A Digital RF Transmitter with Background Nonlinearity Correction

Digital RF transmitters (TXs) have gained popularity in recent years for their many advantages. They
dispense with analog functions, such as baseband filters, variable‐gain amplifiers, and predrivers, while
allowing correction of I/Q mismatches and carrier leakage in the digital domain. Moreover, they avoid
the linearity‐gain trade‐off present in every stage of an analog TX.
The greatest challenge facing RF transmitters, analog or digital, is the trade‐off between the linearity
and power efficiency in the front end, specifically, in the output DAC in a digital design. This issue has led
to many linearization techniques. Since the die temperature varies considerably with the output power,
the nonlinearity correction must continue in real time, a point addressed by no digital TX work reported
to date.
In this presentation, we describe a new TX architecture that automatically corrects static and dynamic
nonlinearities with no need for digital predistortion or adaptation. We begin with the Newton‐Raphson
method of solving equations and show that it leads to Delta‐Sigma modulation as a special case and to a
compact, efficient digital TX in the general case. Owing to a multitude of simplifications, the TX reduces
to only one shift register, four Delta modulators, and a single output DAC. To demonstrate the efficacy
of the proposed concepts, a prototype has been developed for the WCDMA standard in 28‐nm CMOS
technology. The complete TX achieves an overall efficiency of 50% while delivering +24 dBm with an
adjacent channel power of ‐35.4 dB and a receive‐band noise of ‐137 dBc/Hz.