User Manual

Axell Wireless Limited
Technical Literature
L.A. M.T.A. Remote Sites
Document Number 80-301401HBKM Issue No. 1 Date 13/06/2008 Page 104 of 148
4.9.5.4. Automatic Gain Control
17-001101 AGC Detector Assembly
17-001201 AGC Attenuator Assembly
VHF Amplifier Shelf 80-301409 is fitted with two Automatic Gain Control (AGC) systems, one for the
Dwonlink amd one for the Uplink
The Automatic Gain Control system consists of two units, a detector/amplifier and an attenuator. The
detector/amplifier unit is inserted in the RF path on the output of the power amplifier, and the
attenuator is situated in the RF path before the amplification stage(s)
The attenuator comprises a 50ȍ P.I.N diode, voltage-variable attenuator with a range of 3 to 30dB.
The attenuation is controlled by a DC voltage which is derived from the associated detector controller
board.
Normally the attenuator is at minimum attenuation. The detector/amplifier unit monitors the RF level
being delivered by the power amplifier, and when a certain threshold is reached it begins to increase
the value of the attenuator to limit the RF output to the (factory set) threshold. Therefore overloading
of the power amplifier is avoided.
The factory set threshold is 1dB below the Enhancer 1dB compression point. Some adjustment of this
AGC threshold level is possible, a 10dB range is mostly achieved. It is not recommended under any
circumstances to adjust the AGC threshold to a level greater than the 1dB compression point as
system degradation will occur.
The detector comprises of a 50ȍ transmission line with a resistive tap which samples a small portion
of the mainline power. The sampled signal is amplified and fed to a conventional half wave diode
rectifier, the output of which is a DC voltage proportional to the RF input signal.
This DC voltage is passed via an inverting DC amplifier with integrating characteristics, to the output,
which drives the attenuation control line of the corresponding AGC attenuator. This unit is fitted at
some earlier point in the RF circuit.
For small signals, below AGC onset, the output control line will be close to 12V and the AGC
attenuator will have minimum attenuation. As the signal level increases the control line voltage will
fall, increasing the attenuator value and keeping the system output level at a constant value.