Harmonic Filter; Antenna Switch - Motorola APX 5000 Service Manual
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3-14
3.1.3.4 Harmonic Filter
The harmonic filter is a high-power, low-loss, low-pass filter. Its purpose is to suppress transmitter
harmonics. The filter also improves receiver out-of-band rejection. The appropriate shield over the
filter must be in place to achieve the required stop band rejection.
VHF: The harmonic filter uses discrete components. The pass band is up to 190 MHz, and the stop
band is above 260 MHz.
UHF1: The harmonic filter applies discrete components as the circuit line up. The pass band is up to
470 MHz while the stop band is above 740 MHz.
UHF2: The harmonic filter applies discrete components as the circuit line up. The pass band is up to
520 MHz while the stop band is above 1000 MHz.
700–800 MHz: The harmonic filter uses both discrete components and transmission lines. The pass
band is up to 870 MHz, and the stop band is above 1500 MHz.
3.1.3.5 Antenna Switch
NOTE: Refer to
listing of schematics that will aid in the following discussion.
The antenna switch consists of a single pole triple-throw IC designated U1102. The IC is connected
to the output of the two harmonic filters and it is connected to the receiver. The output of the switch is
connected to the directional coupled via the inductor L1108. Control lines V1_V2 and V3 control the
routing of the signal paths. During Rx operation, V3 is set high and V1_V2 is low. During Tx
operation, V3 is set low and V1_V2 is high. This is the same case for VHF, UHF1, UHF2 and 7/800
radios.
There is a second IC designated U1111 at the output of the radio. This switch routes RF power to the
main antenna or to the accessory GCAII connector on the side of the radio. This switch is controlled
by control line DIG_CNTRL_RMT which connects to pin 13 of the IC. When this signal level is high,
the power is routed to the antenna and when it is low the power is routed to the accessory connector.
NOTE: This switch does not exist on VHF radios.
3.1.3.6 Reverse Power Protection
The radio, while in receive mode is constantly monitoring the input power from the antenna. This
power is sensed by the directional coupler and channeled into an RF detector U1106. The matching
network between the coupler and the detector consists of R1107, L1102, C1105 and C1107. Once
the input RF level exceeds a certain limit the detector trips a logic circuit then enables attenuation to
protect the RF front end. This serves as to protect the front end from large signal damage.
3.1.3.7 Transmitter Power Control
In TX mode, the transmitter Automatic Level Control (ALC) section enables the transmitter and
controls TX power in all modes. Power control is based on a unique dual control loop approach
which utilizes voltage control in one loop and current control in the other. The voltage control loop is
normally used in all transmit modes. The only time the current control loop controls TX power is
during the end of a TX slot in TDMA (Phase 2) mode in the event transmitter saturation is detected.
Several other functions included in the TX ALC section of the radio are RX/TX switching, thermal
cutback of power, current cutback of power, and reverse power detection with a means to disable the
receiver in the event of high reverse power at the antenna port.
Table 8-1, "List of Transceiver Schematics and Board Overlays," on page 8-1
Theory of Operation: Transceiver Board
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