Increasing Modulation Frequency Research Articles

Modulation thresholds and temporal modulation transfer functions

Temporal Modulation Transfer Functions (TMTF's) were obtained by measuring the threshold amplitude of sinusoidal modulation as a function of modulating frequency. For modulation frequencies below approximately 800 Hz, TMTF's obtained with a continuous wide-band noise carrier generally show the low-pass characteristic reported previously [J. Acoust. Soc. Am. 53, 314(A) (1973)], That is, with increasing modulation frequency the amplitude of modulation required for threshold remains constant up to approximately 10 Hz and then increases monotonically up to 800 Hz. The interpretation is that at high modulation frequencies the auditory system temporally “smooths” the amplitude fluctuations produced by modulation and the observer therefore requires greater modulation amplitude at the input in order to detect the modulation. For modulation frequencies greater than 800 Hz, modulation threshold is independent of modulation frequency and can be predicted from the increment threshold for wide-band noise. The form of the empirical TMTF generally agrees with that predicted by the familiar model consisting of half-wave rectification followed by “leaky integration.” The time constant of the integrator is estimated to be 3 msec. [Research supported by NIH.]

Dynamic properties of Renshaw cells: Responses to sinusoidal variation of input frequency

Sinusoidally modulated trains of impulses (frequency modulation) were applied to cut ventral roots of intercollicular decerebrate cats in order to determine the frequency response of Renshaw cells. On the average, the gain of the interneurons rose slowly with increasing modulation frequency (about 5 dB/decade), while the phase lag became greater. Gain was proportional to generator modulation depth. Negative feedback by way of Renshaw cells with the characteristics revealed by our experiments would explain the comparative lack of rate sensitivity in spinal alpha motoneurons.

Profiles of the calcium resonance line emitted by a modulated hollow cathode lamp

Profiles of the 422·7 nm calcium line emitted by an electronically modulated hollow cathode lamp (50 % duty cycle, 20 mA square wave current) are measured as a function of modulation frequency. The optical system provides for resolution in time (60 μs) wavelength (3 × 10 −4 nm) and space (0.5 mm). Self-reversal and the line width increase with radial distance from the center of the cathode bore, increase with time during the pulse and decrease with increasing modulation frequency (up to 6400 Hz). Up to 1600 Hz, electronic modulation yields narrower and more intense lines than d.c. operation combined with optical chopping, In an appendix it is shown that the apparent line profile observed with the aid of a lock-in amplifier may change with the phase adjustment of the amplifier.

Measurement of Ferromagnetic Relaxation by a Modulation Technique

A modulation method is introduced as a novel means of studying directly the relaxation process in the case of ferrimagnetic rare earth garnets and ferrites. The method directly measures the relaxation of the spin system of the material which is indentified with the transverse relaxation time by means of the standard equations of motion. A low-power fixed frequency X-band microwave field is amplitude modulated at an rf rate up to 50 Mc while maintaining the sample in ferromagnetic resonance. The resulting precession of the magnetic moment will then contain an rf component which is a function of both the transverse relaxation time and the frequency of the rf modulation. The precession is observed by a single turn pickup loop in which the induced microwave voltage is proportional only to the rotating component of magnetization. The rf signal resulting from demodulation of this voltage then characterizes the response of the precession to the modulation frequency of the driving microwave field. The falloff in this response with increasing modulation frequency is shown to be related to the transverse relaxation time by means of the equations of motion. The predicted form of this falloff is a function proportional to the factor (1+β2τ2)−12 from which the relaxation time τ is obtained. Certain conclusions may be drawn from the excellent agreement of the experimental data with theory.

Backwater

In this paper, the improved analysis for SRS- and XPM-induced crosstalk has been reported. The modified expression for XPM-induced crosstalk has been obtained and SRS- and XPM-induced crosstalks have been reported at varied walkoff parameter, modulation frequency, input optical power and transmission distance. It has been observed that there is exponent decrease in SRS-induced crosstalk with the increase in modulation frequency from 0 to 2.0 GHz. It varies with the increase in length and lie in the range of (−114 to −122.4) dB and (−115.5 to −124.4) dB at 20 and 100 km, respectively. Moreover, it increases exponentially with the increase in input optical power and lies in the range of (−121.6 to −130.6) dB at 10 mW and grows exponentially up to the range of (−114 to −122.8) dB at 60 mW optical powers at walkoff parameter of (13.6, 27.2, 54.4 and 81.6) ps/km. It has been observed that the XPM-induced crosstalk increases exponentially with the increase in transmission distance and modulation frequency for 2OD and 3OD. Furthermore, it has been found that the total SRS- and XPM-induced crosstalk rises exponentially with fluctuations with the increase in modulation frequency and transmission length in the presence of combined effect of 2OD and 3OD at varied walkoff parameters.