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800MHz Tetra Repeater
User
Handbook
Handbook Nō.-55-151501HBK Issue No:-A
Date:-22/04/2005
Page:-
22 of 31
4.6 Wide Dynamic Range AGC (17-001109, det. & 17-001201, atten.)
4.6.1 Description
The equipment is fitted with a wide dynamic range 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 wide dynamic range Automatic Gain Control system consists of two units, a
detector/amplifier and an attenuator. The logarithmic 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.