Vackár oscillator
Encyclopedia
A Vackář oscillator is a variation of the split-capacitance oscillator model. It is similar to a Colpitts oscillator
or a Clapp oscillator
in this respect. It differs in that the output level is relatively stable over frequency, and has a wider bandwidth when compared to a Clapp design.
In 1949, the Czech
engineer Jiří Vackář published a paper on the design of stable oscillators. As a result of his in-depth analysis of (vacuum tube
) oscillators, he proposed a variant of a Colpitts oscillator, where an additional capacitive voltage divider on the grid input reduces the feedback voltage to the necessary amount, and at the same time reduces the unwanted, unstable tube capacitances.
The schematic is the equivalent of Fig. 5 in his paper, redrawn for the use of a junction FET
. L1 with C0 and Ca form the resonant circuit of a Colpitts oscillator, and Cv/Cg is the grid voltage divider. The circuit can be tuned with C0. Example values are from his paper.
It is similar to an earlier Seiler oscillator, the difference is that in Seiler's the C0 is connected to the other side of Ca. Vackář based his design on stability analysis of Gouriet-Clapp
(Vackář claims it is for fixed frequency or a very narrow band, max 1:1.2), Seiler and Lampkin oscillators (in the Lampkin's, an inductive voltage divider on the tuned circuit coil is used instead Cv, Cg, and Ca of Seiler's; schematics in the 1st ref).
The oscillator's stability is due largely to the dependency of the tube's (or transistor's) forward transconductance
on the resonant frequency (ω) of the tuned circuit. Specifically, Vackář found that forward transconductance varied as ω3 for the Clapp oscillator, as 1/ω for the Seiler oscillator, and as ω/Q for his design, where the Q factor
of the coil (L1) increases with ω.
The conditions for a forward tranconductance that varies minimally with respect to ω are met when:
and the Q of the resonator increases in proportion to ω, which is often approximated by real-world inductors
.
Colpitts oscillator
A Colpitts oscillator, invented in 1920 by American engineer Edwin H. Colpitts, is one of a number of designs for electronic oscillator circuits using the combination of an inductance with a capacitor for frequency determination, thus also called LC oscillator...
or a Clapp oscillator
Clapp oscillator
The Clapp oscillator is one of several types of electronic oscillator constructed from a transistor and a positive feedback network, using the combination of an inductance with a capacitor for frequency determination, thus also called LC oscillator.It was published by James Kilton Clapp in 1948...
in this respect. It differs in that the output level is relatively stable over frequency, and has a wider bandwidth when compared to a Clapp design.
In 1949, the Czech
Czechoslovakia
Czechoslovakia or Czecho-Slovakia was a sovereign state in Central Europe which existed from October 1918, when it declared its independence from the Austro-Hungarian Empire, until 1992...
engineer Jiří Vackář published a paper on the design of stable oscillators. As a result of his in-depth analysis of (vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
) oscillators, he proposed a variant of a Colpitts oscillator, where an additional capacitive voltage divider on the grid input reduces the feedback voltage to the necessary amount, and at the same time reduces the unwanted, unstable tube capacitances.
The schematic is the equivalent of Fig. 5 in his paper, redrawn for the use of a junction FET
JFET
The junction gate field-effect transistor is the simplest type of field-effect transistor. It can be used as an electronically-controlled switch or as a voltage-controlled resistance. Electric charge flows through a semiconducting channel between "source" and "drain" terminals...
. L1 with C0 and Ca form the resonant circuit of a Colpitts oscillator, and Cv/Cg is the grid voltage divider. The circuit can be tuned with C0. Example values are from his paper.
It is similar to an earlier Seiler oscillator, the difference is that in Seiler's the C0 is connected to the other side of Ca. Vackář based his design on stability analysis of Gouriet-Clapp
Clapp oscillator
The Clapp oscillator is one of several types of electronic oscillator constructed from a transistor and a positive feedback network, using the combination of an inductance with a capacitor for frequency determination, thus also called LC oscillator.It was published by James Kilton Clapp in 1948...
(Vackář claims it is for fixed frequency or a very narrow band, max 1:1.2), Seiler and Lampkin oscillators (in the Lampkin's, an inductive voltage divider on the tuned circuit coil is used instead Cv, Cg, and Ca of Seiler's; schematics in the 1st ref).
The oscillator's stability is due largely to the dependency of the tube's (or transistor's) forward transconductance
Transconductance
Transconductance, also known as mutual conductance, is a property of certain electronic components. Conductance is the reciprocal of resistance; transconductance, meanwhile, is the ratio of the current change at the output port to the voltage change at the input port. It is written as gm...
on the resonant frequency (ω) of the tuned circuit. Specifically, Vackář found that forward transconductance varied as ω3 for the Clapp oscillator, as 1/ω for the Seiler oscillator, and as ω/Q for his design, where the Q factor
Q factor
In physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....
of the coil (L1) increases with ω.
The conditions for a forward tranconductance that varies minimally with respect to ω are met when:
and the Q of the resonator increases in proportion to ω, which is often approximated by real-world inductors
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
.