Operating instructions

LOCAL OPERATION
3-79
DCS
Overview
Digitally Coded Squelch signaling is a system for addressing mobile radio transceivers from their
base station by transmitting a code unique to the addressed transceiver for the duration of the
transmission. The principles and limitations of the system provide 104 uniquely identifiable codes
for use on any one RF channel.
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 routing and RF input attenuators starting on page 3-71.
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 earlier in this chapter.
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. 3-25, RF DCS ENCODER screen
and menu.
Fig. 3-25 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
Bit Rate 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
TONE LEVEL 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.