Patent Description:
Power amplifiers are used in communications satellites to amplify RF signals. Such power amplifiers are known to exhibit nonlinearity, whereby the gain and phase of the amplified output RF signal varies nonlinearly with the input signal power. To compensate for this, gain and phase distortions can be applied to the input signal before it reaches the amplifier stage, so as to maintain a constant gain and phase of the output signal.

<FIG> illustrates an open-loop control system used in a conventional communications satellite to control the gain and phase of the input signal. The system comprises a power amplifier <NUM>, a variable attenuator <NUM>, a phase shifter <NUM>, an input coupler <NUM>, a detector <NUM>, a processor <NUM> and a memory <NUM>. An initial calibration procedure is carried out in which the response of the amplifier <NUM> is characterised over a range of different signal powers. A lookup table (LUT) is created to record suitable gain and phase predistortions to be applied to the input signal for any given power level. The LUT is stored in the memory <NUM>.

During operation, the detector <NUM> measures the power level of a coupled input signal received from the input coupler <NUM>. The processor <NUM> then determines the power of the input signal RFIN based on a known coupling factor of the input coupler <NUM>. The processor <NUM> searches the LUT to determine how the gain and phase of the input signal should be adjusted, and controls the variable attenuator <NUM> and phase shifter <NUM> to apply the appropriate gain and phase predistortions.

However, a drawback of this approach is the lengthy calibration procedure, which can take up to <NUM> hrs.

<CIT> discloses an RF power amplifier controller circuit including a calibrated phase control loop. <CIT> discloses a distortion compensation circuit incorporated in a communications apparatus that is used for satellite communications, ground microwave communications, mobile telecommunications and the like. <CIT> discloses a method of adjusting the amplitude and the phase of the RF input signal to produce an adjusted RF input signal. <CIT> discloses a polar envelope correction mechanism for enhancing the linearity of an RF/microwave power amplifier. <CIT> discloses a signal processing system which separately processes the amplitude component of the input signal and the component of frequency or phase or both frequency and phase.

According to the present invention, there is provided an apparatus for controlling a gain and phase of an input signal input to a power amplifier, according to claim <NUM>.

The apparatus may further comprise an input coupler configured to receive the input signal and output the third signal, the input coupler having a first coupling factor, and an output coupler configured to receive the amplified signal and output the fourth signal, the output coupler having a second coupling factor, wherein the first and second coupling factors are selected such that when the amplified signal has the predetermined gain, the third signal and the fourth signal have substantially the same power level.

The amplified signal may be clipped by the power amplifier, and the apparatus may further comprise a limiter configured to clip the input signal in correspondence with the clipping of the amplified signal by the power amplifier, such that the third signal received by the first detector and the fourth signal received by the second detector are clipped by substantially the same amount.

The phase control loop may comprise delaying means for delaying the first signal, error signal generating means for generating the error signal based on the second signal and the delayed first signal, a detector for measuring a power of the error signal and processing means configured to control the phase control means based on the measured power of the error signal, wherein the delaying means are configured such that the electrical length of a first path to the error signal generating means via the power amplifier is substantially the same as the electrical length of a second path to the error signal generating means via the delaying means.

The processing means may be configured to control the phase control means in order to minimise the measured power of the error signal.

The current gain and phase of the amplified signal may be dependent on an operational history of the power amplifier.

The power amplifier may be a Gallium Nitride GaN solid-state power amplifier.

A satellite may comprise a power amplifier and the apparatus, the apparatus being configured to control the gain and phase of the input signal input to the power amplifier.

According to the present invention, there is also provided a method for controlling a gain and phase of an input signal input to a power amplifier, according to claim <NUM>.

Referring now to <FIG>, a system for controlling the gain and phase of an input signal of a power amplifier is illustrated, according to an embodiment of the present invention. The system comprises a power amplifier <NUM>, a gain control module <NUM>, a phase control module <NUM>, a gain control loop <NUM> for controlling the gain control module <NUM>, and a phase control loop <NUM> for controlling the phase control module <NUM>. The gain control module <NUM> may, for example, be a variable attenuator such as the one shown in <FIG>, and the phase control module <NUM> may, for example, be a phase shifter such as the one shown in <FIG>. The gain control module <NUM> and phase control module <NUM> can be controlled to change the gain and phase, respectively, of the input RF signal (RFIN) before it is input to the power amplifier <NUM>. Although in <FIG> the RFIN signal is input to the phase control module <NUM> before the gain control module <NUM>, in other embodiments the order of the phase control and gain control modules may be reversed.

The gain control and phase control loops <NUM>, <NUM> each receive a signal derived from the RFIN signal and a signal derived from the output RF signal (RFOUT). Each control loop <NUM>, <NUM> is therefore able to monitor both the RFIN and RFOUT signals.

The gain control loop <NUM> and phase control loop <NUM> are configured to control the gain control module <NUM> and the phase control module <NUM> respectively, to apply gain and phase predistortions to the RFIN signal in order to maintain linearity of the power amplifier <NUM>.

Because the control loops <NUM>, <NUM> are configured to monitor the output signal RFOUT, the gain and phase of the input signal RFIN can be adjusted based on a current value of the output signal, i.e. based on the current performance of the power amplifier. Therefore in the present embodiment, it is not necessary to make assumptions about the behaviour of the amplifier. As such, a calibration procedure is not required for the control system shown in <FIG>.

Also, because the control system of <FIG> monitors the RFOUT signal produced by the power amplifier <NUM>, the control system can accurately control power amplifiers that exhibit hysteresis-type memory effects, in which the current performance of the amplifier is dependent on its operational history. That is, the current gain and phase of the amplified signal may be dependent on the operational history of the power amplifier. The operational history can include recent operating parameters of the power amplifier, such as input signal power and a gain and/or phase applied to the input signal, as well as environmental parameters to which the amplifier has recently been exposed. For example, for any given power level of the RFIN signal or temperature of the amplifier, the amplifier may amplify the signal differently depending on whether the amplifier was recently used to amplify a high-power signal, or to amplify a low-power signal. One such type of power amplifier that exhibits significant memory effects is a Gallium Nitride (GaN) amplifier. Therefore embodiments of the present invention may be particularly suitable for controlling GaN amplifiers. In contrast, the conventional open-loop control system of <FIG> cannot be used with amplifiers that exhibit a hysteresis-type memory effect.

Furthermore, in comparison to the conventional control system of <FIG>, the control system of <FIG> uses separate control loops <NUM>, <NUM> to control the gain and phase of the input signal. Therefore the processing performed in each control loop can be simplified, since each loop controls only one variable, i.e. the gain or the phase. Accordingly, in <FIG> the control system can have a faster response time than the conventional control system of <FIG>.

Referring now to <FIG>, gain and phase control loops for controlling a power amplifier are schematically illustrated in detail, according to an embodiment of the present invention. Although one such structure is shown in <FIG>, the invention is not limited to this arrangement. In general, the gain control loop and phase control loop may have any structure that provides the required functionality.

As shown in <FIG>, the control system comprises a gain control module <NUM> and a phase control module <NUM>, coupled to the input of a power amplifier <NUM>. The system further comprises a first input coupler <NUM> and second input coupler <NUM>. The first input coupler <NUM> is configured to direct a first coupled input signal, which is a coupled portion of the RFIN signal, to the gain control loop <NUM>. The second input coupler <NUM> is configured to direct a second coupled input signal, which is a coupled portion of the RFIN signal, to the phase control loop <NUM>. The system further comprises a first output coupler <NUM> and a second output coupler <NUM>. The first output coupler <NUM> is configured to direct a first coupled output signal, which is a coupled portion of the RFOUT signal, to the gain control loop <NUM>. The second output coupler <NUM> is configured to direct a second coupled output signal, which is a coupled portion of the RFOUT signal, to the phase control loop <NUM>.

The first and second input couplers <NUM>, <NUM> may be formed as a single unit or as separate units, and the first and second output couplers <NUM>, <NUM> may be formed as a single unit or as separate units. The first and second input couplers <NUM>, <NUM> may be configured to have the same coupling factor such that the first and second coupled input signals have the same power level. Alternatively, the first and second input couplers <NUM>, <NUM> may be configured to have different coupling factors such that the first and second coupled input signals have different power levels. Similarly, the first and second output couplers <NUM>, <NUM> may be configured to have the same coupling factor such that the first and second coupled output signals have the same power level, or may be configured to have different coupling factors such that the first and second coupled output signals have different power levels. The coupling factor of each of the first and second input couplers <NUM>, <NUM> and first and second output couplers <NUM>, <NUM> may be chosen to ensure that during normal operation of the power amplifier <NUM> and control system, the first and second coupled input and output signals have power levels that can be detected by the gain and phase control loops <NUM>, <NUM>.

Although in <FIG> separate couplers <NUM>, <NUM> are used to generate the first coupled input signal and second coupled input signal, in other embodiments a single input coupler may be provided. In such embodiments, means for splitting the coupled input signal into the first and second coupled input signals may be provided. For example, a rat-race coupler could be used to split a coupled input signal into the first coupled input signal to be sent to the gain control loop, and the second coupled input signal to be sent to the phase control loop. Similarly, a single output coupler connected to splitting means such as a rat-race coupler could be used to generate both the first and second output signals.

In the present embodiment, the gain control loop <NUM> comprises an input detector <NUM> arranged to receive the first coupled input signal from the first input coupler <NUM>. The input detector <NUM> is configured to measure the power level of the first coupled input signal, and send a signal representing the measured power to a first differential amplifier <NUM>. For example, the input detector <NUM> may be a root-mean-squared (RMS) detector configured to output a voltage that is representative of the RMS power of the first coupled input signal.

The gain control loop <NUM> further comprises an output detector <NUM> arranged to receive the first coupled output signal from the first output coupler <NUM>. The output detector <NUM> is configured to measure the power level of the first coupled output signal, and send a signal representing the measured power to a second differential amplifier <NUM>. Like the input detector <NUM>, the output detector <NUM> may be an RMS detector configured to output a voltage that is representative of the RMS power of the first coupled output signal.

In more detail, the input detector <NUM> includes two matched RMS detectors biased by the same DC bias. One of the detectors receives the RF first coupled input signal, and outputs the measured power level to one input of the first differential amplifier <NUM>. The other detector does not receive the first coupled input signal, and outputs a reference signal to the other input of the first differential amplifier <NUM>. The first differential amplifier <NUM> therefore outputs an amplified signal that is representative of the power level of the first coupled input signal. The output detector <NUM> and second differential amplifier <NUM> are arranged similarly to the input detector <NUM> and first differential amplifier <NUM>. However, in other embodiments other arrangements may be used to detect power levels of the first coupled input signal and first coupled output signal.

In the present embodiment, the gain control loop <NUM> further comprises a scaling amplifier <NUM> coupled to an output of the first differential amplifier <NUM>. The scaling amplifier is configured to amplify the signal from the first differential amplifier <NUM>, to account for any mismatch between the input detector <NUM> and the output detector <NUM>. That is, if the input detector <NUM> and output detector <NUM> are not matched, each detector may output a different voltage for any given signal power level.

Although in the present embodiment an amplifier is provided as a means for scaling the signal produced by one of the detectors, in other embodiments alternative scaling means may be used. Instead of amplifying the signal from one of the detectors, the scaling means could be arranged to pull down the output of one of the differential amplifiers <NUM>, <NUM> by a suitable amount, for example using a resistive divider, to compensate for any mismatch between the detectors. Also, although in <FIG> the scaling means, i.e. the scaling amplifier <NUM>, is coupled to an output of the first differential amplifier <NUM>, the present invention is not limited to this particular arrangement. For example, the scaling means <NUM> could be coupled to the output of the second differential amplifier <NUM>.

Continuing with reference to <FIG>, the scaled output of the first differential amplifier <NUM> and the output of the second differential amplifier <NUM> are coupled to the inputs of another differential amplifier <NUM>, hereinafter referred to as the loop amplifier <NUM>. The loop amplifier <NUM> generates a gain control signal which is representative of a difference between the scaled output of the first differential amplifier <NUM> and the output of the second differential amplifier <NUM>. The gain control signal is sent to the gain control module <NUM>, which determines based on the value of the gain control signal whether to adjust the gain being applied to the RFIN signal. For example, the gain control module <NUM> may be configured to adjust the gain in order to minimise the value of the gain control signal received from the loop amplifier <NUM>.

In summary, the gain control loop <NUM> is configured to control the gain control module <NUM> based on a difference in power between the first coupled input signal and the first coupled output signal. Although one structure of the gain control loop <NUM> is shown in <FIG>, in other embodiments other arrangements may be used.

As shown in <FIG>, the phase control module <NUM> is controlled by a separate phase control loop <NUM>. As described above, the phase control loop <NUM> receives the second coupled input signal from the second input coupler <NUM>, and receives the second coupled output signal from the second output coupler <NUM>. The second coupled input signal and second coupled output signal are combined in a coupler <NUM>. However, before being input to the coupler <NUM>, the second coupled input signal is passed through a delay line <NUM>. The delay line <NUM> is configured to delay the second coupled input signal to ensure the same electrical length across both signal paths for frequencies in the RFIN signal. That is, the delay line <NUM> is configured so that the electrical length of the "through path" including the gain and phase control modules <NUM>, <NUM>, the power amplifier <NUM>, the second output coupler <NUM> and the coupler <NUM>, is the same as the electrical length of the "coupled-through path" including the delay line <NUM> and the coupler <NUM>.

In this way, the second coupled input signal and the second coupled output signal arriving at the coupler <NUM> at any point in time are controlled to correspond to the same part of the original RFIN signal. That is, the delay line <NUM> in the phase control loop <NUM> is arranged to delay the first signal before the error signal is obtained by the coupler <NUM>, such that the delayed first signal and the second signal used to obtain the error signal correspond to the same part of the input signal RFIN. Therefore the phase control loop <NUM> may be referred to as a feed-forward loop, since the second coupled input signal is "fed forward" and compared against the corresponding portion of the output RFOUT signal.

A signal path from the second input coupler <NUM> to the coupler <NUM> via the delay line <NUM> may be referred to as a "feed-forward path". As described above, the signal path through the phase control module <NUM>, gain control module <NUM>, and power amplifier <NUM> to the second output coupler <NUM> may be referred to as the "through path", and a signal path from the second output coupler <NUM> to the coupler <NUM> may be referred to as the "coupled-through path". The delay line <NUM> is therefore configured such that the electrical length of the feed-forward path is substantially the same as the combined electrical lengths of the through path and the coupled-through path. In the present embodiment, the delay line <NUM> is physically embodied as a length of coaxial cable having an appropriate physical length to achieve the required delay. However, other arrangements may be used in other embodiments.

In the present embodiment, the phase control loop <NUM> is configured such that when the amplified RFOUT signal output by the power amplifier <NUM> has the correct phase, the second coupled input signal and second coupled output signal arrive at the coupler <NUM> in-phase. The coupling factors of the second input coupler <NUM> and second output coupler <NUM> may be chosen such that when the power amplifier <NUM> is operating at the desired gain, the second coupled input signal and second coupled output signal have the same power level. Alternatively, an attenuator may be used to pull down the second coupled input signal or the second coupled output signal to the correct power level.

The coupler <NUM> is a <NUM>° coupler, and therefore when the second coupled input signal and second coupled output signal are combined in the coupler <NUM>, they will cancel at an output of the coupler <NUM> if amplified RFout signal outputted from the power amplifier <NUM> has the correct phase. In effect, the phase control loop <NUM> is arranged to subtract the second coupled input signal from the second coupled output signal to obtain a difference between the two signals, as an error signal. However, if the RFOUT signal does not have the correct phase, the second coupled output signal will not be in-phase with the second coupled input signal as they arrive at the coupler <NUM>. In this case, the signals will not completely cancel, and the amplitude of the error signal output by the coupler <NUM> is representative of the phase difference between the signals. The phase control loop <NUM> can therefore detect whether the phase of the RFOUT signal is offset from the desired value, for example as a result of nonlinear phase distortions introduced by the power amplifier <NUM>.

The error signal output by the coupler <NUM> is sent to a detector <NUM>, which may be an RMS detector similar to the input detector <NUM> and output detector <NUM> of the gain control loop <NUM>. The detector <NUM> measure the power level of the error signal, and outputs a signal representing the measured power to a processor <NUM>. The processor is configured to adjust a phase adjustment applied to the RFIN signal by the phase control module <NUM>, so as to minimise the error signal power level measured by the detector <NUM>.

Although in the present embodiment, an error signal is obtained by taking the difference between the coupled input and output signals, in other embodiments the phase control loop <NUM> may be configured to add the coupled signals together to produce the error signal. For example, the second coupled input signal and second coupled output signal could be arranged to be in-phase when they arrive at the coupler <NUM>, such that the signals add together instead of cancelling. In this case, the processor can be arranged to vary the phase applied to the RFIN signal so as to maximise the measured power of the error signal.

As described above, using separate control loops to control the gain and phase applied to the RFIN signal offers the advantage that the processing algorithm can be simplified in comparison to a conventional control system, since each control loop only deals with a single variable. Therefore a control system such as the one shown in <FIG> can operate with a shorter response time compared to a conventional control system. Furthermore, the gain and phase control can be accurately applied even when the gain control behaves as a phase shifter, which typically occurs when the power amplifier is operated at, or close to, saturation. In this case, varying the gain can affect the phase of the RFOUT signal, but the separate phase control loop can detect this change and automatically adjust the phase to compensate.

Although in the embodiment of <FIG>, the gain control loop is implemented using analogue components, in other embodiments the gain control loop may be digitised. For example, a field programmable gate array (FPGA) or application specific integrated circuit (ASIC) may be configured to provide similar functionality to the analogue gain control loop of <FIG>, in order to control the gain control module. Additionally, although in the present embodiment the gain and phase control loops are used to control an RF power amplifier, other embodiments may be used at different frequencies, i.e. not only at RF.

In the embodiment shown in <FIG>, the phase control loop is a feedforward loop similar to a signal cancellation circuit of a feedforward linearization circuit. The phase control loop of <FIG> differs from a feedforward linearization circuit in that the error signal obtained by the coupler <NUM> is not subsequently combined with the RFout output signal of the power amplifier to cancel intermodulation products, as would normally happen in the error cancellation circuit of a feedforward linearization circuit. Instead, a power of the error signal is detected and used to determine a phase to be applied to the RFin input signal.

Referring now to <FIG>, a graph comparing the gain control performance of a conventional open-loop control system with the control system of <FIG> is illustrated. The graph shows the variation in gain error (Delta Gain) over a range of output signal RFOUT power levels. The gain error is the difference between the actual gain and the target gain. A first curve <NUM>, shown as a solid line in <FIG>, illustrates the gain error for a range of output power levels when the control system of <FIG> is used to control a GaN power amplifier. A second curve <NUM>, shown as a dashed line in <FIG>, illustrates the gain error over the same power range when a conventional open-loop control system, such as the one shown in <FIG>, is used to control the same GaN power amplifier. As shown in <FIG>, the control system of <FIG> achieves substantially more stable gain control than is possible with the conventional open-loop control system.

Referring now to <FIG>, a graph comparing the phase control performance of a conventional open-loop control system with the control system of <FIG> is illustrated. The graph shows the variation in phase error (Delta Phase) over the same power range used in <FIG>. A first curve <NUM>, shown as a solid line in <FIG>, illustrates the phase error when the control system of <FIG> is used to control the GaN power amplifier. A second curve <NUM>, shown as a dashed line in <FIG>, illustrates the phase error when a conventional open-loop control system is used to control the GaN power amplifier. As shown in <FIG>, the control system of <FIG> achieves substantially more stable phase control than is possible with the conventional open-loop control system.

Preferably, the input and output detectors of the gain control loop should be RMS detectors. However, if the detectors are not good RMS detectors, a limiter can be used to clip the input signal RFIN before the signal reaches the first input coupler. In more detail, when the RFIN signal has a high peak-to-average ratio (PAR), the amplified signal RFOUT produced by the power amplifier can become clipped when the amplifier is driven to a high gain level. In this event the RFOUT signal will have a lower PAR than the RFIN signal, and accordingly the first coupled output signal will have a lower PAR than the first coupled input signal. If the input and output detectors are not good RMS detectors, the detectors may give a different measured power for signals having a different PAR, even when the RMS power of the signals is the same. Therefore when the RFOUT signal is clipped relative to the RFIN signal and the detectors are not good RMS detectors, different power levels may be measured by the input and output detectors even when the signals have the same RMS power level. This can result in the gain control being incorrectly applied.

To compensate for this, embodiments of the present invention in which the detectors are not good RMS detectors may further include a limiter coupled to an input of the first input coupler. The limiter is configured to clip the RFIN signal to the same extent as the RFOUT signal is clipped by the power amplifier. Accordingly, the first coupled input signal and first coupled output signal are clipped to the same extent, and the gain control error can be avoided.

<FIG> is a graph illustrating the gain error over a range of output power levels, when the input and output detectors of the gain control loop are not good RMS detectors. A first curve <NUM>, shown as a solid line in <FIG>, illustrates the gain error when a limiter is used to clip the RFIN signal. For comparison purposes, a second curve <NUM> is also provided, shown as a dashed line in <FIG>, to illustrate the gain error when the RFIN signal is not clipped. As shown in <FIG>, without the limiter the gain error varies by as much as ±<NUM> dB, but by using the limiter this can be improved to ±<NUM> dB. By way of comparison, if good RMS detectors are used as in the embodiment of <FIG>, the gain error varies by ±<NUM> dB, as shown in <FIG>.

A further embodiment of the present invention will now be described with reference to <FIG>. The control system shown in <FIG> may be included in a satellite, such as a communications satellite, to control a power amplifier of the satellite. In this embodiment, gain and phase control is performed using a single feed-forward control loop <NUM>. Like the embodiment of <FIG>, the present embodiment comprises a power amplifier <NUM>, gain control and phase control modules <NUM>, <NUM> coupled to an input of the power amplifier <NUM>, and input and output couplers <NUM>, <NUM> configured to direct coupled input and coupled output signals, respectively, to the feed-forward control loop <NUM>. Also like the embodiment of <FIG>, the feed-forward control loop <NUM> includes a coupler <NUM> for combining the coupled input signal and coupled output signal, and a delay line <NUM> for delaying the coupled input signal before it is input to the detector <NUM>. The coupler <NUM> outputs an error signal to a detector <NUM>, which measures the power level of the error signal and sends a signal representing the measured power to a processor <NUM>. As with the phase control loop <NUM> of <FIG>, the control loop <NUM> of <FIG> is arranged to delay the first signal before obtaining the error signal, such that the delayed first signal and the second signal used to obtain the error signal correspond to the same part of the input signal RFIN.

A detailed description of the operation of the feed-forward control loop <NUM> will be omitted to maintain brevity, since the error signal is obtained in a similar manner as in the embodiment of <FIG>. However, in the present embodiment, the processor <NUM> determines, based on the measured power of the error signal, whether to adjust the gain and phase applied to the RFIN signal before it is input to the power amplifier <NUM>. As such, the processor <NUM> is coupled to both the gain control module <NUM> and the phase control module <NUM> to control the gain and phase of the RFIN signal. Therefore, in the present embodiment, gain and phase control is performed using a single control loop.

Referring now to <FIG>, a graph comparing the gain control performance of the conventional open-loop control system with the control system of <FIG> is illustrated. The graph shows the variation in gain error over a range of RFOUT power levels. A first curve <NUM>, shown as a solid line in <FIG>, illustrates the gain error for a range of RFOUT power levels when the control system of <FIG> is used to control a GaN power amplifier. A second curve <NUM>, shown as a dashed line in <FIG>, illustrates the gain error over the same power range when a conventional open-loop control system, such as the one shown in <FIG>, is used to control the same GaN power amplifier. As shown in <FIG>, the control system of <FIG> achieves substantially more stable gain control than is possible with the conventional open-loop control system.

Referring now to <FIG>, a graph comparing the phase control performance of the conventional open-loop control system with the control system of <FIG> is illustrated. The graph shows the variation in phase error over a range of RFOUT power levels. A first curve <NUM>, shown as a solid line in <FIG>, illustrates the phase error for a range of signal powers when the control system of <FIG> is used to control the GaN power amplifier. A second curve <NUM>, shown as a dashed line in <FIG>, illustrates the phase error over the same power range when the conventional open-loop control system is used to control the same GaN power amplifier. As shown in <FIG>, the control system of <FIG> achieves substantially more stable phase control than is possible with the conventional open-loop control system.

In comparison with the embodiment of <FIG>, the control system of <FIG> offers less stable gain control over a given range of output signal power, because a single control loop is used to control both the gain and phase. A more complex processing algorithm is required when a single loop controls both gain and phase, and accordingly the response time of the feed-forward control loop <NUM> of <FIG> is slower in comparison to the gain control and phase control loops <NUM>, <NUM> of <FIG>. Nonetheless, the embodiment of <FIG> still offers a substantial improvement over the conventional open-loop control system.

In more detail, when a conventional open-loop control system is used to control the gain and phase, the gain error can only be controlled to within a range of ±<NUM> dB, as shown in <FIG> and <FIG>. When a single feed-forward control loop is used to control both gain and phase, as in the embodiment of <FIG>, this is improved to ±<NUM> dB, as shown in <FIG>. When a separate closed loop is used to control the gain, as in the embodiment of <FIG>, this is improved yet further to ±<NUM> dB, as shown in <FIG>.

Similarly, when the conventional open-loop control system is used to control both gain and phase, the phase error can only be controlled to within a range of ±<NUM>°, as shown in <FIG> and <FIG>. When a single feed-forward control loop is used to control both gain and phase, as in the embodiment of <FIG>, this is improved to ±<NUM>°, as shown in <FIG>. When a separate feed-forward control loop is used to control the phase, as in the embodiment of <FIG>, this is improved yet further to ±<NUM>°, as shown in <FIG>.

The results shown in <FIG>, <FIG> were obtained during preliminary testing of embodiments constructed using relatively low-quality components. Nevertheless, as described above these early embodiments still offer a measurable improvement over the standard open-loop control system of <FIG>, and a greater improvement will be expected following further optimisation.

Although embodiments of the present invention have been described in relation to controlling GaN power amplifiers that exhibit hysteresis-like memory effects, other embodiments may be used to control power amplifiers that do not exhibit such memory effects, for example GaAs-based devices. In these cases, a control system according to the present invention may still offer an advantage over the conventional open-loop control system of <FIG>, by virtue of an improved response time that allows the gain and phase distortions to be more quickly adjusted in response to changes in the input signal power.

Additionally, embodiments of the present invention have been described in which the gain control loop monitors both the input and output signals. However, some embodiments may be configured for use in applications where the input signal has a known constant power, and in such embodiments the gain control loop can determine a current gain of the amplified signal without monitoring the input signal, since the power level of the input signal is already known.

Furthermore, embodiments of the present invention have been described in which the power levels of signals derived from the input and output signals are measured. This can allow the use of low-power detectors even when the input and/or amplified signals are high-power signals. Alternatively, in some embodiments the power levels of the input and/or amplified signals may be directly detected, in which case the first and/or second couplers and first and/or second detectors of <FIG> can be omitted accordingly.

Claim 1:
Apparatus for controlling a gain and phase of an input signal input to a power amplifier (<NUM>), the apparatus comprising:
gain control means (<NUM>) for controlling the gain of the input signal;
phase control means (<NUM>) for controlling the phase of the input signal;
a gain control loop (<NUM>) configured to control the gain control means based on a power level of the input signal and a power level of an amplified signal output by the power amplifier, to obtain a predetermined gain of the amplified signal; and
a phase control loop (<NUM>) configured to obtain an error signal related to a phase difference between a first signal derived from the input signal and a second signal derived from the amplified signal, and control the phase control means based on the error signal to obtain a predetermined phase of the amplified signal,
wherein the phase control loop is arranged to delay the first signal before obtaining the error signal, such that the delayed first signal and the second signal used to obtain the error signal correspond to the same part of the input signal,
wherein the gain control loop is configured to receive a third signal derived from the input signal and a fourth signal derived from the amplified signal,
wherein the gain control loop is configured to compare a power level of the third signal and a power level of the fourth signal and control the gain control means based on the result of the comparison, and
wherein the gain control loop comprises:
a first detector (<NUM>) configured to measure a power level of the third signal;
a second detector (<NUM>) configured to measure a power level of the fourth signal;
characterised in that the gain control loop comprises means (<NUM>) for scaling an output of the first detector or an output of the second detector, so that the outputs of the first and second detectors are substantially identical when the third and fourth signals have the same power.