1. Direct Digital Synthesis: Applications to Radar
EECS 713
Direct Digital Synthesis: Applications to Radar
Manjish Adhikari

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

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

9. 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

10. 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

11. 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

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

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

14. 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

15. References
[1] Winston E.S. Ong, Commercial off the shelf direct digital synthesizers for digital array radar, Naval postgraduate school Monterey, California
[2]
[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