Predistortion and Quadrature Modulator Compensation in Direct-Conversion Transmitters

Key Takeaways

• The basic function of any transmitter is modulation, up-conversion, and power amplification.

• Direct-conversion transmitters consist of a transmit modem (TM), quadrature modulator (QM), local oscillator (LO), and power amplifier (PA).  

• Predistortion (PD) and quadrature modulator compensation (QMC) are the two corrective measures implemented in direct-conversion transmitters. 

RF transceivers are used in wireless communication systems

The growth of wireless communication has established the need for RF transceivers with high capacity and flexibility. Transceivers utilize different architectures of transmitters and receivers to achieve high data rate signal transmission and a high peak-to-average power ratio (PAPR). 

Irrespective of the architecture, the basic function of any transmitter is modulation, up-conversion, and power amplification. Apart from performing basic functions, a transmitter needs to meet the spectral emission mask and provide modulation accuracy without compromising noise and linearity requirements. 

The direct-conversion transmitter is one type of transmitter that has gained popularity due to its simplicity and ease of implementation. Let’s take a closer look at this unique transmitter.

Quadrature Modulation Based, Simple Direct-Conversion Transmitters

The direct-conversion transmitter, which utilizes quadrature modulation, is widely adopted in wireless multimedia communication systems due to its low modulation distortion. This transmitter is applicable for both analog and digital signal systems. The desirable features of a direct-conversion transmitter are low power consumption, increased efficiency, reduced complexity, and compact size. Direct-conversion transmitters are also called zero intermediate frequency (IF) transmitters because they directly execute frequency conversion between an RF signal and a baseband signal without any intermediate frequency stages. 

A simple direct-conversion transmitter consists of a transmit modem (TM), quadrature modulator (QM), local oscillator (LO), and power amplifier (PA). Simple direct-conversion transmitters can perform all the basic functions of a transmitter; input data is fed into the TM and sent to the QM. The QM then performs baseband I/Q modulation. The LO supplies the in-phase (I) and the quadrature (Q) carries signals to the QM. Quadrature modulated signals are amplified using PA and then transmitted via an antenna. 

Limitations of Direct-Conversion Transmitters

Direct-conversion transmitters exhibit difficulties when implemented in real-time. The practical limitations of a simple direct-conversion transmitter include the following:

1. The high power efficiency of the power amplifier makes it non-linear. To meet spectral efficiency demands, the direct-conversion transmitter needs to be linearized. This problem is generally called power amplifier non-linearity.

2. Direct-conversion transmitters are influenced by the quadrature modulator gain imbalance, phase, and DC voltage offset errors. In direct-conversion transmitters, sometimes the power amplifier output signal leaks back to the local oscillator and causes phase modulation. Such a phenomenon is called local oscillator pulling. 

3. The practical difficulties discussed in points 1 and 2 (above) are vulnerable to temperature, channel frequency, device biasing, and component aging. 

Considerations When Designing a Direct-Conversion Transmitter

When designing a direct-conversion transmitter, the goal should be to establish corrective measures in the system to avoid the influence of the practical difficulties discussed above.  

Direct-conversion transmitter designs require:

1. Linearizers to be used for reversing the nonlinearity imposed by power amplifiers in direct-conversion transmitters.

2. Control over quadrature modulator errors. 

3. The monitoring of the performance of the power amplifier and quadrature modulator.

Predistortion and Quadrature Modulator Compensation 

Predistortion (PD) and quadrature modulator compensation (QMC) are the two corrective measures implemented in direct-conversion transmitters.

Predistortion (PD)

Like Cartesian feedback, feedforward, and linear amplification using non-linear components, predistortion is a linearization technique employed in direct-conversion transmitters. During predistortion, the signal to be amplified is passed through a nonlinear system called a predistorter. Predistorters have inverse characteristics of power amplifiers. The input signal is pre-distorted before amplification so that the output signal shares a linear relationship with the input signal. The two non-linearities of the predistorter and the power amplifier cancel each other out and the direct-conversion transmitter architecture becomes linear.

Quadrature Modulator Compensation (QMC)

Quadrature modulator compensation (QMC) is used as a countermeasure against quadrature modulator errors. QMC involves a vector correction of the input. Digital signal processing (DSP) techniques are used for vector correction. The DSP involves matrix multiplication of the complex baseband terms and the addition of suitable DC offsets. 

PD and QMC must adapt to the variations in temperature, biasing, and frequency to maintain the performance of the power amplifier and quadrature modulator within acceptable values.

Direct-conversion transmitters are the first choice of transmitter architecture in wireless communication systems. Cadence’s software supports the modeling and simulation of direct-conversion transmitter architectures and the effect of various compensation techniques on them. 

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