Motorola M90AMNOKV5-K, Service Manual, Page: 40 / 181

125W. Accurate sensors and the radio software keep the output power within the nominal output
power
± 7.5%.
The transmitter includes thermal protections. If, for any reason, the transmitter internal temperature
exceeds the maximum permitted temperature, the output power is automatically reduced to avoid
any fault due to excessive heat.
Figure 5-1. Signal Flow through the Transmitter (LORD Board)
Mismatch protection is also included. If VSWR (Voltage Standing Wave Ratio) is worse than 2:1, the
output power is proportionally reduced to avoid damaging the transmitter.
Seven different multi-section elliptical low-pass filters are inserted between the transmitter's final
output stage and the antenna to reject spurious and harmonic frequencies. The seven filters cover
seven frequency ranges and are automatically switched by PIN diode switches into the circuit
according to the channel selection. The frequency range of these filters are: 1.6 to 2.43 MHz, 2.43 to
3.7 MHz, 3.7 to 5.6 MHz, 5.6 to 8.5 MHz, 8.5 to 13 MHz, 13 to 19.8 MHz and 19.8 to 30 MHz.
The radio has a closed loop automatic level control (ALC) which keeps the output power within the
limits of 125
± 10 Watts.
The automatic gain control loop begins at the output of the harmonic filter near the antenna output
where the output power is sampled (see T6 and its peripheral components in High Power board
schematics). A voltage proportional to the output power is fed back to the power amplifier unit and
enters a comparison amplifier U6 which compares the sample of the output level with the level
determined by the power adjustment word stored in the radio memory and translated to an analog
level by D/A U1104 on the LORD board. For every frequency range, the radio stores the appropriate
power level word. The output of the comparison amplifier is fed to RF attenuator composed of CR54,
CR56. Thus attenuator regulates the input power to the RF amplifier, keeping the output level
constant.
5-2 Cat. No. 6886859J01