2-8
68P02945C75-A
June 1999
Theory of Operation
GTX/GTX LTR/LTS 2000 Portable Radios Service Manual
Controller
routed to the Audio power ampliÞer circuit in the
transceiver board.
Audio Power Amplifier
Resistor R466 sets the input impedance to U409-P2 of
the audio power amp. The audio PA circuit is a
bridged-tied-load (BTL) conÞguration with Þxed gain
of 40dB, developing 500mW (rated audio power) out-
put at less than 5% harmonic distortion into the
16 Ohm internal speaker LS401 with nominal 7.5 V DC
battery supply. Maximum audio power output is
greater than 1.2 W.
Audio PA Muting and Output Protection
PNP transistor Q410, the audio PA power switch,
driven by NPN darlington transistor Q411, the PA
mute amp, controls Vcc supply to Audio PA U409-P1.
U701-A3 is connected to Q411 base, controlling audio
PA Vcc supply. Resistors R489 and R490, PNP transis-
tor Q412 and the current sense circuit monitor current
supplied to audio PA U409-P1. Worst case audio PA
current (at 9 V DC battery voltage, maximum volume
and full system deviation) does not exceed 450 mA at
the nominal 16 Ohm load. Resistor R488 and capacitor
C461 provide an RC time delay for U405-2, a
monostable multivibrator circuit. A 2.5 V DC reference
voltage is fed to U405-2-P6. On radio power-up, and in
normal operation U405-P7 monostable multivibrator
output is logic ÒLOÓ pulling Q411 emitter to Vee with
the audio PA controlled by U701-A3. Should U409-P5
and/or U409-P8 become shorted to each other or to the
ground (Vee), current consumption exceeds 500 mA
(approximately) and Q412 collector. When U405-2-P5
voltage rises higher than the U405-2-P6 reference volt-
age (rise time is less than 50
µ
sec), U405-2 is triggered
and U405-2-P7 dc output voltage is switched to 4Vdc,
effectively biasing Q411 into cut-off and turning off the
audio PA power switch Q410. U405-2-P7 remains in
this state for 15 msec, then reset to logic ÒLOÓ state.
Average power dissipation in the audio PA circuit com-
ponents is helped to a low level by the low duty cycle
(less than 0.3%) of the audio PA protection circuit. The
cycle repeats until the audio PA output short is
removed.
Receive Data Circuits
The ASFIC (U701) decodes all receive data, which
includes PL, DPL, low-speed trunking, MDC, and
high-speed trunking data. The ÒdecodeÓ process for
each data type typically involves low pass or band pass
Þltering, signal ampliÞcation, and then routing the sig-
nal to a comparator, which outputs a logic zero or one
signal. The detected audio from the transceiver board
is routed to U701-H6 and J7 through coupling cap
C435. Inside U701, the data is Þltered according to the
data type (HS data or LS data, then hard-limited to a 0-
5V digital level. The high-speed limited data output
(MDC and trunking high-speed) appears at U701-G4,
where it connects to U709-P11. The low speed limited
data output (PL, DPL and trunking low-speed)
appears at U701-A4, where it connects to U709-P10.
Alert Tone Circuits
When the microprocessor needs to give the operator
feedback (for a good key press or for a bad key press)
or radio status (trunked system busy, low battery con-
dition, phone call, circuit failures), it sends an alert tone
to the speaker. It does so by sending data to U701,
which sets up the audio path to the speaker for alert
tones. The alert tone itself can be generated in one of
two ways: internally by the ASFIC, or externally using
the microprocessor and the ASFIC. The allowable
internal alert tones are 300, 900 and 1800 Hz. For exter-
nal alert tones, the microprocessor can generate any
tone within the 100-3000 Hz audio band. This is accom-
plished by the microprocessor toggling the output line
U709-P7, which is also the same line used to generate
low-group DTMF data. Inside the ASFIC, this signal is
routed to the external input of the alert tone generator.
The output of the generator is summed into the audio
chain just after the RX audio de-emphasis block. The
tone is then ampliÞed and Þltered before passing
through the 8-bit digital volume attenuator. The tone
exits at U701-J4, then is routed to the audio PA circuitry
in the transceiver board.