Direct Digital Synthesis: Applications to Radar EECS 713 Direct Digital Synthesis: Applications to Radar Manjish Adhikari 2870257

Why Digital Waveform Synthesis Why Digital Waveform Synthesis? Direct digital synthesis to generate analog waveforms is more robust and repeatable Methods Arbitrary Waveform generator -Digitized waveform stored in memory -Using address counter, memory is accessed at high speed, converted to analog by DAC Fig. Arbitrary Waveform Generator

2. Direct Digital Synthesis Uses Frequency accumulator, Phase accumulator and lookup table to generate multiple frequencies from single LUT Requires less memory, High frequency resolution and sub degree phase tuning Fast frequency hopping Different types of waveforms can be generated

DDS Phase accumulator produces linear phase and in steps given by tuning word Based on that particular address in memory is accessed to generate sine waves Higher the tuning word or steps, faster a complete sine cycle is completed ie. High frequency and vice versa

DDS If M is tuning word, bigger the steps, higher the frequency Derived from clock frequency maximum useful output frequency is limited to about 40% of the sample clock frequency

DDS DAC converts digital waveform to analog but it’s sinx/x interpolation and requires some averaging by LPF to remove image and harmonic components Finer the DAC better the resolution, 6dB SNR per bit Can be used to generate square/or clock signals or othersignals

Applications in RADAR Chirp Generation Single tone requires linear phase but chirp has quadratic phase Uses Frequency accumulator to change phase accumulation rate to generate chirp

Amplitude Modulation LUT output is multiplied by desired amplitude data to produce Amplitude Modulated Signals

Limitations DDS can produce signals usually upto 400 MHz but radar transmit frequencies range from MHzs to GHzs So either DDS can be used in association with PLLs or up converted to radar frequency using mixers and Local Oscillators Analog Devices AD9854 DDS and AD8346 Quadrature modulator can be used to generate SSB up converted signals with good LO rejection and spurious signal suppression

AD9854 DDS - 300 Mhz clock rate - Linear or Chirp generation - 12 bit DAC - I and Q synthesis - 3.3 V supply - 80dB SFDR(spurious-free dynamic range

AD8346 SSB Quadrature Modulator - Output Frequency range 800 MHz to 2.5 GHz - I/Q Base-band Frequency range DC – 70 MHz - Output Power –3 dBm P1 dB - Noise Floor –147 dBm/Hz - Single 2.7 V – 5.5 V Supply

Radar Signals Generation - Can be used to generate CW, FMCW signals

Pulsed Signals Generation Can be used to generate pulsed waveforms by using Switching at the output of AD9854

DDS Synchronization - When multiple DDS are used synchronization is necessary - Each DDS has REFCLK pin that needs to be timed properly to reduce phase variations between signals

References [1] Winston E.S. Ong, Commercial off the shelf direct digital synthesizers for digital array radar, Naval postgraduate school Monterey, California [2] http://www.analog.com/media/en/technical-documentation/data-sheets/AD9858.pdf [3] Eric D. Adler, Edward A. Viveiros, Tuan Ton et al, Direct Digital Synthesis Applications for Radar Development [4] C. Allen, EECS 725 Introduction to Radar System Slides