A two-transistor circuit controls the bias of the driver and final amplifier transistors; as heatsink
temperature increases, the circuit reduces the bias correspondingly.
5-5.2.2 Harmonic Filter
The harmonic filter is used to attenuate the unwanted spurious and harmonics of the transmitted and
received signals. During transmission, the Tx/Rx switch is in transmit state, transferring the RF output
from the PA to the antenna via the harmonic filter. In the receive mode, the Tx/Rx switch is in the
receive state, transferring the received signal from the antenna to the LORD board via the harmonic
filter.
The harmonic filter contains:
• Seven selectable low pass filters.
• Seven electronic switches. Only one switch is on at any time, while the others remain off.
• Tx/Rx electronic switch (includes CR33, CR34, CR35 and CR36).
• Forward/reflected power detector (includes T7, CR4 and CR5).
The filter ranges and switches are listed in Table 5-2.
Table 5-2. Harmonic Filter Bands/Ranges and Controlling Switches
Filter Range (MHz) Switch PIN Diodes
1 1.6 to 2.43 CR40, CR47
2 2.43 to 3.7 CR37, CR44
3 3.7 to 5.6 CR39, CR46
4 5.6 to 8.5 CR42, CR49
5 8.5 to 13 CR41, CR59, CR48, CR59
6 13 to 19.8 CR38, CR45
7 19.8 to 30 CR37, CR44
The harmonic filter contains seven selectable low pass filters, each covering a section of the 1.6 to
30 MHz range. Seven control lines are used to select an appropriate filter according to the operating
frequency. When a control line is pulled down, the corresponding PIN diode switch is turned on and
inserts the corresponding filter in the signal path.
A forward/reflected power detector is inserted between the filters and the antenna. During
transmission it generates two DC signals: one proportional to the forward RF power delivered to the
antenna, and the other proportional to the RF power reflected from the antenna. The signals are sent
to both the exciter and the Control board for controlling the level of the power output as a function of
the following parameters:
• VSWR value. If the VSWR increases above 2:1, the output power is gradually decreased, to
protect the radio from high reflected power.
• Harmonic filter insertion losses. If the harmonic filter insertion loss is too high, the output power
level is decreased.
Cat. No. 6886859J01 5-15
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