Antenna Switch And Harmonic Filter - Motorola ASTRO Digital Spectra Plus Service Manual

Theory of Operation: Power Amplifiers

3.7.2.2.2 Antenna Switch and Harmonic Filter

Antenna Switch
The antenna switch's impedance inverter circuit, made up of C5923 and L5921, takes the place of a
quarter-wave microstrip line. During transmission, Keyed 9.4 V forward-biases CR5921, producing
low impedance on CR5921's anode and high impedance on the C5923/L5921 node. Effectively, this
isolates the transmitted power from the receiver, C5922 couples the power to the harmonic filter and
on to the antenna.
Total TX to RX isolation exceeds 45 dB from 450-512 MHz. The impedance inverter contributes
approximately 35 dB to transmit isolation. A second shunt switch, made up of CR5922, L5922, and
C5924, provides additional isolation. C5926 and C5927 block DC.
During RX, CR5920 has an OFF capacitance of approximately 1 pF, which is tuned out by L5904.
CR5921 and CR5922, incorporated in the RX match, have similar OFF capacitances.
Harmonic Filter
The 40-Watt harmonic filter is a 7-pole, low-pass filter, consisting of screened plate capacitors and
discrete inductors (1,5924, L5925, and L5926) on a 35-mil alumina substrate. The filter's ground
plane is attached to the PA printed circuit board with solder, while input and output connections are
made via MP5901 and MP5902. The filter's primary function is to attenuate harmonic spurs
generated by the transmitter. It also adds low-pass selectivity for the receiver. L5910, grounded
through MP5903, protects the PA from static discharge.
NOTE: When removing any of the discrete coils, take care to avoid leaching the plate capacitor
metallization. Removal of the entire hybrid is best accomplished by heating the hybrid/PC
board assembly with a heat gun or heat blower until the solder joint reflows.
6881076C25-D
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October 28, 2002