UHF Delta-Sigma Waveform Generator

Introduction: A laboratory bench model of a UHF delta-sigma waveform generator was developed and tested. The waveform generator, which will eventually consist of only a few components, can generate both high-fidelity transmit waveforms and local oscillator signals for UHF radars. The application would be for digital radars. These radars require a large number of small, inexpensive digital waveform generators that have very good performance but not ultra-high performance because some performance improvement is obtained by paralleling a large number of waveform generators.

Delta-Sigma Background: Conventional digital-to-analog (D/A) conversion methods require a large number of bits for high resolution. Delta-sigma digital-to-analog converters (DACs) use only one bit. However, they can provide the same resolution as a 16-bit (or higher) conventional DAC, at a cost of increased overall system complexity and a higher sampling rate. Although delta-sigma coding circuitry is relatively simple for small signal bandwidths at baseband (namely, audio bandwidths of 20-24 kHz), capturing higher frequencies with large signal bandwidths (which are typically needed for radar transmission waveforms and/or radar local oscillators (LOs)), is much more complex and is still a significant field of research. In the search for low-cost, high-performance frequency generation for digital radar, it is useful to investigate the potential of delta-sigma waveforms since they can provide the high resolution needed by radars.

Creating a Delta-Sigma Waveform Generator: This research involved designing, constructing, and successfully demonstrating a UHF Delta-Sigma Waveform Generator. The waveform generator is DAC hardware only; the actual delta-sigma A/D is encoded through a MATLAB program, and the digital waveforms are then stored in an Agilent Logic Analysis System. Although the delta-sigma encoding produces a single binary bitstream, the code is stored as a 16-bit parallel waveform in the Agilent system because it cannot operate at the high serial bitrate needed (2.5 Gbps). The stored waveforms can then be replayed through the Delta-Sigma Waveform Generator hardware to recreate the analog waveform. The main components of the waveform generator are the Agilent pattern generator, a serializer, and a very high speed flip-flop to buffer the output (Fig. 1). The only other components of the system are the various clock signals required and filters for the analog signal band. The simplicity of the architecture is the main highlight of this design, showing that high-quality waveforms can be produced with a minimal amount of hardware.

FIGURE 1
UHF delta-sigma waveform generator.

The waveforms tested on this system were a sinusoid and a linear FM, representing a LO and transmission type waveform, respectively. Figure 2 shows typical measurements of these delta-sigma created waveforms (in this figure, the out-of-band noise has not been filtered out). Although some issues with spurious signals must be resolved, the waveform generator produces low noise waveforms. The overall noise profile is difficult to measure accurately because of high frequency PRF lines that appear in the signal band, but created sinusoids have a noise floor as low as -140 dBc/Hz over an 80 MHz bandwidth. Since the waveforms are digitally synthesized in a software A/D process, the frequency and bandwidth are easily variable (up to UHF frequencies, 100 MHz bandwidth).

FIGURE 2
Measured waveform spectra from the UHF delta-sigma waveform generator at 2.5 Gbps sampling rate (signal band is unfiltered).

A slightly different architecture of the UHF Delta-Sigma Waveform Generator was constructed using a field-programmable gate array (FPGA) evaluation board (to store and process waveforms), which produced waveforms nearly as high in quality. This architecture effectively replaces the Agilent pattern generator and the serializer with an FPGA, creating a low-cost design using a small number of parts that can all fit onto a small printed circuit board (Fig. 3, filters and simple complementary transmission line circuitry not shown).

FIGURE 3
Key components of the delta-sigma waveform generator.

Applications and Significance: This work can be directly applied to waveform and LO synthesis for UHF radar systems. Using a higher delta-sigma sampling rate will allow larger signal bandwidths and/or higher signal center frequencies to be produced. The delta-sigma sampling rate of 2.5 Gbps chosen for this project corresponds to the I/O rate of our current generation of FPGAs. Flip-flops with a much higher sampling rate are already on the market, and the next generation of Xilinx FPGAs will allow us to use a sampling rate of 10 Gbps, which will be suitable for working with L-band or S-band waveforms. Since the circuit can also be easily scaled down to a very small physical size, an ideal application is use in digital phased array radars in which a self-contained exciter circuit is needed at each radiating element. The Delta-Sigma Waveform Generator shows the potential to meet the strict cost requirement of such a system and provide the ultra-high quality waveform needed through paralleling or ensemble averaging.

As this project demonstrates, delta-sigma encoding does not have to be restricted to very low frequencies and small bandwidths. The value of high-speed delta-sigma D/A conversion is that it requires only a small number of relatively low-cost parts to produce a high quality waveform.

[Sponsored by ONR]