Operating instructions

LOCAL OPERATION
3-72 46882-311D
The coded signal consists of a repeating 23 bit word which has error correcting information and
timing using 14 bits, leaving nine bits available to carry address code.
The digital signal is formatted as a simple non-return to zero (NRZ) signal which is used to
directly modulate the transmitter. On frequency shift modulating systems a ‘1’ is usually made to
increase the carrier frequency and a ‘0’ to decrease the carrier frequency. An ‘Invert’ facility is
included to reverse the polarity of the digital signal before it is applied to the modulator.
The most often used bit rate for generating the 23 bit code words and that which is specified in
most standards is 134 bits/s. This allows the system to operate below the communications audio
pass band.
See Signal routeing and RF input attenuators starting on page 3-65.
DCS receiver decoder testing
To test the DCS decoder function of a mobile transceiver, set up and connect the Service Monitor
with the mobile as described in the Rx testing section of the operating manual.
Access the DCS facility within the RF tones mode by using the sequence, [Tones], [DCS]. The
RF DCS Encoder screen and menu is displayed as shown in Fig. 3-23, RF DCS ENCODER
screen and menu.
Fig. 3-23 RF DCS ENCODER screen and menu
Check that the bit rate is correct for the unit to be tested. If a different bit rate is required to that
which is shown on the display, enter the required value as follows.
Press the [Bit Rate] key to highlight the
legend on the display. Use the data entry
keys to enter the correct value and press the Hz terminator key. Note. The most common
bit rate for DCS systems is 134 bits/s.
Check that the tone (modulation) level is correct for the system specification. If a different level
is required to that which is shown on the display, enter the required value as follows:-
Press the [Tone Level] key to highlight the
legend on the display. Use the
data entry keys to enter the correct value and press the [kHz] or [Hz] terminator key as
appropriate. Note. The level of modulation for DCS signal modulation is usually within
the range 10% to 20% of the system maximum level.
Check the modulation polarity of the transceiver. For positive modulation the polarity should be
set to
Normal
and for negative modulation, set to
Inverted.
The [Polarity] key toggles between these two options.
To enter or change the DCS code for the transceiver to be tested, first press the [Code] key to
highlight the legend
Key in the code using the data entry keys. The Service Monitor
will accept any octal sequence in the range 0 to 777.
Complete the entry by pressing the [Enter] key. Leading zeros will be suppressed.