An improved CAD procedure for oscillator circuit design using fast-approximation formulas describing the voltage-dependent non-linear characteristics of the intrinsic equivalent circuit elements of the active devices

Abstract

This article presents a selection of the calculated and measured results of an oscillator circuit using two source-coupled junction field-effect transistors (JFETs), which are investigated at a number of different operating points. These results illustrate vividly some strange but typical problems of the oscillator design, such as "spurious" and "wrong" or "jumping" oscillation frequency.

Oscillator design for suppressing spurious signals

“…These equations are inserted here and produce a good forecast of the electrical behavior of the oscillator circuit with regard to the common operating voltage and the values of several necessary resistors in the oscillator circuit, which determine the different operating points of the active devices, respectively. The dc circuit is shown in Figure 11 [6] and clarifies the physical context of the dc quantities. Due to the voltage drop (V r ) at the drain resistor (R D ), the dc values of the two drain-source-voltages (V ds1 ) and (V ds2 ) are always different, whereas those of the two gate-sourcevoltages (V gs1 ) and (V gs2 ) are always the same.…”

“…The main reason for this is the severe voltage dependence [4], [5] of the intrinsic transistor equivalent circuit elements, which represent the physical properties of the semiconductor substrate. To get a proper description of the nonlinear course of the characteristic parameters of the JFET's intrinsic equivalent circuit elements, a number of measurements for a wide range of appropriate values of the drain-source-voltage and the gate-sourcevoltage and for different values of the concerning resistors must be done [6], [7]. Then, the measured and calculated values are approximated by fast computable formulas.…”

“…To determine the context of the particular values of the prevailing dc quantities, an adequate number of measurements had to be done with varying values of the supply voltage (V C ) and the two resistors (R D , R S ), respectively [7]. The details of the measurement setup and the approximation formulas are given in [6] and [7]. For completeness, some fundamental formulas from [6], using a proven curve fitting method [13] to obtain fast computable approximation formulas from the measured values, are repeated here.…”

Conversion matrix extraction by commercial CAD software for the stability design of autonomous circuits

“…In this way, it is possible to evaluate the matrix representation of the reflection coefficient as in (3).…”

“…As said, in this case spurious frequencies appearing only when the oscillating signal becomes large cannot be detected. When the large-signal behavior of the oscillator is investigated, harmonic balance or time domain analysis are the usual approaches [2][3][4][5]. In the former case, a probing technique allows the detection of a spurious frequency, if it is present [6].…”