IEEE Access (Jan 2023)
Image Enhancement Technique Using Manchester Coding and RF Tripler for 1-bit Bandpass Delta Sigma Direct Digital RF Transmitter
Abstract
1-bit band-pass delta-sigma modulator (BP-DSM) can generate RF signal directly from 1-bit data stream in high dynamic range by oversampling technology without local oscillator (LO) and mixer, so the miniaturization of transmitter can be realized. Meanwhile, to generate RF signal in high frequency band, direct digital RF transmitter using image components of 1-bit signal folded in high Nyquist zone has been studied. In a previous study, an image enhancement 1-bit digital-to-analog converter (DAC) based on a high-speed inverter has been proposed to regenerate or enhance the high order image components of 1-bit BP-DS modulated signal which attenuate severely because of the transmission loss. However, a high-speed inverter is very complex and difficult to implement. Consequently, only low order ( $1^{\textrm {st}}$ or $2^{\textrm {nd}}$ ) image components can be used in direct digital RF transmitter. In this paper, firstly, we propose an image enhancement technique using Manchester coding and RF tripler instead of 1-bit DAC based on inverter and illustrate its principle. By the theoretical calculation, simulation and measurement, the feasibility has been proven. In the measurement, 1-bit data in Manchester code is applied whose amplitude and datarate are set to 1Vpp and 2 Gbps, respectively. It is confirmed that, because of the nonlinearity of RF tripler, both in the continuous wave (CW) and 5Msps-QPSK condition, the high order image components of 1-bit BP-DS modulated signal in Manchester code can be regenerated with a good characteristics of noise shaping. In CW condition, the measured output power of regenerated high order image components at $6^{\textrm {th}}$ (5.5 GHz) and $7^{\textrm {th}}$ (6.5 GHz) Nyquist zone are −22.4 dBm and −24.9 dBm, respectively. After the peak-to-peak amplitude of measured signal is normalized in MATLAB, the output power of regenerated high order image components are −9.1 dBm and −11.6 dBm, respectively, which is very close to ideal 1-bit data in Manchester code. The measurement results have a good agreement with the theoretical calculation and simulation. In the QPSK condition, the measured output power of regenerated high order image components at $6^{\textrm {th}}$ (5.5 GHz) and $7^{\textrm {th}}$ (6.5 GHz) Nyquist zone are −25.3 dBm and −27.8 dBm, respectively. After the peak-to-peak amplitude of measured signal is normalized in MATLAB, the output power of regenerated high order image components are −11.9 dBm and −14.5 dBm, respectively. It is proven that the illustrated principle of image enhancement is reasonable and proposed image enhancement technique using Manchester coding and RF tripler is effective. Compared with direct digital RF transmitter only using low order ( $1^{\textrm {st}}$ or $2^{\textrm {nd}}$ ) image components in previous work, the proposed image enhancement technique can regenerate or enhance the high order (( $5^{\mathrm{th}}$ or $6^{\textrm {th}}$ )) image components and contribute to direct digital RF transmitter operating in higher frequency with lower datarate.
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