Rx Audio Path; Audio Processing (Audio Filter Ic); Pl Decoder; Rx Audio Mute - Motorola GP68 Service Manual

GP68 Portable Radios Service Manual
Transmit and Receive Audio Circuitry
path must settle within 1 millisecond of PTT activation to
prevent center frequency offset.

Rx Audio Path

Audio Processing (Audio Filter IC)

The recovered Rx audio from the rf board (IF detector IC
U51) is coupled through to U451-7 and U451-8 on the Audio
Filter IC. Rx audio at U451-7 is processed first by the PL
rejection filter, which is characterized by a 2-pole, 300 Hz
corner frequency high-pass response. Audio de-emphasis is
a single pole low pass filter with a corner frequency of 231
Hz. Audio then passes through the digital volume attenuator
and buffer amplifier output to U451-23. For standard test
modulation, the audio level at U451-7 is 255 mV rms, and
output audio level at U451-23 is 765 mV rms with the digital
volume attenuator set to minimum attenuation.

PL Decoder

Recovered Rx audio at U451-8, the PL decoder input, first
passes through the Tone PL filter, or the Digital PL filter,
depending on the PL option selected for the current operat-
ing mode. Filtered PL is then coupled to the PL detector cir-
cuit, with detected PL output at U451-27 to microcontroller
U401-64 where algorithms perform the final PL decoding.
Data for the Tone PL frequency or Digital PL code for each
mode is programmed through the Radio Service Software.

Rx Audio Mute

The Rx audio mute is controlled by microcontroller U401
via U451-3. The chip disable U454-1 is used to power down
audio PA to conserve standby current and mute. When at a
logic "0" (0 V to 0.8 V), the U454 is enabled for normal oper-
ation. When pin 1 (U454-1) is at a logic "1" (2.0 V to Vcc V),
the U481 is disabled (muted).
Audio Power Amplifier
U454 (MC 34119) is a low amplifier capable of low voltage
operation (Vcc=2.0 V minimum). The circuit provides a dif-
ferential output (U454 pins 5 & 8) to the speaker (24 ohms)
to maximize the available voltage swing at low voltages. The
internal configuration consists of two identical operational
amplifiers. Open-loop gain is above or equal to 80 dB (at f<=
100 Hz), and closed-loop gain is about 46 dB set by R485
(feedback resistor) and input impedance (R480, R481, and
SW482-2). Variable resistor SW482-2 and R482 provide Rx
audio volume adjustment. R482 will set the minimum vol-
ume level. The chip disable pin (U454-1) permits powering
down the U454 IC for muting purposes and to conserve
power.

Noise Squelch Attenuator

The Audio Filter IC U451 contains a 16 step programmable
digital squelch attenuator between U451-16 and U451-18.
Noise squelch setting is a user programmable option (see
March, 1997
user manual), with open squelch at step 0, and tight squelch
at step 15.

Option Interface

The option interface provides the ability to interface Motor-
ola designed and third party designed option boards to the
radio. The following is a description of the signals available
on the option interface.
J403 -1 interfaces with pin 42 of U401 and one end of
PB401, the PTT switch. This is a wire-or connection with the
internal PTT signal. It can either be used as an internal PTT
sense or as a PTT input from the option board. This connec-
tion is pulled to 5 volts through a 51k ohm resistor. When this
connection is shorted to ground the radio microcontroller
processes an internal PTT request. This request normally
keys the radio unless overridden by other enabled features.
J403-2 interfaces with pin 54 of U401. Pin 54 of the micro-
controller is bi-directional port D bit 4. In the GP60 Series
Radio, this connection option board enables output from the
microcontroller. This pin is used to enable option boards or
to enable a serial transfer in more complex option boards.
When used as a simple option board enable the radio micro-
controller sets the output either high or low on power-up to
reflect the programmed selection in the radio's wide data.
When used in more complex option boards this line enables
serial transfers between the radio microcontroller and the
option board. Since the serial data out of the microcontroller
is used for multiple internal devices, including an option
board, this signal indicates when serial data is for option
board use and not other electrical subsystems within the
radio.
Tx Audio input to the radio is available at J403-3 with a sen-
sitivity of 40 mV rms pre-emphasized at a 6dB/octive, and
less than 200 ohm output impedance (from the option board).
If "flat" audio response is required, the audio output from the
option board must be de-emphasized at a -6dB/octive rate,
300 Hz-3 kHz with 0 dB gain at 1 kHz. The low option board
output impedance is required to achieve better than 40 dB
isolation between main board input (J403-4) and output
(J403-3) audio.
This is the audio from the internal or external microphone of
the radio. J403-4 provides MIC audio output to an option
board at a level of 45 mV rms and a 10k ohm input imped-
ance.
J403-5 is the logic board ground.
6881086C09-O
Theory of Operation

J403-1

J403-2

J403-3
J403-4

J403-5

2-3