Rfpa; Output Network; Power Control; Circuit Protection - Motorola ASTRO XTL 1500 Detailed Service Manual

Product Overview: Frequency Generation Unit
2.5.3 700–800 MHz Radios
The 700–800 MHz transmitter consists of an RF power amplifier (RFPA), output network (ON), and

power control.

2.5.3.1 RFPA

The RFPA is a three-stage, discrete-LDMOS transistor amplifier consisting of the following stages:
first, driver, and final.
The first stage acts as a variable-gain amplifier and feeds the driver stage, which, in turn, feeds the
final stage. All of the stages are matched using transmission lines, capacitors, and inductors. Stage
drain biases are supplied via A+ or K9.1V and DRV_9.3V (DRV_9.3V is present for UHF Range 1
and UHF Range 2 only). Stage gate biases are supplied via a digital-to-analog converter (DAC) or
the RFPA control voltage.

2.5.3.2 Output Network

The output network (ON) consists on the antenna switch, harmonic filter, and power detector. The
antenna switch operates in two modes: RX and TX.
In TX mode, the RFPA final stage is connected to the antenna through the harmonic filter and power
detector and is isolated from the RX path.
In RX mode, the antenna is connected to the RX front-end through the power detector and the
harmonic filter and is isolated from the TX path. The harmonic filter attenuates harmonics generated
by the RFPA when the antenna switch is in TX mode and provides extra selectivity in RX mode.
The power detector senses forward and reverse power and generates a detected voltage
proportional to each.
2.5.3.3 Power Control
The forward-power and reverse-power detected voltage is fed back to the power control section
where it is added to a DAC voltage determined via power tuning and compared to a reference
voltage. A control loop corrects the control voltage adjusting the first stage gain to maintain the
reference.

2.5.3.4 Circuit Protection

Final-stage current and temperature as well as radio A+ voltage and RFPA control voltage are
sensed. If a fault condition is determined, power is cut back to a level that is safe for the particular
conditions.
2.6

Frequency Generation Unit

This section discusses the frequency generation unit (FGU) components and basic operation for
each band.
2.6.1

VHF MHz Radios

The VHF (136-174 MHz) frequency generation unit consists of the following:
• Low-voltage fractional-N synthesizer IC
• 16.8 MHz reference oscillator IC
• Two receiver (RX) voltage-controlled oscillators (VCOs)
• Two transmitter (TX) voltage-controlled oscillators (VCOs)
• VCO buffer/amplifier circuits
6815854H01-A
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June 15, 2005