Antenna Switch And Harmonic Filter; Power Control Circuitry - Motorola ASTRO Digital Spectra Plus Detailed Service Manual

3-60
Final Stage
The final device is a 3- to 33-Watt device and is driven by the driver through a low-pass matching
circuit that consists of C3815, C3816, C3817, L3811, C3819, C3821, C3822, C3823 and associated
transmission lines. Base network, L3852, L3851, and R3815, R3819 provide the zero-DC bias
required by the final device's Class-C operation. L3852 and L3851 provide the DC path from base to
ground. R3815 and R3819 help lower the network's Q at low frequencies. The collector DC network
consists of L3875, L3876, R3876, R3877, C3880, C3885, C3881, C3882, and CR3875. This network
provides the A+ voltage to the final while blocking RF from getting up the DC line. L3875 and L3876
provide the DC path and block RF. R3876 and R3877 resistively load down the final's collector at low
frequencies and prevent unwanted oscillations. C3881, C3882, C3880, and C3885 are all bypass
capacitors ranging from very low frequencies up to VHF frequencies. R3875 is the current-sense
resistor. CR3875 protects against reverse polarity. Finally, the RF signal goes through a low-pass
matching network (C3875, C3877, C3878, C3879, L3877, and associated transmission lines) to the
rest of the output network (Directional Coupler, Antenna Switch, and Harmonic Filter).

3.7.1.2.1 Antenna Switch and Harmonic Filter

Antenna Switch
The antenna switch's impedance inverter circuit, made up of C3920 and L3920, takes the place of a
quarter-wave microstrip line. During transmission, Keyed 9.4 V forward-biases CR3921, producing
low impedance on CR3921's anode and high impedance on the C3920/L3920 node. Effectively, this
isolates the transmitted power from the receiver. C3910 couples the power to the harmonic filter and
on to the antenna.
Total TX to RX isolation exceeds 50 dB from 136-174 MHz. The impedance inverter contributes
approximately 30 db to transmit isolation. A second shunt switch, made up of CR3922, L3921,
C3922, and C3921, provide additional isolation. C3926 and C3923 block DC.
During RX, CR3920 has an OFF capacitance of approximately 1 pF. CR3921 and CR3922,
incorporated in the RX match, have similar OFF capacitance.
Harmonic Filter
The 25/10-Watt harmonic filter is a 7-pole, low-pass filter, consisting of high-Q chip capacitors
(C3911, C3913, C3912, and C3914) and discrete inductors (L3911, L3912, and L3913). The filter's
primary function is to attenuate harmonic spurs generated by the transmitter. It also adds low-pass
selectivity for the receiver. L3914 protects the PA from static discharge.

3.7.1.2.2 Power Control Circuitry

Command Board Circuitry
Inside U500, the Regulator Power Control IC (see
amplifier that has four inverting inputs, and one non-inverting input (at pin 44) which is the reference
input for the entire power control loop of the power amplifier. The 3.2-V reference voltage at U500-44
is produced by dividing SW +5-V with the voltage-divider circuit, R514 and R515.
September 5, 2008
Theory of Operation: Power Amplifiers
Figure 3-21 on page 3-61), is an operational
6881076C25-E