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Theory of Operation
Controller Audio & Signaling Circuits
routed to the accessory connector (J400-16 & 1) and, via
the jumper plug connecting J400-16 to 13, to the control
head (connector J101-1 & 2).
Handset Audio
Certain hand-held accessories have a speaker within
them which require a different voltage level than that
provided by U401. For those devices HANDSET
AUDIO is available at control head microphone jack
J903-8.
The received audio from the output of the ASFIC's dig-
ital volume attenuator (U201-J4) is also routed to
U202-9 where it is amplified 15 dB; this is set by the
10k/68k combination of R233 and R232. This signal is
routed from the output of the op amp U202- 8 to J101-
14. The control head sends this signal directly out to the
microphone jack. The maximum value of this output is
6.6Vp-p.
External Rx Audio
RX Audio is also routed to the accessory connector
J400-11. This audio level is fixed and not affected by the
setting of the digital volume attenuator. The source of
this audio is selectable as follows:
In the Data Modem mode (switch S401, 1-3 off), U402-
9 is high, and FLAT_ RX_AUDIO, directly from detec-
tor U5201-28 via C230 and R236, is routed from U402
pin 10 to pin 5, amplified by U403 and applied to J400-
11 via R419 and C419.
In Normal mode (switch S401 1-3 on), U402-9 is low,
and GATED_RX_AUDIO, from ASFIC U201-H5 via
DET AUDIO
DISCRIMINATOR AUDIO
FROM RF SECTION
(IFIC)
8-14
G1
HIGH SPEED
CLOCK
DATA FILTER
LIMITER
& DEEMPHASIS
J7
PL
ASFIC U201
IN
FILTER
LIMITER
LOW SPEED
LIM CAP
C5
Figure 8-6. Receive Signalling Path
6880905Z99-O
GTX LTR/Privacy Plus 800 MHz Mobile Service Manual
C237 and R235, is routed from U402 pin 6 to pin 5,
amplified by U403 and applied to J0400-11 via R419
and C419. The values of R235 and R236 provide the
correct audio levels for GATED and FLAT_RX
_AUDIO paths, respectively. The GATED_RX_AUDIO
signal is PL-filtered, de-emphasized, and controlled by
the squelch mute gate. The FLAT_ RX_AUDIO signal is
non-de-emphasized, unmuted, and is flat between
0.6 Hz and 3 kHz. The radio is shipped configured for
Normal mode operation.
Receive Signalling Circuits
Sub-audible Data (PL/DPL) and High Speed Data
Decoder
The ASFIC (U201) is used to filter and limit all received
data. The data enters the ASFIC at U201-J7. Inside U201
the data is filtered according to data type (HS or LS),
then it is limited to a 0-5V digital level. The trunking
high speed data appears at U201-G4, where it connects
to the µP U101-11.
The low speed limited data output (PL, DPL, and LST)
appears at U201-A4, where it connects to the µP U101-
10. While receiving low speed data, the µP may output
a sampling waveform depending on the sampling
technique to U201-C3 between 1 and 2 kHz.
The low speed data is read by the µP at twice the fre-
quency of the sampling waveform; a latch configura-
tion in the ASFIC stores one bit every clock cycle. The
external capacitors C223, C225, and C226 set the low
frequency pole for a zero crossings detector in the lim-
C3
LOW SPEED
CLOCK
G4
RX
LIM
OUT
A4
PL
LIM
RX LIM
CAP
J3
6
5
11
MICRO
CONTROLLER
U101
10
June, 2000
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Troubleshooting
