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

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The final stage output network serves the dual purpose of impedance matching and power
combining of the two final devices. R3872 and R3873 help balance the load impedances presented
to the collectors of the final devices. Filtered A+ is routed to the final amplifier devices via the current
sense resistor R3841, the ferrite bead L3881, and the coil L3880. The final stage output network
terminates at C3889, which is the input to the antenna switch. The circuit impedance is 50 ohms at
this point.

3.7.1.1.2 Antenna Switch and Harmonic Filter

Antenna Switch
The antenna switch utilizes PIN diodes to form a low loss, high isolation RF relay. During transmit,
PIN diodes CR3901, CR3902, and CR3903 are forward biased during transmit via the K9.4 supply
and resistors R3900, R3901, R3902, and R3903. In this state, a low loss path exists from the final
amplifier through PIN diode CR3901 and into the harmonic filter. PIN diodes CR3902, and CR3903
effectively shunt the path to the receiver front-end which protects the preamp or mixer device from
excessive RF levels. A properly functioning switch will pass less than 10 mW of transmit power to the
receiver front-end.
During receive, all three PIN diodes remain unbiased. This opens a low loss path from the harmonic
filter to the receiver.
Harmonic Filter
The harmonic filter is a 7-pole low-pass filter consisting of screened plate capacitors and air-wound
coils on a 0.035 inch thick ceramic substrate. The filter's primary function is to.attenuate harmonic
energy generated by the amplifier stages. The filter also adds some selectivity for the receiver.
September 5, 2008
Theory of Operation: Power Amplifiers
6881076C25-E