Crescend C5 Series Manuale - Pagina 13

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PRINCIPLES OF OPERATION

4.1.

INTRODUCTION

This section contains principles of operation of the C5 Series of RF Power Amplifiers.

4.2.

RF INPUT SIGNAL

The amplifiers are set at the factory to work over different input power ranges. The

available range of inputs for different frequency ranges may be found in Table 1-2.

4.3.

RF OUTPUT LOAD

The load impedance should be as good as possible (VSWR <1.2:1) in the working band for

good power transfer to the load and maximum amplifier efficiency.

4.4.

FUNCTIONAL DESCRIPTION

The amplifier is a single channel CW amplifier that operates in various frequency bands

from 30 MHz to 942 MHz. The amplifier specifications are listed in Tables 1-1 and 1-2.

4.4.1.

MAIN RF PATH

The typical main RF path consists of an input attenuator, driver, splitter, final stage(s),

combiner, rf detectors and harmonic filter. The circuitry becomes active upon the

sensing of rf input power. This occurs when rf input reaches approximately 3 dB below

the low end of the input power range. This keeps standby current at a minimum (< 20

mA) when rf input is not present (or is below the turn on level).

4.4.2.

LOOP CONTROL

The loop control circuitry includes circuitry to works to maintain a constant output power

versus variations in DC voltage, rf input power, frequency and load. A coupled detector

at the output monitors the final RF output level and generates a DC voltage proportional

to the RF output level. This voltage is fed back to the controller. The circuitry compares

this voltage to a reference voltage and varies the gain of the amplifier to maintain a

constant output.

4.4.3.

VSWR PROTECTION

Reflected power is detected at the output of the amplifier and compared to forward

power. When reflected power reaches an equivalent VSWR of approximately 2.5:1, the

amplifier output is reduced to protect the finals and the front panel HIGH VSWR LED

activates. The higher the load VSWR, the more the output power is reduced.

4.4.4.

THERMAL PROTECTION

The heat sink temperature is monitored with a thermostat. When the heat sink

temperature reaches an unsafe level, the output power of the amplifier is reduced by

approximately 50%. This keeps the channel on the air while still providing some short

term protection until the customer can address the underlying cooling issue.

4.5.

AMPLIFIER COOLING

The amplifier is cooled with forced air over a large heat sink. The fan, located on the rear of

the amplifier and switched on and off with rf input power (3 seconds delay from detection of

RF and 6 seconds hang after RF is removed), draws ambient air in from the back of the

amplifier and forces it over the heat sink. The rear baffle forces the air to run over the fins

and exit on each side of the amplifier. It is important to keep a minimum clearance of 3

inches behind the fan and allow the coolest air possible to circulate through the fan.

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