Receive Audio Circuits - Motorola M11URD6CB1_N Service Manual

Theory of Operation
Controller Audio & Signaling Circuits
There are seven frequencies, with four in the low group
(697, 770, 852, 941Hz) and three in the high group
(1209, 1336, 1477Hz).
The high-group tone is generated by the µP (U101-5)
strobing U201-G1 at six times the tone frequency for
tones less than 1440Hz or twice the frequency for tones
greater than 1440Hz. The low group tone is generated
by the µP (U101-7) strobing U201-G2 (DTMF CLCK) at
six times the tone frequency. Inside U201 the low-
group and high-group tones are summed (with the
amplitude of the high group tone being approximately
2 dB greater than that of the low group tone) and then
pre-emphasized before being routed to the summer
and splatter filter. The DTMF waveform then follows
the same path as was described for high-speed data.
H5
H6
J6
J7
FROM
RX SECTION
(IFIC)
8-12
1
9
ATTEN.
J4
RX AUD OUT
UNAT RX OUT
VOLUME
ATTEN.
RX IN
FILTER &
DEEMPHASIS
AUX RX IN
PL IN
LIMITER, RECTIFIER
FILTER, COMPARATOR
SQ IN
H7
DET AUDIO
(DISCRIMINATOR AUDIO)
Figure 8-5. Receive Audio Paths
GTX LTR/Privacy Plus 800 MHz Mobile Service Manual

Receive Audio Circuits

Squelch Detect
The radio's RF circuits are constantly producing an
output at the discriminator (U5201-28). This signal
DET_AUDIO is routed to the ASFIC's squelch detect
circuitry input SQIN (U201-H7). All of the squelch
detect circuitry is contained within the ASFIC. There-
fore from a user's point of view, DET_AUDIO enters
the ASFIC, and the ASFIC produces two CMOS logic
outputs based on the result. They are CHACT
(U201-H2) and SQDET (U201-H1).
The squelch signal entering the ASFIC is amplified, fil-
tered, attenuated, and rectified. It is then sent to a com-
ACCESSORY CONNECTOR
J400
INT
SPKR+
U401
SPKR+
4
AUDIO
PA SPKR-
6
INT
SPKR-
CONTROL HEAD CONNECTOR
J101
ASFIC U201
PL
LIM A4
PL FILTER
LIMITER
SQUELCH
CIRCUIT
SQ DET
CH ACT
H2 H1
25 68
MICRO
10
CONTROLLER
U101
6880905Z99-O
11
EXT RX AUDIO
13
INT SPKR+ JUMPER
16
EXTERNAL
1
SPEAKER
2
INTERNAL
1
SPEAKER
14
HANDSET
AUDIO
June, 2000