R. Saedi

Theory of subharmonic synchronization of nonlinear oscillators

General expressions for the theoretical prediction of the subharmonic injection-locking range of oscillators with a nonlinear input-output relationship are presented. This analysis is extended to describe oscillators realized using real nonlinear input voltage and output current relationships. In particular, analytical results for an oscillator represented by a fifth-order polynomial describing a nonlinear current-voltage relationship for subharmonic factors of 2, 3, and 4 are given. Experiments relating the subharmonic locking range of a field-effect transistor oscillator at 18 GHz to the injected signal level are reported.<<ETX>>

High-speed fiber-optic links for distribution of satellite traffic

Low-loss fiber-optic links are designed for distribution of data and the frequency reference in large-aperture phased-array antennas based on the transmit/receive-level data mixing architecture. In particular, design aspects of a fiber-optic link satisfying the distribution requirements of satellite data traffic are presented. The design is addressed in terms of reactively matched optical transmitter and receiver modules. Analog and digital characterization of a 50-m fiber-optic link realized using these modules indicates the applicability of this architecture as the only viable alternative for distribution of data signals inside a satellite at present. It is demonstrated that the design of a reactive matching modules enhances the link performance. A dynamic range of 88 dB/MHz was measured for analog data over 500-1000-MHz bandwidth. >

Phase control of optically injection locked oscillators for phased arrays

The most suitable architecture for millimeter-wave frequencies is based on the transmit/receiver (T/R)-level data mixing architecture, where a frequency reference is provided to local oscillators in the subarrays to have them frequency and phase synchronized. The indirect subharmonic optical injection locking has the benefit of a high degree of frequency synchronization up to millimeter-wave frequencies; however, the local oscillators suffer from phase inaccuracy over the locking range, first formulated by R. Adler (1946). A scheme is proposed to measure this phase error and correct for it by adjusting the free-running oscillation frequency of a voltage controlled oscillator (VCO). Experiments supporting this approach are reported for two optically injection locked oscillators at 18 GHz, where controlled phase shifts over -90 degrees to 78 degrees are achieved by adjusting the bias current to a yttrium-iron-garnet (YIG) tuned VCO.<<ETX>>

Comparison of CPU level data mixing to T/R level data mixing architectures in optically controlled phased arrays

A performance comparison of a CPU level data mixing architecture and a T/R (transmit/receive) level data mixing at C-band is presented. The fiberoptic link used for transmission of the modulated carrier at 5.65+or-0.15 GHz has demonstrated insertion loss of approximately=11 dB, a compression dynamic range of 72 dB-MHz, and a spurious free dynamic range of 53 dB-MHz/sup 2/3/. The achieved results for the CPU level data mixing architecture using InGaAsP/InP-based components are higher than the results obtained for the AlGaAs/GaAs-based components for the T/R level data mixing architecture. This difference is a result of advances in the 1300 nm based components as opposed to the components available at the obsolete wavelength of 850 nm. In fact, when a T/R level data mixing architecture was constructed using the same type of devices used in the CPU level data mixing, a higher dynamic range was attained.<<ETX>>

Optically controlled K-band oscillator

PIN photodiodes play an important role in detection of modulated light in fiberoptic links, however, these devices are essentially junction devices, with bias-dependent capacitance very similar to those of varactor diodes. Particularly under proper biasing conditions, junction capacitance of these devices can be changed by light illumination. Results are reported of tuning and frequency modulation experiments of a 24.5-GHz oscillator configured with a PIN photodiode as the tuning element. Both electrical and optical tuning were investigated, resulting in tuning range of few hundred megahertz at K-band.<<ETX>>

Fiber optic fed C-band active phased array antennas

Experimental results are presented demonstrating the operation of optically controlled phased array antennas at C-band. More specifically, experimental results on optically controlled 2*4 MMIC (monolithic microwave integrated circuit) based active T/R (transmit/receive) modules over the frequency of 5.5 to 5.8 GHz are presented. Custom-designed fiber-optic links have been used to provide distribution of data and frequency reference signals to phased array antennas used in the T/R Level Data Mixing architecture system.<<ETX>>

Recirculating fiberoptic link for memory loop

Fiber-optic links can be used as delay elements in microwave frequency memory loops. The authors present the analysis and experimental results of a recirculating memory loop operating over 2-4 GHz, leading to a 100 mu sec delay. The reactively matched optical transmitter and actively matched optical receiver are designed to achieve optimum loop performance. New gain equalization techniques are discussed which permit a long time delay in the range of milliseconds. The experimental results demonstrate recirculation of pulses at microwave frequencies in the range of sub-milliseconds. The analytical calculations indicate that, for a fiber-optic based memory loop, the total recirculation time for a flat frequency response reduces from 2300 to only 32 with a gain flatness of +or-1 dB, where a minimum 10 dB signal-to-noise ratio is specified.<<ETX>>

High-speed digital fiber optic links for satellite traffic

Large aperture phased array antennas operating at millimeter wave frequencies are designed for space-based communications and imaging platforms. Array elements are comprised of active T/R modules which are linked to the central processing unit through high-speed fiberoptic networks. The system architecture satisfying system requirements at millimeter wave frerquency is T/R level data mixing where data and frequency reference signals are distributed independently before mixing at the T/R modules. This paper demonstrates design procedures of a low loss high-speed fiberoptic link used for transmission of data signals over 600-900MHz bandwidth inside satellite. The FO link is characterized for transmission of analog and digital data. A dynamic range of 79dB/MHz was measured for analog data over the bandwidth. On the other hand, for bursted SMSK satellite traffic at 220Mbps rates BER of 2x10-7 was measured for Eb/No of 14.3dB.

Fiberoptic based recirculating memory loop

The authors discuss the design of a low-loss, low-noise figure, fiber optic delay element at 1300 nm, capable of recirculating a short electrical pulse as long as a millisecond over the 2-4 GHz frequency band by using an adaptive gain equalizing technique. While the RF pulses are recirculating in the system the noise level starts to build up. By looking at the spectrum of the output signal the effect of the recirculation on the spectral purity of the microwave signals can be studied. The spectral purity characteristics of recirculated signals were quantified for various recirculation times.<<ETX>>

Optical Control of MMIC Based Phased Array at C-Band

Experiments performed on a 2×4 active MMIC based phased array antenna system are described. The active MMICs operating at C-band are controlled and fed via fiber optic links. The T/R level data mixing scheme for up-conversion (down-conversion) of the IF (RF) to RF (IF) is demonstrated. The phased array at C-band is characterized in an anechoic chamber. Optical true-time delay measurements to overcome beam squint are proposed.