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

3-78
Final Stage (35-Watt Only)
On the 15-Watt radio, the transmit RF signal from U9850, pin 5, is applied to the 50-ohm microstrip
directional coupler. On the 35-Watt radio, the transmit RF signal is applied to the emitter of the final
power amplifier Q9880 through the coupling capacitor C9856, the 50-ohm quarter-wave matching
transmission line, and the matching capacitors C9875 and C9876. The 100-ohm coupling line,
L9930, R9930, R9931, CR9930, and C9930 form an interstage power detector between U9850 and
Q9880 to limit the drive into Q9880 to about 17 Watts. L9875, the emitter choke, is also the emitter
DC return. The final power amplifier, Q9880, is a 45-Watt, 800 MHz, common-base NPN devise. The
Q9880 output match consists of C9877, C9878, a section of the 50-ohm microstrip line, C9879 and
the DC blocking capacitor, C9883. L9876 isolates the RF signal from A+. C9880 and C9884 are
signal frequency bypass capacitors. L9877 presents a high impedance at low RF frequencies;
therefore the collector of Q9880 is resistively loaded by R9876 at low frequencies where the gain is
much greater. C9881 and C9882 are low frequency bypass capacitors.

3.7.3.1.2 Antenna Switch and Harmonic Filter

Antenna Switch
35-Watt Power Amplifier:
The antenna switch's impedance inverter circuit, made up of C9922 and L9921, takes the place of a
quarter-wave microstrip line. During transmission, K9.4-V forward-biases CR9921, producing a low
impedance at its anode end, and a high impedance at the node of C9922 and L9921, to effectively
isolate the transmitted power from the receiver. C9921 couples the power to the harmonic filter and
on to the antenna.
The impedance inverter contributes approximately 30 dB to transmit isolation. Additional isolation is
obtained by the series switch made up of CR9922, L9923, and associated DC bias components.
During transmit, CR9922 is reverse-biased, thus creating a small series capacitor that is tuned out
by L9923. C9925 is a DC blocking capacitor. The high impedance of the series arm works against
the low impedance of the shunt arm (CR9921) to provide approximately 10 to 15 dB additional
isolation. Total TX to RX isolation is in excess of 45 dB from 851-870 MHz. The preselector provides
over 50 dB isolation from 806-824 MHz.
When receiving, CR9920 has an off capacitance of approximately 1 pF, which is tuned out by L9926.
CR9921, with similar off capacitance, is incorporated in the RX match. CR9922 is forward-biased
with an ON resistance of approximately 1 ohm. The signal passes CR9922 and through L9922, a
series inductor used to complete the RX match. Capacitor C9929 blocks DC.
L9910, at the node of the antenna and harmonic filter, protects the PA from static discharge.
15-Watt Power Amplifier:
The theory for the 15-Watt antenna switch is exactly the same as the 35-Watt except that some of
the components are labeled with different numbers. C9921, in the 15-Watt PA, is located after the
harmonic filter.
L9922, at the node of the antenna and capacitor C9921, protects the PA from static discharge.
Harmonic Filter
The 15- and 35-Watt harmonic filters are 7-pole, low-pass filters implemented with screened plate
capacitors and discrete inductors (L9911, L9912, and L9913) on a 35 mil (0.035") 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 "J"-straps MP9856 and MP9857. The filter's primary function is to
attenuate harmonic spurs generated by the transmitter and to provide additional low-pass selectivity
for the receiver.
October 28, 2002
Theory of Operation: Power Amplifiers
6881076C25-D