User's Manual

800MHz & VHF Cell Enhancers
Maintenance Handbook
H/book Number:-50-078001HBKM
Issue No:-
A
Date:-02/09/2004
Page:-
71 of 88
5.22 Wide Dynamic Range (Log.) 12V AGC (17-001109, det. & 17-001201, atten.)
5.22.1 Description
The equipment is fitted with a wide dynamic range (logarithmic detector) 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
input amplifiers 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 50 transmission line with a logarithmic amplifier 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 logarithmically
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.
The AGC onset level is adjusted by the choice of sampler resistor R1 and by the setting of
potentiometer VR1, (factory set @ time of system test) do not adjust unless able to monitor
subsequent RF levels.
The attenuator comprises a 50 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. Note that the log detector module is used with an associated controller
board, in the duplex shelves for detecting the presence of signals to be muted/amplified.