8525B/8528 Technical Service Manual Technical Description 4-39
Publication No: 15-02036 Issue 6
When the Tune pad on the transceiver control panel is pressed the
microprocessor causes some RF, at the required transmission frequency, to
be applied to the antenna. At the same time the microprocessor prepares the
IO device, IC3, to respond to the VSWR measurement. Voltages
proportional to the forward and reflected power at the antenna are applied
from the PA and Filter Board to J3 of the Antenna Driver PCB.
The forward power is compared in IC4a with a preset voltage to establish
whether the RF is adequate for the VSWR measurement to be made. If the
voltage proportional to the forward power exceeds the preset voltage the
output of IC4a goes to the low logic level to indicate
. The
result of the comparison is passed, via IC3, to the microprocessor.
The microprocessor has recorded the last used frequency and is given the
new frequency and it uses this information to drive the antenna tuning motor
in the appropriate direction to improve the VSWR. It does this by sending
data to IC3 to switch on one of the transistors, V4, V6, V8 or V10. These
switch on the corresponding Darlington transistors V5, V7, V9 or V11. The
antenna motor is a 4-phase, uni-polar stepper motor and is driven in the
wave mode (i.e. one phase at a time). +12V is applied to one end of all four
windings of the motor and, when switched on, each Darlington transistor
grounds the other end of one winding.
The forward and reflected power are compared in IC4b, IC5a and IC5b. One
input to each of these comparators is from a potential divider on the forward
power voltage, while the other input to each is the reflected power voltage.
The potential divider values are such that the output of IC4b will go low
when the VSWR is less than 3:1, IC5a will go low when it is less than 2:1
and IC5b will go low when it is less than 1.5:1.
The outputs of the comparators are sent to the microprocessor via IC3 and,
as the antenna tuning changes, the microprocessor detects any improvement
in the VSWR. As soon as an improvement is detected, provided the RF has
been found to be adequate, the microprocessor stops the tuning and a
positive voltage is applied to the Load input to the board (J1 pin 7). This
switches on V2 so that the Load output (P1 pin 4) goes low.
The Load output is applied to a circuit in the antenna which controls a 2-coil
latching type relay (K2 on the Auto Antenna PCB). The low level on the
Load input to the antenna causes the load presented by the antenna to be 22
ohms, while a high level on the input causes the load to be 35 ohms. The
microprocessor assesses whether the change of load has improved the
VSWR.
The microprocessor continues the process of changing the antenna tuning
and the load until either the VSWR is found to be less than 1.5:1 or no
further improvement can be made. Provided the final VSWR is less than 3:1
a Pass indication is given. If it is greater than 3:1, or the RF is inadequate, a
Fail indication is given.
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