It is known practice to process radiocommunication signals according to a first mode amplifying these signals with a constant gain or according to a second mode amplifying these signals up to a constant output power of the amplifier irrespective of the input power of these signals.
Certain items of telecommunication equipment, such as satellites, alternate these two amplification modes. Therefore, radiocommunication signals received by certain satellites may be amplified according to the first constant gain mode, this method being called hereinafter FGM for Fixed Gain Mode, or according to the second constant output power mode, this method being called hereinafter ALC for Automatic Level Control.
US 2006/0006940 relates to an amplification device having a plurality of operating modes. DE 196 23 930 describes a system of amplifiers, each amplifier having an adjustable attenuator receiving control signals. JP 57030429 relates to an automatic restoration system for an automatic level regulator. US 2003/055590 describes an amplifier operating in an ALC mode or in an FGM.
FIG. 1a represents schematically an amplification device 100 making it possible to alternate the two modes, FGM or ALC, in order to amplify an input power Pin of a radiocommunication signal up to an output power Pout.
For this purpose, this device 100 comprises a string 101 of amplifiers 112 whose gain is partially controlled by a gain controller 113, the operation of this controller 113 being determined by a setpoint which varies according to the amplification mode of the device 100. In this example, this setpoint is a voltage Vatt.
A control circuit 104 delivers such a setpoint, or voltage, Vatt of fixed value Vfgm transmitted by a source REF FGM, in order to obtain a constant gain in the string 101 of amplifiers 102, or of variable value Valc, in order to obtain a constant power Pout at the output of the device 100.
In this latter case, the variable setpoint Valc is obtained at the output of a regulator 106 forming a control loop in order to regulate this voltage as a function of the difference between the desired output power Pout and a setpoint REF ALC of the regulator 106 transmitted by a device not shown. To do this, this regulator 106 may comprise an operational amplifier 141—and other electronic components not shown—while the measurement of the output power Pout is taken through a detector 108 correcting the effects of temperature on the measurement.
Such a device 100 has considerable variations in the gain of amplification and output power Pout during the transition from an FGM mode to an ALC mode as illustrated in FIG. 1b which represents the variation (ordinate axis 120, in dBm) in the output power Pout of a device 100 as a function of the time (abscissa axis 122, in ms) wherein there is a change 124 in the amplification mode for the device 100.
Actually, when the device 100 is in FGM mode, the regulator 106 supplies a maximum voltage Valcmax in order to bring the output power Pout to a constant power such that, during the switchover to ALC mode via a switchover of a switch 105, the control voltage Vatt reaches this maximum voltage Valcmax before tending toward a value specific to an operating point of the circuit.
Such a power variation is problematical because it risks causing signal losses, notably between a satellite and a ground station. As a result, a circuit 204 (FIG. 2a) controlling a string 201 of amplifiers 212 may comprise means so that the voltage Valc at the output of a regulator 206 is fixed at a value that is fixed and close to a predetermined setpoint value Vfgm and suitable for an FGM mode.
Considering a regulator 206 comprising an operational amplifier 241, this result is achieved by incorporating a set of resistors 244 and 246 fixing the output voltage of the regulator 206 at a precise value when the device 200 is operating in FGM mode, a capacitor 240 and a resistor 242 forming an integrator element with the amplifier 241 in order to supply a variable setpoint based on the setpoint REF ALC in ALC mode.
Such a configuration makes it possible to obtain, as shown in FIG. 2b, a variation of amplified power Pout that is relatively limited during a change 224 from an FGM mode to an ALC mode (axis of the ordinates 220, in dBm) as a function of the time (axis of the abscissae 222, in ms).
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