Order Intermodulation Distortion Research Articles

Nonlinear behavior of thin film SrTiO3 capacitors at microwave frequencies

The voltage-dependent dielectric constant (ε) of SrTiO3 (STO) thin films is the basis for developing cryogenic capacitors for tunable microwave applications. In this study, the effect of microwave signal level on nonlinear response at 1.7–1.9 GHz was examined by measuring the level of the third order intermodulation distortion (IMD) signal relative to the input signal level. Small signal dielectric properties such as capacitance, tuning, and loss (tan δ) were also measured at 1 MHz, 3 GHz, and 10 GHz, at temperatures from 4.2 to 300 K. Planar capacitors were comprised of highly (100)-oriented, 1 μm thick STO films deposited via magnetron sputtering onto CeO2-buffered (11_02)-oriented sapphire substrates, with 10 μm gaps between the electrodes. Deviations from the anticipated cubic dependence of the third order IMD product on incident power, for incident power ranges from −10 to 22 dBm, were attributed to conductivity nonlinearity. At incident power levels of 22 dBm and with no dc bias applied to the capacitor, the level of the third order IMD product was 21 dB below the fundamental signal level. Application of a 107 V/m dc electric field bias across the capacitor suppressed the third order IMD by an additional 10 dB. The nonlinear properties of thin film STO capacitors as a function of microwave voltage were determined by comparing the experimental and theoretical dependencies of the IMD products.

Calculating the CSO/CTB spectrum of CATV amplifiers and optical receivers

The discrete CSO/CTB intermodulation distortion has a big influence on the CATV net quality. This article describes a new method to calculate the composite second order (CSO) and composite triple beat (CTB) intermodulation distortion spectrum for the full frequency range of a coaxial cable CATV amplifier for any input frequency plan. The calculation takes into account the output level ripple and preemphasis, using frequency dependent intermodulation coefficients. These coefficients have been established through CSO/CTB measurements on multichannel test systems for standard frequency plans at 5-8 intermodulation frequencies or on distortion arrays for the two (three) oscillator measuring test system. The defined intermodulation coefficients are unrelated to the frequency plan and the input/output level ripple. It is also possible to calculate the CSO/CTB spectrum caused by analogue channels within digital QAM channels. The results equally apply to optical CATV receivers. Simulation results have been verified using extensive data from multichannel measurement systems.

Nonlinear properties of SrTiO3 films at microwave frequencies

Third order intermodulation distortion (IMD) measurements were performed on SrTiO3 thin film capacitors at frequencies of 1.7–1.9 GHz. The output power of the third order IMD product was 30 dB below that of the output fundamental signal when the level of the input signals were 0 dBm. Upon increasing the input signal levels to +22 dBm, the difference between the output fundamental and third order IMD signals dropped to 20 dB. Applying a DC bias to the varactors decreased the level of the third order IMD substantially; with a 100 volt bias, the third order IMD product was 31 dB lower than the output level of the fundamental signal.

Effect of bias and load on MESFET nonlinear characteristics

An investigation of the variation of measured and simulated nonlinear characteristics of a MESFET with bias and load resistance is reported. A discrete MESFET exhibits simultaneous low 2nd and 3rd order intermodulation distortion while yielding high gain and operating in its low noise region.

Theoretical and experimental study of second-order distortions in CATV DFB laser diodes

Calculation of linearity performances of a given DFB laser is a well-known problem when working with CATV AM-VSB optical links. In this paper, a nonlinear model of the DFB laser is presented. The bias and frequency dependence of second order intermodulation distortion is analyzed by combining Volterra models of the laser leakage current and rate equations. The prediction of the model is then compared to distortion measurements with a maximum discrepancy of around 1.5 dB. >

Analysis of the second- and third-order intermodulation distortion in DFB lasers including dynamic spatial hole burning effect

Analysis of second and third order intermodulation distortion characteristics of semiconductor DFB lasers is, for the first time, performed, including dynamic longitudinal spatial hole burning (LSHB) effect. We have shown theoretically that the third order intermodulation distortion can be lower than the calculated curve without LSHB effect, which is confirmed with experiments.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Signal distortion and noise in AM-SCM transmission systems employing the feedforward linearized MQW-EA external modulator

In subcarrier multiplexing (SCM) optical video distribution systems, the nonlinear signal distortion generated by the combined action of laser chirp and fiber dispersion limits the transmission distance. This paper for the first time shows that low-chirp MQW (multiquantum well)-EA (electroabsorption) external modulators are applicable to AM-SCM transmission systems. The feedforward linearization technique is used to compensate the signal distortion due to the nonlinearity in the modulator's L-V (light power versus voltage) characteristic. The effectiveness of this type of modulator for suppressing dispersion-induced distortion is clarified experimentally. A feedforward linearized transmitter with composite second order (CSO) intermodulation distortion 50.5 dB is successfully constructed. It is shown that the transmitter can achieve a CSO 49.5 dB for a 32 channel AM signal and 10 km transmission. Furthermore, estimations of the signal distortion cancellation and the noise characteristic achieved with feedforward circuit modeling are shown. The results obtained here emphasize that MQW-EA external modulators are applicable to AM-SCM transmission systems. >

High modulation frequency low distortion 1.3 μm MQW-DFB-LDs for subcarrier multiplexed fibre-optic feeder systems

A remarkable improvement has been attained for intermodulation distortion and spectrum linewidth performance for 1.3μm DFB LDs by introducing thin barrier MQWs and short cavity configurations. Less than −85dBc in 3rd order intermodulation distortion and more than 50MHz in spectral linewidth have been simultaneously obtained from devices modulated by two 1.5 GHz signals.

Mechanisms determining third order intermodulation distortion in AlGaAs/GaAs heterojunction bipolar transistors

The third-order intermodulation distortion (IMD3) mechanisms of heterojunction bipolar transistors (HBTs) are analyzed using Volterra series theory. A T-equivalent circuit is used for the large-signal model of the HBT. The third-order nonlinear currents generated by the device nonlinearities are evaluated for this purpose and current cancellation is discussed. It is found that, even though the C/sub je/ and g/sub je/ related currents do not show pronounced cancellation, the total base-emitter current and the total base-collector current cancel partially. Second harmonic loading is addressed in view of IMD3 optimization while at the same time maintaining high gain through conjugate matching at the fundamental frequency. IMD3 is very sensitive to the nonlinear currents generated by g/sub je/ and alpha . Optimum IMD3 occurs at high second-harmonic reflection coefficients corresponding to open load conditions. Minimum and maximum IMD3 occurs for second-harmonic load reflection coefficient phases close to analogous extremes of the dominant nonlinear current of the device.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Effect of varying threshold gain on second order intermodulation distortion in distributed feedback lasers

Bias-dependence of second order intermodulation distortion (IMD2) of distributed-feedback laser-diodes (DFB-LD) has been investigated both theoretically and experimentally. A new analytical model proposes that varying the threshold gain is responsible for the bias-dependence of IMD2. The experimental behaviour is explained using this theory.

Digital and quasi-linear electrooptic modulators synthesized from directional couplers

By synthesizing from cascaded directional couplers, we propose new designs to achieve digital and quasi-linear electrooptic modulations. For the digital modulator, a nearly perfect on-state or off-state can be obtained in a broad range of modulation voltages. The calculated switching voltage can be as low as the drive voltage of a conventional directional coupler. The quasi-linear modulator is achieved by adding a set of compensating electrodes to a foreshortened directional coupler. Deviation from a linear voltage-intensity relationship of less than 1 percent can be obtained over a 93.5 percent modulation depth. The calculated second harmonic and the third order intermodulation distortions are -18.3 and -32.6 dB, respectively.

Intermodulation distortion in a directly modulated semiconductor injection laser

A most important quantity in high-frequency analog transmission is the intermodulation distortion product. Experimental studies of the third order intermodulation distortion products in the modulation response of high-speed semiconductor lasers give very low values (&amp;lt;−60 dB) at low frequencies, an increase at a rate of 40 dB/dec as the modulation frequency is increased, and a leveling off at one-half of the relaxation oscillation resonance frequency. These experimental results can be well explained by a theory based on a perturbative analysis of laser dynamics.

Investigation of Intermodulation in a Tuning Varactor

2nd and 3rd order intermodulation distortions of a tuning varactor are analyzed by using nonlinear parameters P2 and ¢3 which are determined by the measured values of the diode capacitance. The calculated values by using ¢agree quite well with the experimental results for actual diodes. In addition, the methods of compensating the distortion by a fixed capacitor or another diode with appropriately divided bias voltage are discussed and it is shown that the nonlinearity is compensated even in the case of n greater than 0.5. These methods are particularly effective for a circuit with greater Q.

Intermodulation distortion test

Errors in test results are minimized in a test system (FIG. 1) employing digital data acquisition units (121) when measuring intermodulation distortion of a voice frequency communication facility (105) by employing a unique 3-tone test signal. To this end, the three tones have predetermined amplitude, frequency and phase relationships to obtain a test signal having a desired optimum probability density function which is substantially Gaussian. Additionally, circuit noise components are eliminated from the test results by obtaining a measure of noise with a single tone test signal and subtracting it out from the three tone test results. In a specific embodiment, a measure of the power spectrum resulting from the three tone signal and the power spectrum of the single tone signal are obtained, combined and utilized to obtain measurements of second and third order intermodulation distortion products compensated for noise.

Radiation Effects on Distortion Characteristics of Power GaAs MESFET Amplifiers

A nonlinear MESFET model that explains the degradation in power GaAs MESFET amplifier third order intermodulation distortion (IM3) characteristics following neutron irradiation is presented. The model verifies the experimental data and shows that the significant increase in IM3 observed at a neutron fluence of 1.4 × 1015 n/cm2 and higher is mainly due to the non-linearity in device transconductance gm and output conductance go. The model also indicates that IM3 is dominated by the device gm or go non-linearity depending on applied gate bias.

A System for Automatically Recording Third and Higher Order Intermodulation Distortion Components

A problem arises in measuring certain intermodulation distortion components when measurements are made at more than one set of input frequencies. When two input frequencies with a constant frequency difference between them are swept through the frequency range of interest, only some intermodulation components remain fixed in frequency. Others. which may be of significance, particularly in push-pull systems, change in frequency as the input frequencies are varied. A system is described in which some of these variable frequencies may be tracked by a slightly detuned reference frequency, which is derived by suitable heterodyning of the radio-frequency signals of two beat frequency audio oscillators. This signal serves as the reference signal in a non-phase-sensitive synchronous detector, the output of which is filtered, detected, and automatically recorded. (This work was done under the sponsorship of the Veterans Administration.)