6881094C31-E
November 16, 2006
Theory of Operation:
Transceiver Board
3-13
The power level is set by programming an internal DAC to a calibrated reference voltage. D/A
settings for the power set points were determined during radio tuning and stored in EEPROM. An
internal op-amp compares the D/A reference voltage to the detector voltage at pin 1(RFIN) (TP101)
and produces an error signal output. This signal is buffered by another op-amp, configured as a low-
pass filter, or integrator, to produce the INT output at pin 4 (TP104). This signal drives the base of
voltage follower Q108.
Transistor Q108 supplies current to drive the gain control pins of amplifiers U102 and Q107.
Resistors R105 and R106 determine the voltage ratio between U102 pin 2 (VCNTRL) and the Q107
gate. Transient response during key-up and key-down is controlled by the power amplifier rise and
fall times. External capacitors at pins CI, CJ, and CL, along with internal programmable resistors,
determine the ALC time constants.
3.1.3.9.4 Temperature Cut Back
The PCIC contains a temperature cut-back circuit to protect the power amplifier (PA) from thermal
damage that might result from incorrect assembly of the radio. External sensor U103 is a linear
temperature-to-voltage transducer, placed near the hottest spot in the radio: power module Q107.
The output is a DC voltage at pin 2 (VOUT) proportional to the temperature at pin 3 (GND). VOUT is
750 mV at 25°C and increases by 10 mV/°C. The PCIC temperature cut-back threshold is
programmed to correspond to 85 or 90°C. Above this threshold, the ALC gradually cuts back the
transmitter until it is fully turned off at 125°C. The slope of cut-back versus temperature is set by
external resistor R111. Diode D104 clamps TEMP to a voltage not much less than VG (pin 15), about
1.3 V, to improve the transient response of the cut-back circuit.
3.1.3.9.5 D/A Outputs
In RX mode, the PCIC shuts down the transmitter, turns on the receiver, and tunes the RX front-end
pre-selector filters.
VHF:
Signal VAR2 supplies the voltage used to tune both front-end preselector filters. The voltage
range varies from 1.2 V to 2.4 V across the VHF band.
UHF Range 1
: Signal VAR2 tunes both receiver preselector filters. Control voltage is in the 1 to 4 V
range.
UHF Range 2:
Signal VAR2 supplies the voltage used to tune both front-end preselector filters. The
voltage
range varies from 1.2 V to 3.6 V.
700–800 MHz:
Signals VAR1 and VAR2 are D/A outputs to the RX front-end preselector filters.
Output voltage is in the range of about 1 to 4 V over the frequency band. In TX mode, VAR1 and
VAR2 disconnect from the D/A and go to 0 Vdc, thus tuning the RX front-end filters out of band.
3.1.4
Frequency Generation Unit (FGU)
The frequency-generation function is performed by several ICs; multiple, discrete, voltage-controlled
oscillators (VCOs); and associated circuitry. The reference oscillator provides a frequency standard
to the fractional-N frequency synthesizer (FracN) IC, which controls the VCOs and VCO buffer IC
(VCOBIC). The VCOBIC amplifies the VCO signal to the correct level for the next stage.
VHF:
Two VCOs are employed—one to generate the first LO and the other to generate the transmit-
injection signals.
UHF Range 1:
Four VCOs are employed: two transmit and two receive.
UHF Range 2:
Three VCOs are employed: one transmit and two receive.
Summary of Contents for ASTRO XTS-5000
Page 7: ...vi Table of Contents November 16 2006 6881094C31 E Notes ...
Page 11: ...x List of Figures November 16 2006 6881094C31 E Notes ...
Page 17: ...November 16 2006 6881094C31 E xvi CommercialWarranty Notes ...
Page 31: ...November 16 2006 6881094C31 E xxx Portable Radio Model Numbering System Notes ...
Page 83: ...November 16 2006 6881094C31 E 3 44 Theory of Operation Encryption Module ...
Page 135: ...November 16 2006 6881094C31 E 5 44 Troubleshooting Charts Secure Hardware Failure Notes ...
Page 175: ...November 16 2006 6881094C31 E 7 32 Troubleshooting Tables List of Board and IC Signals Notes ...
Page 367: ...November 16 2006 6881094C31 E Glossary 10 Notes ...
Page 373: ...Index 6 November 16 2006 6881094C31 E Notes ...