Codan 8525B Series Technical & Service Manual

8525B/8528 Technical Service Manual Technical Description 4-27
Publication No: 15-02036 Issue 6
4.9.8 ALC Control
ALC control is provided from the following sources:
• Forward power
• Reflected power
• Battery voltage
• Low Power control (8528S only)
• Heatsink over-temperature
• Output stage collector swing.
The ALC control inputs are applied to IC2a which provides a fast attack,
slow decay control voltage to the base of V7. In the absence of any ALC
input, the negative input of IC2a is held at 3.9V by voltage divider R16, R17
and R19 and the voltage on C59 and V7 base is held at a similar level. V7 is
cut off and V9 then operates at maximum gain.
The R16, R17 and R19 voltage divider chain is used to control ALC action
for heatsink temperature. When the heatsink temperature exceeds 80°C,
PTC resistor R16 rapidly increases in value, reducing the ALC threshold
level and reducing the power output such that the heatsink does not rise
above 90°C.
The output voltage from the RF bridge T1, L1, R2, R3, C1 and C2 is
rectified by D1 as forward power and by D2 as reflected power. The forward
power output is divided by R9, R10 and R11 and ORed by D8 with the
output of the reflected power detector into the positive input of the ALC
level comparator IC2a. The power output is set by R10.
The output voltage of the forward power detector is averaged by R68, C92
and ORed into the ALC comparator. The value of R68 is chosen so that a
average forward power of 60W will provide ALC control.
When the DC supply drops below 12.5V, V5 starts to conduct, increasing
the voltage across R11. This effectively raises the forward power detector
level, resulting in increased ALC, reducing the output power.
A low on the LO PWR input (P3 pins 1 and 2) will cause V20 to conduct
also raising the current in R11 and reducing the power output.
The peak positive swings of the PA output transistors V18 and V19 are
rectified by D13 and D14, divided by R61 and R62 and buffered by V22
before being ORed into the ALC comparator. Thus excessive swings of the
PA collector voltage reduces the output power to prevent damage to the PA
transistors. The output from V22 is also applied to one input of IC2b where
it is compared with the forward power to the antenna. If the forward power
is very low (or absent) indicating a fault in the antenna circuit, the output
power is reduced to prevent damage to the PA transistors.

Codan 8525B Series Specifications

General

Brand	Codan
Model	8525B Series
Category	Transceiver
Language	English