A Delay Spread Cancelling Waveform Characterizer for RF Power Amplifiers

Abstract

A two channel 65 nm CMOS RF-waveform characterizer is presented that enables multi-harmonic Adaptive Matching Networks (AMN) or Adaptive Digital Pre-Distortion (ADPD) in RF-power amplifiers. The characterizer measures the DC component and the first 3 harmonics of RF signals by applying a DFT to 8 (ideally) equally spaced quasi-DC output voltages. Conventionally in these types of systems accuracy is limited by sample timing accuracies, which in our case are mainly due to delay cell mismatch. We introduce a novel way to cancel delay cell mismatch, that significantly increases measurement accuracy at the cost of only a small power and area increase. The RF-waveform characterizer achieves 6.8-bit measurement linearity together with a (clock feedthrough limited) 24 dB SFDR. The measured power consumption for our proof-of-principle demonstrator is 18.6 mW at a maximum input signal frequency of 1.1 GHz under continuous operation.

Reconstructing Aliased Frequency Spectra by Using Multiple Sample Rates

“…Simultaneously it allows an aggregated sample rate close to the aggregated bandwidth of 2f o |S|. Sampling the signal at different sample rates creates unique aliasing patterns for the same input signal spectrum as described by (5). A system of independent linear equations can be formulated that describes these aliasing patterns, allowing for reconstruction of the input spectrum.…”

“…Another use case for the MRU algorithm is the characterization of the harmonics of an RF-waveform to improve the reliability and pre-distortion in an RF-PA as mentioned in Section I. The work in [5] uses 8 equally spaced samples over a single period of the RF-waveform, allowing characterization of the DC component and the first 3 harmonics. However, when the RF-waveform contains more harmonics aliasing will occur.…”

“…This allows the signal to be sampled without any spectral leakage, which ensures that every spectral contribution occupies a single distinct frequency after sampling. The proposed algorithm could find applications in areas such as RF waveform characterization [5] (Appendix C) and built-in self-test of transceiver signal chains to improve digital pre-distortion and reliability, IQ reconstruction and other application scenarios as long as the frequencies of interest are known and the signal is orthogonal.…”

“…Aliasing pattern when the OFDM signal is sampled at 4.86 MS/s as described by(5). Only aliasing to the first Nyquist zone is shown (r=-1 and r=0).…”

Preserving Polar Modulated Class-E Power Amplifier Linearity under Load Mismatch

“…In such systems, accuracy is generally limited by sample timing inaccuracies which in this case is mainly due to delay mismatch in a 4-stage differential inverter ring oscillator inside the phase-locked loop (PLL). To significantly enhance the accuracy of sampling, statistical properties of the delay mismatches are exploited to cancel the impact of delay spread [11]. The characterizer consumes 3 mW power under continuous operation at 2 GHz.…”