Service manual

3.12 TP0332/2
Signalling Facilities
The baseband chip has facilities for CTCSS, SELCALL, DTMF and FFSK signalling. The way in
which these signalling modes are controlled and how to they relate to the receive and transmit audio paths
is described in the following sections. Figure 3.8 is referred to for receive signalling and Figure 3.9 for
transmit signalling.
-CTCSS
The baseband chip has a CTCSS encoder which is programmed by the processor with the
appropriate frequency via the IIC serial bus lines SCL and SDA. The CTCSS tone is summed into the
internal transmit path before the 3kHz high-pass filter F8.
The CTCSS detector works from the signal before the de-emphasis filter F1. The detector consists
of a narrow bandpass filter followed by a comparator that generates a zero-crossing signal at the SUB_IO
pin. The processor continuously monitors the time interval between the zero-crossing events and decides
whether the correct CTCSS frequency is present.
- Selcall/DTMF
The baseband chip has two tone generators which are used for SELCALL and DTMF tone
generation. The tone generators are programmed by the processor with the appropriate frequency via the
IIC serial bus lines SCL and SDA. The tones are summed into the internal transmit path before the 3kHz
high-pass filter F8.
There is no DTMF decode capability in the baseband chip.
The SELCALL decoder works from the signal after the de-emphasis filter F1. The detector consists
of a broad bandpass filter (450Hz to 3kHz) followed by a comparator that generates a zero-crossing signal
at the TXS pin. The processor continuously monitors the time interval between the zero-crossing events
and decides which SELCALL tone is present.
- FFSK
In transmit mode the processor supplies the transmit FFSK clock on the TXS pin and the transmit
FFSK data on the TXDAT pin from which the FFSK signal is generated. The FFSK signal summed in to the
transmit audio path before the 3kHz low-pass filter F8.
The FFSK decoder works on the signal from either before or after the de-emphasis filter F1
depending on the FFSK system used. The decoder consists of analogue filtering, carrier detect, clock and
data recovery circuitry. The chip provides the processor with a carrier detect signal (CD), a receive clock
(RXCLK) and receive data (RXDAT). The chip carries out no interpretation of the FFSK data; this is left up
to the processor to decode.
- DTMF Encoding
DTMF Encoding is achieved using the baseband chip tone generators.