User Manual

Axell Wireless Limited
Technical Literature
L.A. M.T.A. Remote Sites
Document Number 80-301401HBKM Issue No. 1 Date 13/06/2008 Page 53 of 148
3.5.5.7. DC/DC Converter, 24V in, 12V 8A out (13-003011)
The DC/DC converter fitted is a high power PCB unit with an 8 amp at 12V output capability. The
circuit is basically an O.E.M semiconductor regulator (one side of which has a heatsink mounting
plate, usually bolted to the casing of the unit) and smoothing components built onto a printed circuit
board with screw block terminations.
In event of failure this unit should not be repaired, only replaced.
13-003011 Specification
PARAMETER SPECIFICATION
Input Voltage range 18-28V DC
Output voltage 12V±0.5V
Max. current load 8.0Amps
operation -10°C to +60°C Temperature
range
storage -20°C to +70°C
Size(PCB) 190 x 63mm
Weight (Loaded PCB) 291g
3.5.5.8. Automatic Gain Control
17-001101 AGC Detector Assembly
17-001105 AGC Detector Assembly (Logarithmic)
17-001201 AGC Attenuator Assembly
UHF BDA UNIT(80-301404) is fitted with two differing types of Automatic Gain Control (AGC) system,
one linear, and one logarithmic. The Downlink UHF Midband and Highband paths are each fitted with
logarithmic detector (17-001105), and attenuator (17-001201) the Uplink UHF Midband and Highband
paths are each fitted with linear detector (17-001101) and attenuator (17-001201) The 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 before the amplification stage(s)
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 detector controller
board. 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 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.