Dr. -Ing. Habil Ajay Poddar of Indian Institute of Technology Jammu

The latest low-noise millimeter wave frequency synthesizers builds upon OEOs to produce tunable, low-noise output signals at X- and K-band frequencies. They use long fiber-optic delay lines to reduce phase noise close to the carrier. The synthesizers also employ self-phase-locked-loop (SPLL) and self-injection-locking (SIL) techniques to reduce phase noise otherwise located close-in and far-from the carrier, respectively. The X- and K-band frequency synthesizers are truly subsystem designs, incorporating several different technologies to provide variable-frequency output signals with low phase noise. The frequency synthesizers are suitable for applications in test systems, wireless-communications systems, radar systems, and remote-sensing systems, or wherever microwave receivers require high sensitivity not limited by phase noise.

The frequency synthesizers orchestrate SIL and double-sideband PLL techniques simultaneously, with multiple signal paths within the synthesizers in support of enhanced signal stability as well as application of modulation as needed. Signals are combined within the synthesizer with the aid of a custom-designed, double-balanced frequency mixer and low pass-filter-amplifier (LPFA) assembly. The synthesizer design also incorporates operational-amplifier (op-amp) circuits that work as the phase detector and low pass portion of the PLL.

The high resolution and wavelength-sensitive tuning is due to the fine tuning made possible by an optical transversal filter. The optical filter uses a chirped fiber Bragg grating (CFBG) as a dispersive component to achieve narrowband filtering. A current-tuned YIG filter is used in cascade along with the optical filter and CFBG to provide coarse frequency tuning across wide tuning ranges of X- and K-band frequencies. At X-band frequencies from 9 to 11 GHz, for example, the YIG filter tunes with a response of about 25 MHz/mA. Since the resolution of the current supply feeding the YIG filter is about 1 mA, the effective frequency-tuning resolution of the YIG filter is 25 MHz. This combination of optical and electronic technologies results in relatively wide frequency-tuning ranges with outstanding phase noise, both close in and far from the carrier.

Case in point: at X-band, the single-sideband phase noise is −109.97 dBc/Hz offset 1 kHz from the carrier and −136.45 dBc/Hz offset 10 kHz from the carrier for carrier frequencies from 9 to 11 GHz (Fig. 2). In the time domain, this translates to 4.395 fs measured at sidemode markers of 35 and 200 kHz from the carrier. At higher K-band frequencies, the SSB phase noise is −102.30 dBc/Hz offset 1 kHz from the carrier and −127.37 dBc/Hz offset 10 kHz from the carrier, or time-domain response of 6.961 fs measured at sidemode markers of 35 and 200 kHz from the carrier. The main current consumption in the frequency synthesizer assembly takes place due to the YIG filter, which draws 150 mA at +10 V dc and about 1.5 W power. The amplifier, with two channels, draws 80 to 160 mA currents at +10 V dc and as much as 1.6 W power. The mixer LPFA, which uses a combination of frequency translation and filtering to extract the RF/microwave signals from higher-frequency optical signals, draws about 60 + 5 + 45 mA, or 110 mA current, from respective supplies of +15, +5, and −5 V dc.In stark contrast, the photodetector used in the frequency synthesizer operates at very low current and power, with its three cells each drawing about 10 mA current or 30 mA current at +5 V dc and about 0.15 W power as part of the frequency synthesizer dominated in terms of size and power by the YIG filter. The broadband dual-channel amplifier draws roughly 80 mA current per channel or 160 mA current from a +10-V dc supply, or about 1.6 W total power as part of the frequency synthesizer. In terms of size and power, the YIG filter is the dominant component in these optoelectronic ally driven frequency synthesizers. 

Biography:

(IEEE SM 2005, IEEE Fellow 2015) Ajay Kumar Poddar graduated in Electronics & Communication Engineering from NIT-C (National Institute of Technology Calicut), India; M. Tech (Master of Technology) from IIT-D (Indian Institute of Technology Delhi) India; Doctorate (Dr.-Ing.) from TUB (Technical University Berlin), Germany; Post Doctorate (Dr.-Ing. habil) from BTU (Brandenburg Technical University) Cottbus, Germany. He has received over dozen awards for scientific and technological innovations and meritorious services; holds over two dozen patents in his credits for the scientific invention, published over 250 scientific papers in journals and international conferences, and co-authored 3 technical books. From 1991-2001 he has worked as Senior Scientist in DRDO (Defense research & Development Organization) in India. From 2001, he has been working as a Chief Scientist at Synergy Microwave Corp, New Jersey, USA; responsible for design and development of signal generation and signal processing electronics for industry, medical and space applications. He is also serving as a visiting professor in Oradea University in Romania, Technical University Munich in Germany, and Indian Institute of Technology Jammu in India. In addition to technical and academic contributions, he has been involved for more than 30 years in helping the underprivileged and underserved community.

Email:

Address:Indian Institute of Technology Jammu, Jagti, PO Nagrota, NH-44, Jammu, India, 181221