CALLUM Linear Transmitter - Architecture and Circuit Analysis

Abstract: This doctoral dissertation presents a study of linear radio transmitters based on the combined analog locked loop universal modulator (CALLUM) approach. Linear architectures such as CALLUM are very attractive for power-efficient operations, since they have no fundamental limitations prohibiting a 100% efficiency for all envelope levels, without sacrificing the potential of a linear transmission. This issue is becoming increasingly important in modern communication standards, where the information content is present not only in the signal phase, but in its amplitude as well. Such modulation schemes, while improving the data rate for a given signal bandwidth, pose tough demands on the linearity of the transmitter. As the power amplifier (PA) in the transmitter handles the largest signals and is the main power consumer in the radio, sufficient linearity should be achieved in conjunction with a high power efficiency of the PA, especially if the equipment is battery operated, as is the in case of mobile applications.

In this work, three different CALLUM architectures (i.e., CALLUM 1, CALLUM 1lin, and CALLUM 2) are studied in terms of loop gain, bandwidth, stability, and frequency compensation. A simplified baseband model of a general CALLUM is presented for efficient simulation of the system, and to gain knowledge about performance differences between the CALLUM derivatives. The investigated CALLUM architectures make use of Cartesian feedback, as it provides better matching between the I and Q signal paths than polar feedback. From baseband simulations with different signal component generator (SCG) implementations, the spectral performance of CALLUM 1 and CALLUM 1lin for an EDGE modulated signal is significantly better than that of CALLUM 2, for a given maximal loop gain. It can be concluded that the radical simplifications leading to CALLUM 2 have severe effects on the spectral properties of the output signal, while the actual implementation of the SCG becomes much simpler than for example CALLUM 1. The effect of propagation delay in the feedback loop is also included in the model, and indeed this delay appears to be the limiting factor in the achievable closed-loop signal bandwidth. A lag-lead frequency compensation network is used to trade bandwidth for increased insensitivity to time delays. The frequency compensation is quite efficient for all CALLUM versions studied when they operate on a 3pi/8-shifted 8PSK, as acceptable delays may in this case become much larger without jeopardizing the stability. It is worth noting that CALLUM 1lin performs very well in terms of maximum acceptable time delay for a certain standard.

Implementations of the circuits for the CALLUM 2 architecture are presented together with simulation results of the fundamental blocks. A differential analog SCG realizing the control equations for CALLUM 2, and a variable-gain amplifier (VGA) were simulated together with other functional blocks to form a complete baseband-modeled CALLUM 2 transmitter. From the simulated spectral performances, the SCG and VGA implementations proved to be appropriate for an EDGE-modulated signal.