User Manual

Fibre Optic Master and Remote Site
User Handbook
Document Number: 60-212701HBK - Issue No. 1 Page 33 of 50
4.2.4.11. Downlink AGC Components
AGC Detector /Amplifier (17-001117)
AGC Attenuator (17-001201)
The Remote Site Uplink/Downlink Tray (60-212802) Downlink path is fitted with an Automatic Gain
Control (AGC) system. The AGC system consists of two units, a detector/amplifier (part No. 17-
001117) and an attenuator (part No. 17-001201). 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 before 1st stage 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 amplifier 1dB compression point. Adjustment of this AGC
threshold level is possible to reduce the maximum power; 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 and signal distortion 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.
The AGC onset level is adjusted by the choice of sampler resistor R1 and by the setting of
potentiometer VR1.
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 AGC detector unit.
Technical Specifications
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