Theory Of Operation; Functional Description; Atu Function; Atu Operation - Motorola F2265A Owner's Manual

3.1

FUNCTIONAL DESCRIPTION

3.1.1 ATU FUNCTION

The ATU allows for an effective energy transfer
between the radio and the antenna. This is basically
achieved by meeting the following requirements:
•
Minimizing the insertion loss of the matching
network.
•
Preventing
mismatch
appropriate impedances to the radio power
amplifier and the antenna.

3.1.2 ATU OPERATION

The DC power is supplied to the ATU via the RF cable.
After changing the frequency, the radio interrupts the
DC power supply via the RF cable for about 50
milliseconds. After the power returns, the ATU
performs the power-on tuning process. During the
power interruption, a special backup power is supplied
to the tuning data storage, to prevent data loss.
The Tuning Process: When the DC power returns (50
msec after changing the frequency), the Tuner Control
circuit (which initiates and controls the ATU operation)
turns on. The tuner control circuit sends a signal to the
radio to generate an output signal at a tune level
(3-6 W) and starts the tuning process.
First, the ATU verifies that the tune power at its input
(V-FORWARD) is within the predefined range
(3-6 W). Power above this range can damage the relays
during the Hot Switching, while power below the
predefined range can cause inaccurate tuning.
by presenting
losses

THEORY OF OPERATION

If the tune power is within the specified range, the ATU
captures a 20ms sample of the signal. The sample
signal frequency is divided by 128 and measured by a
counter. Based on the above data, the frequency
measurement resolution is as follows:
Resolution =
20
The ATU reads the tuning data corresponding to the
measured frequency from its internal memory (such
data exists if tuning was previously performed for this
frequency). The ATU sets the tuning network according
to that data and measures the resulting VSWR. If
VSWR<2 the tuning is completed. If it is higher, a new
tuning cycle is started.
The tuning process principle is based on crossing the
G=1 circle in the positive part of the Smith Chart using
a series inductance, and then moving towards
VSWR=1.5 using a parallel capacitance at the input.
When the ATU completes the tuning process, it stores
the tuning network data in memory in a location
corresponding to the current frequency.
After the tuning process is completed, the ATU shorts
its sensor terminals to ground to protect them from high
power, and its microprocessor enters an idle state to
prevent noise from the receive channel. The relays'
state is maintained by using latches on the relay control
lines.
Radio Switching out of TUNE Mode: A sensor in the
radio measures the reflected power at the radio output
every 100ms. If this power is stable and low enough
over a period of 300ms, the radio will switch out of
TUNE mode assuming that the tuner has completed the
tuning. If the tuning has not been completed after 3
seconds, the radio will automatically switch out of
TUNE mode.
CHAPTER 3
1
× =
128 6400 Hz
ms
3-1