User Manual

STTRS DOCUMENTATION
Document Number 80-330501HBKM – Issue A - Draft Page 405 of 500
21.3.2.11. AGC System
AGC Detector Unit (17-001109)
AGC Attenuator Unit (17-001201)
EQUIPMENT 700MHz Uplink Amplifier/ Duplexer (50-132104) is fitted with a wide dynamic range
Automatic Gain Control (AGC) system. This is fitted in the Uplink path 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.
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 at the time of system test) do not adjust unless able to monitor
subsequent RF levels. 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.
Wide Dynamic Range AGC Specification
PARAMETER SPECIFICATION
Frequency Range up to 1000MHz
Attenuation Range 3 to 30dB
Attenuation Steps continuously variable
VSWR better than 1.2:1
RF Connectors SMA female
attenuator 1W Power
Handling
detector/amp >30W (or as required)
operation -10°C to +60°C Temperature
Range
storage -20°C to +70°C
attenuator pcb 50 x 42 x 21mm
Size
detector/amp pcb 54 x 42 x 21mm
attenuator 90gm
Weight
detector/amp 100gm