Radar related progress in RF technology over the past decade (A short trip down Memory Lane)

1. Radar related progress in RF technology over the past decade(A short trip down Memory Lane) PW van der Walt Reutech Radar Systems University of Stellenbosch

2. Introduction • We’re on the threshold of the last year of the decade – good time to look back! • The first decade of the 21st century has witnessed the maturation of important high speed device technologies • Hetero-junction Bipolar Transistors and HBT ICs • HEMPT and pHEMPT devices and ICs • LDMOS power devices • Ever smaller and faster silicon CMOS devices • During the past decade devices using these technologies have matured, driven mainly by cell phone and other communications systems • In many cases radar applications benefit because the devices also meet radar requirements • Looking back, it has been an exciting 10 years! • In this paper a brief outline is given of some of the more spectacular advances in RF devices that have made them useful in the implementation of radar systems

3. Contents • Frequency synthesis • DDS • PFD • Signal conditioning • Amplification • Frequency translation • Modulators, demodulators and mixers • Data conversion • RF Power • Packaging microwave components • PCB materials • PCB design

4. Frequency synthesis • DDS • A DDS consists of a phase accumulator (= a wrap around counter), a sine look-up table and a digital to analog converter (DAC) • The direct digital synthesizer has become a key element in frequency synthesizers • Radar requirements are strict in terms of bandwidth, phase noise and spurious signals. The bandwidth and spurious responses of earlier DDSs limited the application of early generation DDSs in radar systems. • DDS spurious caused by • DAC harmonics that alias back to baseband • Mitigated by improved linearity • Phase truncation spurs when the frequency tuning word msb’s > DAC resolution • Situation improves with ADC resolution

5. DDS Application • Because the DDS output is a sampled signal, aliasing of harmonics occurs • DDS wide band spurious can be quite high • DDS narrow band spurious can be quite low • A PLL acts as a tracking narrow band filter and is the ideal companion for a DDS • The following slide shows the preferred application of a DDS in a wide-band frequency synthesizer for radar

6. DDS Application

7. DDS then and now • 2000: • State of the art represented by AD9854 • 300 MHz clock, 12 bit ADC, 48 bit FTW • 80 dB narrow band SFDR at 100 MHz • Falls short of many radar requirements • Digital Phase Frequency Detectors (PFD) fall short on phase noise • 2008: • Single IC state of the art represented by AD9912 • 1000 MHz clock, 14 bit ADC, 48 bit FTW • 86 dB narrow band SFDR at 400 MHz • Backed by a matching PFD • The DDS qualifies excellently for radar applications • Agile LO Synthesizers • Chirped pulse generators • FMCW sources with excellent chirp linearity

8. Frequency Synthesis • PFD • The phase-frequency detector is at the heart of a PLL • The first decade of the 21st century saw the introduction of a low noise phase-frequency detector by Hittite, bettering its predecessors by about 20 dB in one spectacular leap • The HMC439 has a noise floor of -140 dBc/Hz at 1.3 GHz • This is comparable to the noise performance of well-designed analogue synthesizers for radar • The PFD made possible the use of simple PLL architectures for high performance radar frequency synthesizers • And it happened in the past decade • As a case in point, RRS now builds a synthesizer optimised for its X-band pulsed MTD radars on two PC boards with phase noise performance better than that of analogue synthesizers costing R1 million a decade ago

9. Amplification • Progress with amplifiers was steady rather than spectacular • Gain block performance has improved with lower noise figures and higher third order intercept points (IP3) • Gain stability has reached an impressive ±0.2 dB over -40º to 85º C temperature range • Bandwidths of 20 GHz • Modern HBT based amplifiers have extremely low phase noise at high drive levels, as exemplified by the measured phase noise at 1 GHz of a ×10=×2×5 frequency multiplier implemented with a new generation of amplifiers.

10. 1 GHz Multiplier phase noise

11. Frequency Translation • Modulators can produce single sideband RF output signals up to about 4 GHz from baseband IQ inputs from a single integrated circuit which incorporates the wideband LO quadrature polyphase phase shift filter

12. Modulators • At the turn of the century, baseband bandwidths were limited to 250 MHz and noise floors were at a level of about -152 dBc/Hz • Today modulators with baseband bandwidths of 700 MHz and noise floors of -158 dBc/Hz • Carrier suppression without trimming can exceed -50 dBc and sideband rejection is better than 40 dB • The translinear multiplier cells far outperform diode mixers regarding spurious performance • These devices have proved to be high performance and versatile elements in radar frequency synthesizers and transmitter exciters • With demodulators to match

13. Frequency translation • Another welcome component is the balanced diode mixer whith incorporated LO amplifier, reducing external LO drive requirements to less than 0 dBm, while internally driving the mixer at a level of 20 dBm • This comes as a blessing to everyone who has had the pleasure of suppressing leakage from high level LO signals • At the same time offering mixer/demodulator with very large dynamic range

14. Data Conversion • The performance of ADCs has improved by leaps and bounds during the past 10 years • 2000: • 80 MS/s, 10 bits, 300 MHz bandwidth (AD) • 50 Ms/s, 12 bits, 300 MHz bandwidth • 2009: • 1 GS/s, 12 bits, 2.1 GHz bandwidth (TI) • Rate of increase in speed: • 1 decade/decade!

15. Bad and Good News • The Bad News for RF engineers • The digital domain is encroaching on their analogue world • One of these days we will be left with nothing but antennas and LNAs! • The Good News for RF engineers • Mixed circuit technology has become an RF problem • PCB design is now done by a multi-disciplinary team

16. Power Transistors • Prior to about 2000, bipolar transistors were the only devices on offer for L-band radar transmitters • Up to 200 W peak power per device • 7 dB power gain • LDMOS transistors are now the devices of choice • Up 500 W of peak power per device (IFF) • 12 dB power gain • 50 V supply • These devices dramatically impact on the complexity of radar transmitters, reducing the number of stages and the number of devices per stage

17. Power transistors • The next decade promises to be equally exciting • GaN device technology is maturing • The first devices have reached production status • Output power of 100 W per device at X band frequency have been reported by laboratories • Currently 25 W from MESFET devices • The devices hold the promise that powerful solid-state transmitters will become reality at frequencies up to X-band in the next decade

18. PCB Materials • Rogers introduced its 4000 series of laminates in the early 90’s. • This substrate material is compatible with fiber-glass FR4 PCB and can be stacked up with fiber-glass to make multilayer PCB’s handling signals up to mm wave frequencies • This material is steadily impacting the way professional microwave systems are built • Slowly but surely extending RF construction techniques to microwave and millimetre wave frequencies

19. Microwave systems • Traditionally, microwave components such as mixers and amplifiers are individually packaged in connectorised enclosures • These modules are expensive, with the enclosure contributing most of the cost • At the same time they provide a very good shielding solution to contain radiation from microwave components • With materials such as Rogers R4003, the PCB can become a microwave enclosure at a much reduced cost

20. A PCB-housed 6-8 GHz doubler

21. Construction • Top layer: Rogers R4003 • The cover is bent from thin sheet metal and soldered into place as a shield • Bottom shielding is provided by ground plane layers inside and on the surfaces of the PCB • DC and BIT circuitry is housed outside the RF shield • All connections into the RF volume is made through LC feed-through components via an internal layer • The key to a successful design is to know something of antennas…

22. Antennas 101

23. Take care! • Example of how not to launch (ca 2000) Poor Earthing Gap! Slot Antenna Oops!

24. Launchers • There is obviously a serious leakage problem at the connector transitions • Totally unacceptable for use in a complex system Launchers should be chosen with care • The transition must be modeled and simulated • Not only for S11 and S21 • Also for leakage!

25. PCB Design • The PCB electromagnetic problem is a 3D problem • Transitions must be carefully designed to prevent radiation (or susceptibility to outside radiation) • The board layout must be carefully planned from a functional and also from an EMC view • Look at the board from all sides! • Following is an example of a recent low-leakage (but not leakage free) edge launcher design on an evaluation board

26. Low Leakage Launcher

27. PCB design • PCB enclosures potentially offer big cost savings in microwave circuitry • The PCB design problem is far from trivial. • Too complex and time-consuming to simulate EM fields fully, forcing us to make judicious use of EM analysis tools • Knowledge of where current flows and good insight into the behaviour of electromagnetic fields is essential for creating good PCB designs • PCB enclosure design is challenging! • A source of interesting research questions for the academics!

28. Finally • The technological advances that had the greatest impact on RF progress? • Undoubtedly the maturation of 3D EM analysis packages • Passive components such as filters and antennas often need only one design iteration • Without these tools, the sophistication reached with RF active and passive components would be unthinkable!