Operating instructions

LOCAL OPERATION
3-60
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. 3-18 Spectrum analyzer setup, Look and Listen function
The Look and Listen facility allows the Service Monitor to be used for ‘OFF-AIR’ monitoring of
transmissions while examining the area of radio spectrum on which the transmission being
monitored is centered. 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 center 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. 3-18, 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.
Center 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 center frequency is set by selecting the [Centre Freq] key to highlight the legend
Centre 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
Centre 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.