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

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Final Stage
The final device is a 12- to 75-Watt device and is driven by the driver through a low pass matching
circuit that consists of C3850 through C3854 and associated transmission lines. Base network,
L3852, L3853, and R3851, 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. R3851 helps lower the network's Q at
low frequencies. The collector DC network consists of L3875, L3876, R3876, C3880, C3885, C3881,
C3882, and CR3875. This network provides the A+ voltage to the final stage while blocking RF from
getting up the DC line. L3875 and L3876 provide the DC path and block RF. R3850 resistively loads
down the final stage's collector at low frequencies and prevents 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, C3876, C3877, C3878, C3879,
L3877, and associated transmission lines) to the rest of the output network (directional coupler,
antennal switch, and harmonic filter).

3.7.1.3.2 Antenna Switch and Harmonic Filter

Antenna Switch
The antenna switches 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 a
low impedance on CR3921's anode and a 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 55dB from 136-174MHz. The impedance inverter contributes
approximately 35dB to transmit isolation. A second shunt switch made up of CR3922, L3921 and
C3921, provide additional isolation. Capacitors C3922 and C3923 block DC.
During RX, CR3920 has an OFF capacitance of approximately 1 pF. CR3921 and CR3922
incorporated in the RX match have a similar OFF capacitance.
Harmonic Filter
The 50-Watt harmonic filter is a 7-pole, low-pass filter, consisting of high Q chip capacitors (C3911
thru C3914) and discrete inductors (L3911 thru 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.
September 5, 2008
Theory of Operation: Power Amplifiers
6881076C25-E