Function generator with electronic regulation of capacitors
The possibility of creating a frequency-tunable function generator with rectangular and triangular pulses of constant amplitude and electronic synchronous control of equivalent capacitances in the master generator and integrator is shown. Such control is possible due to the use of voltage repeaters in the feedback circuit where capacitors are included through the transmission coefficient regulator.
Wow the engineering world with your unique design: Design Ideas Submission Guide
Well-known functional generators contain a master generator of rectangular pulses where an integrator based on an RC-circuit is connected at the output, in which a resistor plays the role of a stable current generator. This provides a linear charge and discharge of the capacitor with the formation of a triangular-shaped voltage on its plates.
The disadvantage of the classical scheme is that such a generator is capable of operating at only one frequency, since when it changes, the amplitude of the triangular output signal changes noticeably. To ensure the possibility of changing the frequency of such a generator while maintaining a stable amplitude of the triangular output signal, it is necessary to use an amplitude stabilization scheme, or adjust the capacitance of the integrator capacitor synchronously with the change in the generation frequency.
It is possible to solve the problem of synchronous frequency change of the master generator and the corresponding proportional change in the capacitance of the integrator capacitor by using electronic regulation of the equivalent capacitance of its capacitors. The principle of electronic control of the parameters of RLC-elements is described in [1–7]. Such control of the equivalent parameters of the active and reactive elements is possible, for example, through the use of voltage repeaters in the feedback circuit of which these elements are included through the transmission coefficient regulator [7].
If a potentiometer is connected to the output of the voltage repeater, Figure 1, and a capacitor C0 is connected between its sliding contact and the input of the repeater, then the equivalent capacitance of the capacitor Cequ will change from nominal to almost zero when adjusting the potentiometer according to the formula Cequ = C0(1 – Ktrans), where Ktrans is the transfer coefficient from the input of the voltage repeater to the sliding contact to the potentiometer, Ktrans = 0…1.
Figure 1 Electrical scheme of a functional generator with electronic regulation of the equivalent capacitance of its capacitors.
The function generator in Figure 1 contains the RC-generator of rectangular pulses on the operational amplifier U1.1 of the LM348 chip. Signals from the generator output with a frequency of 100–1000 Hz and are fed to the integrating chain R3, C2, in which the high-resistance resistor R3 acts as a stable current generator. A triangular-shaped signal is formed on the plates of the capacitor C2. Further, from the output of the voltage repeater on the operational amplifier 1.3, the signal through the separation capacitor C3 enters the amplification stage on the operational amplifier U1.4.
The equivalent capacitance of capacitors C1 and C2 decreases when the feedback voltage is applied to one of its plates. This voltage coincides in phase with the input voltage and is regulated by a dual potentiometer R6.1 and R6.2.
Michael A. Shustov is a doctor of technical sciences, candidate of chemical sciences and the author of over 750 printed works in the field of electronics, chemistry, physics, geology, medicine, and history.
Related Content
Synthesize variable in-circuit Rs, Ls, and Cs
Electronically-variable capacitor with wide range and high value
Inductance comparator
Rectangle and triangle waveform function generator
References
Miller J.M. “Dependence of the input impedance of a three-electrode vacuum tube upon the load in the plate circuit”. Scientific Papers of the Bureau of Standards, 1920, Vol. 15, No. 351, P. 367–385.
Sheingold D.H. “Impedance and admittance transformations using operational amplifiers”. The Lightning Empiricist, 1964, Vol. 12, No. 1, P. 1,2,7,8.
Gaon J. “Feedback turns fixed capacitor into variable capacity”. Electronics, 1966, Vol. 39, No. 24 (Nov. 28). P. 80.
Korshunov A.I. “Smooth regulation of capacitance of capacitors”. Power Electronics (RU), 2014, No. 4 (49), pp. 36–40.
Plasoianu Gh. “Electronically-variable capacitor with wide range and high value”. EDN. June 20, 2018. https://www.edn.com/electronically-variable-capacitor-with-wide-range-and-high-value
Shustov M.A. “Vernier stable current generator of drip type for the range 10–6…10–11 A”. Radiolotsman (RU), 2018, No. 9, pp. 42–44. https://www.rlocman.ru/shem/schematics.html?di=538827
Woodward S.W. “Synthesize variable in-circuit Rs, Ls, and Cs”. EDN, February 19, 2019. https://www.edn.com/synthesize-variable-in-circuit-rs-ls-and-cs/
Shustov M.A. “Electronic regulators of parameters of RLC-elements”. Radiolotsman (RU), 2023, No. 3–4. https://www.rlocman.ru/review/article.html?di=658565
googletag.cmd.push(function() { googletag.display(‘div-gpt-ad-native’); });
–>
The post Function generator with electronic regulation of capacitors appeared first on EDN.
