Colin G. Englefield

Electrical measurement of the junction temperature of an RF power transistor

An enhanced electrical method is presented for measuring the average junction temperature of an RF bipolar transistor. A two-step procedure, previously developed for DC operation, is extended to include the junction temperature measurement for an RF power transistor in a tuned amplifier circuit. The measurement technique is convenient, since it can be used with normal, packaged devices, and does not require a complex heat flow model or ambient temperature measurements. >

Power loss, modal noise, and distortion due to microbending of optical fibers

Theoretical and experimental investigations are described for determining the transmission characteristics of a multimode fiber with microbending for coherent and partially coherent illumination. The measured values of the average excess power loss are shown to be in close agreement with the theory. Also, an estimate of the excess transient loss due to mode coupling is found to be in good agreement with previously published data. Mode-mode interference is shown to be the cause of temporal fluctuations in the microbending loss, from which expressions for modal noise and baseband/subcarrier nonlinearity are derived on a statistical basis. For a given overall loss, the results show that many uniformly distributed small amplitude microbends cause much less modal noise and distortion than a few large amplitude microbends.

Modal noise and distortion caused by a longitudinal gap between two multimode fibers

The modal noise and distortion caused by a longitudinal gap between two multimode parabolic-index fibers are discussed and quantified by means of modal analysis. It is found that, for coherent illumination and a given loss, the modal noise and distortion due to a gap are generally as severe as or worse than those due to an axial offset between two fibers. It is also shown that, when a partially coherent source is used, the transmission characteristics of a gap can be improved by increasing the operating wavelength. Experimental results have been obtained for modal distortion and are shown to be in close agreement with the theory.

Real-time monitoring of laser diode facet reflectivity while being coated with SiOx

A simple real-time technique is described for monitoring the change in reflectivity of a laser diode facet while evaporating silicon monoxide onto it. The output power is studied as a function of coating thickness. (AIP)

Intermodulation Distortion in Microwave MESFET Amplifiers

The third-order intermodulation distortion (IM/sub 3/) of a MESFET amplifier has been analysed using Volterra series representation. A model is described that takes into account the MESFET nonlinearities and their interaction with the surrounding microwave circuit. A theoretical and experimental study of the amplifier intermodulation products has been conducted. Their dependence on the input frequency and power-level of a two-tone test signal is investigated. For power inputs less than -10 dBm, the agreement obtained between measured and predicted IM/sub 3/ is within 3-dB over an amplifier bandwidth of 400 MHz.

Adaptive digital linearization of RF power amplifiers

The performance of an adaptive digital technique for the linearization of RF power amplifiers is investigated. Cubic spline interpolation is used to estimate the amplifiers AM-AM and AM-PM characteristics. Using the computed characteristic coefficients, the baseband input signal is appropriately predistorted to compensate for the amplifier nonlinearity. This method has significantly better suppression of the intermodulation products than other predistortion techniques. The out-of-band power emission is also significantly reduced. The performance of baseband predistortion linearization techniques is adversely affected by modulator and demodulator impairments. A digital correction technique is presented to compensate for these imperfections. In this technique, part of the RF signal is fed to an envelope detector. The detector output and the baseband signal are used to estimate the impairment values, using the Newton-Raphson method. The estimated impairment values are then used to compensate for the modulator/demodulator impairments. Spurious signals can be suppressed by more than 30 dB using this technique.

Up-Conversion in IMPATT Amplifiers

A simple technique has been presented to determine the up-conversion characteristics of IMPATT-diode amplifiers. The method uses the admittance characteristics of the device and the circuit of the amplifier. Theoretical results are given for a typical example and a comparison is made with experimental results obtained for actual amplifiers. Qualitative agreement is obtained.

Real-time implementation of a decision-aided error correction algorithm for digital cellular radio

A decision-aided error correction algorithm is described and evaluated. The receiver based on this algorithm is tested experimentally in real time using a digital signal processor. Its performance is evaluated for a signal based on the North American cellular standard, transmitted over a Rayleigh flat fading channel. The test results show that the algorithm significantly reduces the irreducible error floor. Under adequate SNR conditions, this method of error correction is quite attractive compared to other techniques such as coding, insertion of pilot symbols, or the diversity method, because it requires no overhead bits. Also, since the algorithm does not require any modification to the frame structure of the signal, it may be preferred if the transmitting signal format has already been standardized.

Performance of an optical DFSK system using optical self-heterodyne detection

The performance of an experimental 45 Mb/s optical differential frequency shift keying (DFSK) system using optical self-heterodyne detection (OSHD) has been evaluated. In OSHD, the intermediate frequency is obtained by mixing the received optical signal with its delayed version. Therefore, in such a receiver, an optical local oscillator is not required, which greatly simplifies the receiver design. It has further been demonstrated that a DFSK system can use transmitting laser diodes with linewidths up to 0.5 times the bit rate. A two-channel DFSK system has also been studied; the two channels were wavelength division multiplexed using an optical coupler. The bit error rate as a function of received optical power has been measured for a one-channel and a two-channel DFSK system.

Microbending dependence of phase in single-mode fibers.

The dependence of the radio-frequency phase shift on microbending loss for single-mode fibers has been investigated theoretically. Using representative source and fiber cable parameter values, the frequency stability of a prototype fiber link is found to be about 10(-16) for an averaging time of 10(3) sec.