User's Manual

VHF Headstation & In-Line Amplifiers
User Handbook
Handbook Nō.-50-127201FCC Issue No:-A
Date:-14/06/2005
Page:-
30 of 45
4.8 Automatic Gain Control (17-001101, det. & 17-001201, atten.)
4.8.1 Description
The equipment is fitted with an Automatic Gain Control (AGC) system. This is generally
fitted in the Uplink path (not usually needed in the downlink path, as the signal here is at an
almost constant level), to avoid overloading the amplifiers (with the associated performance
degradation) should a mobile be operated very close to the unit.
The AFL 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 between the 1st and 2nd stages of
amplification.
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 50Ω 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.
The unit contains a 12V DC regulator in the detector module, which supplies stabilised
voltage to the DC amplifier and via an external cableform to the AGC attenuator.
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.
The AGC onset level is adjusted by the choice of sampler resistor R1 and by the setting of
potentiometer VR1.
The attenuator comprises a 50Ω 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
AGC detector unit.