Post-Limiter Filter (Audio Filter Ic); Pl Encoder; Dtmf Encoder; Deviation Attenuators (Audio Filter Ic) - Motorola GP68 Service Manual

Theory of Operation
Transmit and Receive Audio Circuitry

Tx Audio Path

Internal PTT, MIC Bias Switch and External PTT
Sense Circuits
The internal PTT switch (SW402) is connected direct to the
input port of the microcomputer (pin 42) to ground. This port
is an active low. One of the R415 resistor is used to pull up
the line to 5 volt. While PTT (internal) this line will be pulled
low. Internal MIC bias is supplied from the +5VTX (switch
by microcomputer) through R467 and R468. Internal MIC is
connected to the controller board via top keypad flex.
When connecting an external MIC through connector J406,
the external PTT sense transistor Q403(pins 2, 3, and 4)
switches "ON" when the external PTT switch is closed. Col-
lector voltage Q403-4 is monitored by U401-34. When col-
lector is logic "HI" state, the microcontroller configures the
radio for transmit mode. In PTT equipped accessories, the
PTT switch is series connected with the external MIC ele-
ment.
MIC Amplifier
MIC audio from internal MIC MK401 is coupled through
C468, J406, and L402 to the MIC amp circuit U452-1. Exter-
nal MIC plug insertion mechanically disconnects the internal
MIC. External MIC audio is coupled through L402 to the
MIC amp input. Capacitors C458, C460 and C461, and resis-
tors R458, R459 and R460 provide a low audio frequency
roll off with a high-pass corner frequency of 1 kHz to
improve transmit audio clarity. Crossover gain is 12 dB (at 1
kHz). Reference deviation is obtained with 11.0 mV rms
input to the external MIC connector J406.
Tx Audio Mute Gate
Pins 1, 5, and 6 of dual PNP transistor Q403 and resistors
R465 and R456 comprise the Tx audio mute gate. Audio Fil-
ter IC U451-40, expanded output port, controls PNP transis-
tor Q403 (pins 1, 5, and 6). When U451-40 is logic LO state,
a small dc current flows from U452-1-1 MIC amp output into
Q403-6 emitter, through Q403, and out of the collector
(Q403-1) through R456. A fraction of the emitter current
flows out of the base (Q403-5) through R465 to ground (Vss
of Audio Filter IC). MIC audio at U452-1-1 passes through
the Tx audio mute gate. When U451-40 is logic "HI", Q403-
6 is 2.4 Vdc, biasing the device well into cut-off. No current
flows through emitter to base/collector, and no MIC audio
passes. The mute function is enabled (Q403 pins 1, 5, and 6
is "OFF") when modulating DTMF Signalling.
Pre-emphasis Amp
The Audio Filter IC U451, contains a Tx audio pre-emphasis
amp, with external gain setting resistor R451 and C452. Con-
nections are made at each end of resistor R451 to provide
interconnection of option board Tx audio through connector
J403. Pre-emphasis is 6 dB/octave with a corner frequency of
6600 Hz. Crossover gain is 0 dB at 1 kHz, with passband
gain (head-room) of 17.5 dB.
2-2
Limiter (Audio Filter IC)
The audio filter IC U451 contains the limiter circuit, which
prevents over-deviation of the RF carrier by symmetrically
clipping the peaks of the modulating voltage. Audio from the
pre-emphasis amplifier circuit is coupled to the limiter. Gain
of the limiter stage is adjustable in four 3 dB steps, from -3
dB, 0 dB, +3 dB, and +6 dB. Therefore, Tx audio path gain,
or MIC gain, can be adjusted to compensate for different
sound environments through the Radio Service Software.

Post-Limiter Filter (Audio Filter IC)

Clipped modulating voltage from the limiter output is cou-
pled to the post-limiter filter. Filtering attenuates the spuri-
ous products generated by the limiter. The post-limiter filter
is programmable to operate in the following modes:
• EIA mode

PL Encoder

Private Line (CTCSS) is generated by the PL encoder circuit
in the Audio Filter IC U451. Tone PL or Digital PL data is
user programmable (see user manual) for each mode. On
entering transmit mode, TPL or DPL data is programmed to
U451 via the serial DATA and CLOCK lines. U401-57
microcontroller output strobes & U451-32 PL CLOCK input
at a constant rate during DPL encoding, or at a rate deter-
mined by the PL encoder algorithm in the microcontroller
for TPL encoding corresponding to tone frequency. The
encoded PL is summed with MIC audio at the post-limiter
filter input. Digital attenuators are employed to adjust the
balance of MIC radio and PL to prevent over deviation of the
carrier.

DTMF Encoder

Resistors R423, R424, R425, and R427, and summer U452-
2 form the DTMF encoder. Output from U452-2 pin 7 is cou-
pled to U451-13 Audio Filter IC auxiliary Tx modulation
input. DTMF encoded signals pass from this input to the
post-limiter filter input.
Output from U452-2 pin 7 is also coupled to U451-6 and
coupled through Rx audio path to the audio PA for sidetone
audio.

Deviation Attenuators (Audio Filter IC)

Carrier deviation is set by programming the digital deviation
attenuators of the Audio Filter IC. Deviation data for each
mode is entered through the Radio Service Software, and
then programmed into U451 from microcontroller U401 on
entering transmit mode. U451-19 and U451-20 deviation
attenuator outputs are combined through resistors R454,
R455 and R457 and dc-coupled to U201-5 (on rf board), the
synthesizer modulation input. Capacitor C218 provides a
high frequency roll-off corner at 20 kHz to further attenuate
spurious signals from U451. The dc voltage at the combined
attenuator outputs sets the center frequency for the modu-
lated carrier. Any transient (R x C) voltages in the Tx audio
6881086C09-O
GP68 Portable Radios Service Manual
March, 1997