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GP350 Portable Radios Service Manual
Frequency Generation Circuitry
Limiter (Audio Filter IC)
The audio filter IC U402 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 to +6 dB. Therefore, TX audio path gain, or MIC gain,
can be adjusted to compensate for different sound environ-
ments 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:
• CEPT/EIA mode
• Japan mode
• FTZ (Germany) mode
PL Encoder
Private Line (CTCSS) is generated by the PL encoder circuit
in U402, the Audio Filter IC. Tone PL or Digital PL data is
programmed for each mode from the Radio Service Soft-
ware. On entering transmit mode, TPL or DPL data is pro-
grammed to U402 via the serial DATA and CLOCK lines.
U401-35 microcontroller output strobes &402-32 PL clock
input at a constant rate during DPL encoding, or at a rate
determined by the PL encoder algorithm in the microcontrol-
ler 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. PL deviation is adjustable in three "coarse" steps of
500 Hz, 750 Hz, and 1 kHz, for 25 KHz models and steps of
250 Hz, 375 Hz, and 500 Hz for 12.5KHz models with com-
pensation of MIC audio level.
DTMF Encoder
Resistors R424, R425, R426, R428 and R484, and summer
U405A form the DTMF encoder. U405A-1 is coupled to
U402-13 Audio Filter IC auxiliary TX modulation input.
DTMF encoded signals pass from this input to the post-lim-
iter filter input. U405A-1 is also coupled to U402-12 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 U402 from microcontroller U401 on
entering transmit mode. U402-19 and U402-20 deviation
attenuator outputs are combined through resistors R478 and
August, 1996
R479 and dc-coupled to U201-8, the synthesizer modulation
input. Capacitor C218 provides a "high" frequency roll-off
corner at 20 kHz to further attenuate spurious signals from
U402. The dc voltage at the combined attenuator outputs sets
the center frequency for the modulated carrier. Any transient
(R x C) voltages in the TX audio path must settle within 1
millisecond of PTT activation to prevent center frequency
offset.
RX Audio Path
PL Rejection Filter (Audio Filter IC)
The recovered RX audio from the IF detector IC U51 is cou-
pled through capacitor C435 to U402-7 and U402-8 on the
Audio Filter IC. RX audio at U402-7 is processed first by the
PL rejection filter, which is characterized by a two pole, 300
Hz corner frequency "high-pass" response. Audio then
passes through the digital volume attenuator and buffer
amplifier output to U402-23. Unattenuated RX audio is cou-
pled to U402-22 and fed to the center-slicer circuit for detec-
tion of 5/6 tone (European) signals. F or standard test
modulation, the audio level at U402-7 is 255 mVrms, and
output audio level at U402-23 is 765 mVrms with the digital
volume attenuator set to minimum attenuation.
PL Decoder
Recovered RX audio at U402-8, 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 U402-27. The detected PL
signal is coupled from U402-27 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 pro-
g r a m m e d t h r o u g h t h e R a d i o S e r v i c e S o f t w a r e .
Center-Slicer
The center-slicer circuit U406A detects Quick-Call and 5/6
tone signals. Unattenuated RX audio from U402-22 is dc
coupled to the two inputs of U406A. The non-inverting input
U406A-3 is fed through resistor R433. Capacitor C415 sets
a low-pass corner frequency of 3.3 kHz. The inverting input
U406A-2 is fed through resistor R434. Capacitor C416 sets
a low-pass corner frequency of 16 Hz. During operation,
R434 / C416 establish an averaged dc offset level at U406A-
2 dependent on the average dc level of the undetected signal
to set the "trigger" threshold of U406A. R433 / C415 provide
"high" audio frequency roll-off to improve falsing immunity.
The detected output from the center slicer circuit is coupled
to microcontroller U401-43 where algorithms perform the
final data decoding.
Option Interface Connector P1 (Keypad/Display
Models)
P1 provides interconnection of "front cover" option pc
boards to the GP350 radio main board. Filtered "flat" RX
audio output is available at P1-7, at a level of 765 mVrms at
6880904Z07-O
Theory of Operation
4-5
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