2. LITERATURE REVIEW
2.9 Common Linearization Methods
2.9.2 Modulation feedback
Modulation feedback techniques use some form of detection or demodulation to recover representations of the baseband modulating signal and power amplifier output signal. The difference between the two signals provides error signals which are used to apply correction to the amplifier control signals [82].
49 2.9.2.1 Envelope feedback
Envelope feedback corrects for AM-AM distortion and often applied to automatic gain control (AGC) loops to compensate for PA gain variations and to control pulse shaping in TDMA transmitters [83]-[84]. The RF input signal is sampled at the input and the output by a coupler and the envelope of the signals are obtained. The envelope are compared and subtracted from each other using a differential amplifier. The difference signal represents the AM distortion of the amplifier and this signal is amplified, filtered and used to modulate the driver stage of the PA.
Peak Detector RF In
Peak Detector + -
PA AGC
Linearized Output
Video Amp Attenuator
Coupler Coupler
Figure 2.22: Modulation feedback system
Fig. 2.22 is an example of an envelope feedback system. This feedback method has been a mainstay of mobile communications industry for many years as a means of getting a few valuable dB of IM performance for VHF and UHF solid state power amplifiers [85]. Unfortunately, simple amplitude correction cannot increase the intrinsic power saturation of the device, so the effectiveness of the procedure will decrease significantly as the envelope swings into the compression region.
This technique does not correct for AM-PM effects. In fact, when implementing this technique, creating AM-PM distortion must be avoided. The delays in the detection and video signal processing can cause a video phase difference between the AM and PM processes, which reduce or even eliminate whatever correction may have been obtained by the amplitude feedback process. Techniques have been reported in
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[86], which help optimize the feedback loop and avoid spurious oscillations and enables new ways for feedback linearization design.
2.9.2.2 Polar Loop
The polar loop is essentially an extension of the envelope feedback system described earlier but having both phase and amplitude correction. This technique is based around the principle of Envelope Elimination and Restoration (EER) but modified to allow feedback to be applied [87]. In [88], a polar loop implementation of the correction for an SSB transmitter has been reported. Although the system is a simple extension to the basic envelope feedback loop shown in Fig. 2.22, the block diagram is more complex. This is due mainly to the practical difficulties associated with measuring differential phase changes at a microwave signal frequency. The implementation shown in Fig. 2.23 uses a phased locked loop to maintain a constant amplifier phase transfer characteristic.
Peak Detector
+ - PA
AGC
Linearized Output
Video Amps VCO
LPF Peak
Detector Phase
Comp.
(IF)
Down conversion
Signal (IF) RF
Source
Figure 2.23: Polar loop system
One of the key issues with the polar loop is the different bandwidth requirements for the amplitude and phase error amplifiers. In practice, it is reported that the phase amplifier will require higher bandwidth. Therefore, differing levels of improvement for AM-AM and AM-PM characteristics usually occur and this causes poorer overall performance than the Cartesian loop technique. As with the envelope feedback loop,
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the bandwidth limitations of the video circuitry will limit the usefulness of such systems to single-carrier applications.
2.9.2.3 Cartesian Loop
Cartesian correction has been reported to have some benefits over the polar loop technique [89]-[91]. It makes use of the fact that modulated RF signal can be represented in complex in-phase (I) and quadrature (Q) baseband form as well as by amplitude and phase functions. In a modern digital system, it is most likely that the baseband signal will already be available in I and Q format. Thus, the resulting I and Q channels can be processed in well-matched paths, eliminating the problems of the different bandwidth and signal processing requirements for magnitude and phase paths in the polar loop. Fig. 2.24 shows the essentials of a Cartesian loop linearization system.
+ -
PA 90o
+ -
90o
LO
Linearized Output
Attenuator IOut
QOut
IIn
QIn
I-Q Modulator
I-Q Demodulator
Figure 2.24: Cartesian loop system
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The separate I and Q signal inputs will be the filtered, or smoothed, binary symbol sequences. The signals are fed through differential correcting amplifiers into vector modulators that form the actual RF signal S(t), where ωc being the RF carrier frequency, resulting in
t t
Q t t
I t
S( ) ( )cosc ( )sinc (2.48) The signal S(t) is then fed into the RF power amplifier, emerging with some distortion. A small portion of the output is coupled into a downconverter and retrieves the now distorted I and Q signals, which are then directly compared with the undistorted input baseband signals. The gain of the input differential amplifiers will force the loop into generating an output signal that closely tracks the original I and Q signals. The effectiveness of the Cartesian loop depends on the ratio of the feedback loop bandwidths to the I and Q input bandwidths and the linearity of the demodulators.
One of the benefits of the Cartesian loop over the polar loop is the symmetry of gain and bandwidth in the two quadrature signal processing paths. This will reduce the tendency to introduce phase shifts between the AM-AM bandwidth and stability will limit the capability to handle multicarrier signals. With the widespread availability of low-cost quadrature modulators and demodulators, the overall system becomes a simple linearized transmitter architecture. Cartesian loop transmitters that operate up to 900 MHz for relatively narrow band signals (<5 kHz bandwidth) with excellent results have been constructed [92]. The Cartesian loop also forms an entry point for digital linearization techniques.