PA250 Power Amplifier Technical Manual © sbs. Release 31 May 2001 Page 10
7Installation
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Installation should be straight forward. It is strongly recommended that the unit
is mounted on support rails within the rack. The case will get warm in normal
operation. A space of 1U (44mm) above and below the unit should provide
adequate ventilation to the case. To cool the PA, two fans take in air at the
front panel and blow the heated exhaust air out the back directly into the rack.
If this air flow is restricted in any way the PA250 will have to operate at a
higher internal temperature which may have a detrimental effect on its service
life. In extreme cases the thermal cutback may operate to keep the internal unit
temperature at a sustainable level by reducing the RF output power.
All cables, particularly RF should be of a high quality low loss type.
Terminations should be secure and follow the instructions supplied with the
individual connector. Control cables should be screened.
Due consideration should be given to the safety of the installation, in particular
to the AC power wiring. Attention is drawn to the section of this manual
relating to electrical safety. Where necessary, the antenna down lead should be
grounded to a safety earth via an adequately rated grounding plate.
A drive of minimum 5W is required at the RF input. Maximum permissible input
is 8W. No internal adjustment of the unit is required during installation. A green
Power LED indicates that the unit has an electricity supply input and that the
switch mode regulator, responsible for supplying power to the RF amplifier, is
operating correctly. Output power is adjusted by the use of a small flat bladed
screwdriver or trimming tool. Turn the Power O/P Adjust control with the meter
set to read P. FWD on the selector. Set the required power level. If the P.REF
LED turns red the reflected power cutback has come into operation so the power
adjust may have limited effect. Turn off the unit and correct the fault with the
antenna or down leads.
Once the required power is being displayed on the meter, push in the small Push
To Display switch and the meter will indicate the forward power alarm level. If
the power should drop to this level during operation, a forward power fault will
be flagged by the front panel LED turning red and at the control/monitor socket.
While holding in the push switch, turn the P. FWD Alarm Level Adjust control
until the meter shows the level at which this alarm is required to operate.
Similarly the reflected power alarm level can be set by turning the P. REF Alarm
Level Adjust with the push switch held in and the meter set to display P. REF on
the selector. If the reflected power should rise to this level a fault will be
flagged by the P. REF LED turning orange and through a signal output from the
control/monitor socket. This alarm is designed to give an early indication of an
antenna fault. The reflected power cut back will operate above 25W reflected.
The P. REF LED will turn red in this condition and a separate failure signal will be
output. Adjustment of the P.REF Alarm Level will not effect the cutback circuit.
An indication of the efficiency of the amplifier can be gained from the PA.V and
PA.A ranges on the meter.
