Receive Signalling Circuits; Sub-Audible Data (Plidpl) And High Speed Data Decoder; Alert Tone Circuits; Figure 2-6. Receive Signalling Paths - Motorola CDM1550 LS+ Detailed Service Manual

2-16 Theory of Operation
2.3.6 Receive Signalling Circuits
Refer to Figure Figure 2-6 for the descriptions that follow.
DETAUDIO
DISCRIMINATOR AU
FROM RF SECTIO
(IF IC)
DIO
N
2
I
DATA FILTER
HSIO
AND DEEMPHASIS
LIMITER
DISC
ASFIC_CMP
U0221
Lj
FILTER
LIMITER
LS IO
PLEAP PLCAP2
~ ~5
T T
19
18
Figure 2·6. Receive Signalling Paths
2.3.6.1 Sub-Audible Data (PLIDPL) and High Speed Data Decoder
82
,
7
MICRO
CONTROLLER
U0101
80
~
The ASFIC CMP (U0221) filters and limits all received data. The data enters the ASFIC CMP at
input DISC (U0221, pin 2). Inside U0221 the data is filtered according to data type (HS or LS), then
it is limited to a 0-5V digital level. The MDC and trunking high speed data appear at U0221, pin 19,
where it connects to the 1J P UO 1 01 , pin 82
The low speed limited data output (PL, DPL, and trunking LS) appears at U0221, pin 18, where it
connects to the IJP U0101, pin 80.
The low speed data is read by the IJP at twice the frequency of the sampling waveform; a latch
configuration in the ASFIC CMP stores one bit every clock cycle. The external capacitors C0236,
and C0244 set the low frequency pole for a zero crossings detector in the limiters for PL and HS
data. The hysteresis of these limiters is programmed based on the type of received data.
2.3.6.2 Alert Tone Circuits
When the software determines that it needs to give the operator an audible feedback for a good key
press, or for a bad key press, or radio status (trunked system busy, phone call, circuit failures), it
sends an alert tone to the speaker. It does so by sending SPI BUS data to U0221 which sets up the
audio path to the speaker for alert tones. The alert tone itself can be generated in one of two ways:
internally by the ASFIC CMP, or externally using the IJP and the ASFIC CMP.
The allowable internal alert tones are 304, 608, 911, and 1823Hz. In this case a code contained
within the SPI BUS load to the ASFIC CMP sets up the path and determines the tone frequency, and
at what volume level to generate the tone. (It does not have to be related to the voice volume
setting).
For external alert tones, the IJP can generate any tone within the 100-3000Hz audio band. This is
accomplished by the IJP generating a square wave which enters the ASFIC CMP at U0221-19.
Inside the ASFIC CMP this signal is routed to the alert tone generator
The output of the generator is summed into the audio chain just after the RX audio de-emphasis
block. Inside U0221 the tone is amplified and filtered, then passed through the 8-bit digital volume
attenuator, which is typically loaded with a special value for alert tone audio. The tone exits at
U0221-41 and is routed to the audio PA like receive audio.