzer
display. The listen demodulator obtains its signal from a non-sweeping IF circuit and is tuned to
the centre frequency of the display. The output from it is routed through an AF amplifier to the
internal loudspeaker, to the accessory socket and to the ‘demod out’ socket. See Fig. 3-16,
Spectrum Analyzer setup, Look and Listen function .
Setting up
Pressing the [Look & Listen] key from the
SPECTRUM ANALYZER menu selects this option. The
menu shown in fold out diagram (6-6) is displayed.
Centre Freq and Span setting
When operating in the ‘Look & Listen’ mode, the spectrum display is defined only by the
Centre Freq and Span settings.
The centre frequency is set by selecting the [Centre Freq] key to highlight the legend
at
the bottom of the display and then entering a value using the data input keys. Fine adjustment
can be made using the variable control when the
legend is displayed as above.
With an input signal displayed, adjusting the centre frequency will allow the listen detector to be
tuned to an observed signal.