A 2 Gb/s ΔΣ Directly Driven Wireless Link Qianli Mu, Luke Sankey and Zoya Popovic Department of Electrical and Computer Engineering University of Colorado at Boulder, 425 UCB, CO 80309 Emails: {Qianli.Mu, Luke.Sankey, Zoya.Popovic} @colorado.edu Abstract— One-bit ΔΣ modulation in a wireless transmitter is studied as a potential candidate for multifunction broadband transmit front ends. Characterizing a single ΔΣ modulated transmit element is the first step towards a transmit array for radar/communication applications. A wireless link directly driven by a 2 Gb/s ΔΣ bit sequence is analyzed through simulation and measurement. Effects of clock jitter, asymmetrical rise/fall edges and asymmetrical rise/fall times of the digital signal are studied. A Xilinx Virtex II-Pro FPGA is used to generate the ΔΣ bit sequences. The transmit and receive antennas simultaneously perform bandpass filtering of the noise in the ΔΣ spectrum. Index Terms— Delta-Sigma Modulation, Multifunction Array, Wireless Transmitter I. I NTRODUCTION As a result of the increased use of the RF spectrum for communication and sensing, the density of antennas on a variety of platforms has been continuously increasing. For example, nuclear aircraft carriers (CVNs) have as much as 150 antennas on board, resulting in interference, increase of radar cross-section and emissivity, signal masking and obstruction, and increasing maintenance [1]. A possible solution for this collocated antenna problem is a multifunctional and/or recon- figurable transmit and receive system. Multifunctional antenna arrays that can perform radar, electronic warfare, information warfare and communication functions simultaneously are de- sirable. A number of receiving multifunctional arrays have been dis- cussed, e.g. the Advanced Multifunction RF Concept (AMRF- C) has shown the ability to form multiple simultaneous beams in receive mode [2][3]. In transmit mode, however, the com- bined need for high power, linearity and bandwidth make the problem more challenging. ΔΣ modulation provides a promising approach because it can in theory achieve high linearity by spectral shaping of quantization noise from a low-resolution (2-level) quantizer [4][5]. The coded output waveform is thought to be less sensitive to nonlinearities which allows use of nonlinear amplifiers to achieve high power, high efficiency and high linearity of the transmitted signals. This approach also follows the trend to move the digital/analog boundary closer to the antenna element. Most of the research in this field to date are simulations, due to the difficult requirements on the electronic hardware in terms of digital clock rates [6]-[9]. This paper presents sim- ulations and measurements for a directly-transmitted RF fre- quency ΔΣ modulated signal. A high performance commercial Field Programmable Gate Array (FPGA) is used to generate 2 Gb/s ΔΣ bit sequences. Retiming circuitry is implemented to reduce jitter, and a variety of hardware effects on the SNR are studied. Since many potential applications, such as radar and arbitrary waveform generation, require pulsed or repeated waveforms, the effect of finite length ΔΣ sequences on output signal spectra are examined here. Fig. 1. Digitized three-tone signal s(dt) is ΔΣ encoded in software resulting in a one-bit digital signal q(dt). The digital signal is converted to a higher power analog signal y(t) through a driver circuit. A sketch of the time domain signal and noise shaped spectrum are shown in the inset. The antenna that follows is a bandpass filter which filters the noise and radiates the signal y ′ (t). II. I DEAL ΔΣ DAC TRANSCEIVER SINGLE CHANNEL The single channel system, which is the focus of this paper, is shown in Fig. 1. The data presented here is obtained without the high power driver (D). The quantization noise due to one bit quantization is shaped in the frequency domain to be out of band of the signal that is radiated by the antenna. The noise- shaping function h and its Fourier transform H are defined