G. Sarafian

Biological applications of the “Filtered” Van der Pol oscillator

The present article deals with the development of an oscillatory model, which generates waveforms corresponding to ECG patterns. The present oscillatory system relies on coupling of oscillators derived from the famous VDP oscillator. We demonstrate that inducing a relaxation type of dynamics in the models contributes to their successful generation of ECG like signals. Furthermore, an interesting affinity is found, which associates the present models with a version of the well-known practical Wien Bridge oscillator. The presently discussed system relies on coupled elementary oscillator units. The present coupling is due to merely two units. The model, however, is likely to become even more realistic by coupling in the same manner an assembly of relatively many oscillators.

Is the Colpitts oscillator a relative of Chua's circuit?

The topological resemblance of the Colpitts oscillator to that of Chuas circuit suggests that the two circuits may also be similar in their dynamic behavior. The similarity in the dynamics is demonstrated in the present communication by resorting to computer simulation. We deal in particular with the unknown chaotic behavior of the Colpitts oscillator. It is shown by referring to recently published articles that the Colpitts oscillator and Chuas circuit are in fact topologically conjugate one to another. >

A new approach to the modeling of the dynamics of RF VCO's and some of its practical implications

An approach to the modeling of practical radio frequency (RF) voltage controlled oscillators (VCOs) is presented. The usual VCO analysis employs a second-order model. The authors resort to a third-order representation that is more accurate mathematically. The representation is practically advantageous because practical RF VCOs are based on the Colpitts circuit, which is a third-order system. The model is investigated using an approximate linear approach, which is then improved through nonlinear analysis based on the harmonic balance method. The analyses reveal the existence of a relatively unknown mode of a shifting bias superimposed on the oscillation, which may cause interference at the system output. Furthermore, the analytical treatments bring forth a recommendation for a useful procedure of changing the oscillator parameters in attempting a frequency change. This leads to a major improvement of the circuit performance, when employed as a VCO. The analytical and simulation findings are supported favorably by experimenting with a practical circuit. >

Magnesium administration may improve heart rate variability in patients with heart failure

BACKGROUND AND AIM Intracellular magnesium (icMg) depletion may coexist with normomagnesemia. Mg deficiency (serum and/or intracellular) and decreased heart rate variability (HRV) are common in heart failure (HF). Since both are predictors of poor prognosis, it was of interest to evaluate the effect of Mg supplementation on HRV in patients with HF. METHODS AND RESULTS We investigated the effect of Mg administration on HRV in normomagnesemic patients with systolic HF. HRV, serum Mg and icMg were determined before and after 5-week 300 mg/day Mg citrate treatment in 16 patients (group 1). The control group included 16 Mg-non-treated HF patients (group 2). HRV was determined by a non-linear dynamics analysis, derived from the chaos theory, which calculates HRV-correlation dimension (HRV-CD). After 5 weeks, serum Mg (mmol/l) increased more significantly in group 1 (from 0.78+/-0.04 to 0.89+/-0.06, p<0.001), than in group 2 (from 0.79+/-0.07 to 0.84+/-0.06, p=0.042). IcMg and HRV-CD increased significantly only in group 1 (from 59+/-7 to 66+/-9 mmol/g cell protein, p=0.025, and from 3.47+/-0.42 to 3.94+/-0.36, p<0.001, respectively). In group 2, the differences in the respective parameters were 63+/-12 to 66+/-9 mmol/g cell protein (p=0.7) and 3.59+/-0.42 to 3.55+/-0.4 (p=0.8). CONCLUSION Mg administration to normomagnesemic patients with systolic HF increases serum Mg, icMg and HRV-CD. Increasing of HRV by Mg supplementation may prove beneficial to HF patients.

Application of HRV-CD for estimation of life expectancy in various clinical disorders

BACKGROUND Low heart rate variability (HRV) was found in various medical conditions including heart failure and acute myocardial infarction. Decreased HRV in these conditions predicted poor prognosis. METHODS HRV was estimated in 133 unselected inpatients with relevant clinical bedside conditions by non-linear analysis derived from chaos theory, which calculates the correlation dimension (CD) of the cardiac electrophysiologic system (HRV-CD). RESULTS Mean HRV-CD in the entire group was 3.75+/-0.45. Heart failure, coronary artery disease, cardiac arrhythmia, low serum potassium, renal dysfunction, and diabetes mellitus were significantly associated with reduced HRV-CD compared to their counterparts [3.6 vs. 3.9 (P<.001), 3.65 vs. 3.87 (P=.005), 3.58 vs. 3.8 (P=.01), 3.38 vs. 3.81 (P=.02), 3.59 vs. 3.8 (P=.04), and 3.66 vs. 3.82 (P=.04), respectively]. Stepwise logistic regression showed heart failure to be the condition most significantly associated with low HRV-CD (odds ratio 4.2, 95% confidence interval 1.90-9.28, P<.001). In the entire group, decreased HRV-CD (< or =3.75 vs. >3.75) was associated with lower survival (P=.01). Mortality of diabetic patients with HRV-CD < or =3.75 exceeded the mortality in patients with HRV-CD >3.75 (P=.02). Heart failure, renal dysfunction or age over 70 combined with HRV-CD < or =3.75 also appeared to be associated with augmented mortality. CONCLUSIONS Diminished HRV-CD is associated with heart failure, coronary artery disease, cardiac arrhythmia, renal dysfunction, diabetes mellitus and low serum potassium. Among the latter, heart failure is most significantly associated with decreased HRV-CD. Decreased HRV-CD values, especially in diabetics, are also associated with lower survival.

The dynamics of parametric frequency divider and some of its practical implications

The dynamics of a parametric frequency divider is treated rigorously. The complicated process of subharmonic generation is shown to be related to the Mathieu equation. Due to its similarity to the well known RL diode circuit, the system is capable of generating chaos as well as subharmonics. Hence, the circuit can not merely be regarded as a core for constructing frequency synthesizers but it can probably serve as a noise generator as well. Furthermore, recently suggested methods of chaos control may enable the output to be switched rapidly between different types of subharmonics modes of operation.

ON THE QUESTION OF DISSIPATION AND PARAMETRIC EXCITATION IN CHAOTIC NONAUTONOMOUS RESONATORS

Three categories of second order resonators are treated in this work: A. Nonautonomous driven resonators whose reactors are nonlinear and resistances are positive. (The resonators are similar to the RL Diode circuit). B. Nonautonomous driven resonators whose reactors are linear (and maintained constant in time) and whose dissipation is due to a nonlinear resistor which consists of a negative resistance part. C. Autonomous resonators whose reactors are parametrically excited. (The reactances of the resonators of category C are made directly periodically time varied). The introduction of category C is mainly for auxiliary purposes, in order to bridge difficulties that occur in comparing resonators of categories A and B. Resonators of both categories A and B demonstrate similar dynamic behaviors, since both are capable in certain circumstances of exhibiting bifurcation phenomena and chaos. This demonstrated similarity is puzzling. This is due to the fact that the resonators of category A are fully dissipativ...

A Method of Constructing a Pair of Pseudorandom Sequences with a Useful Crosscorrelation Function

The synthesizing of pseudorandom signals which possess an odd crosscorrelation function of useful characteristics is presented. The characteristics are applicable for signal tracking systems such as those associated with ranging instrumentation and spread-spectrum communication. Another property of these signals is that they possess a zero dc component which may be applied for radiating useful carrierless signals.

Is it feasible to observe neutron stars via their direct mode of magnetic-dipole radiation?

SummaryThis communication deals with the observation issue of neutron stars directly via their magnetic-dipole radiation. The bremsstrahlung absorption phenomenon is introduced in this context, although the observation of neutron stars via their magnetic-dipole radiation has already been treated in the literature for more than two decades. The evaluation of the bremsstrahlung absorption indicates that the absorption coefficient is relatively large. As a result, the magnetic-dipole radiation of the prospective neutron stars, from any reasonable distance is strongly attenuated. Therefore, we conclude that the observation of neutron stars via their low-frequency magnetic-dipole radiation is apparently not feasible. Although the bremsstrahlung was already treated by Lipunov, our present conclusion differs significantly from his results. The first part of the article reviews comprehensively the relatively long history of the idea of detecting neutron stars through their direct mode of magnetic-dipole radiation.