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.[1] 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.[2]
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.
In 1949, the Czech engineer Jiří Vackář published a paper on the design of stable oscillators.[1] 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.[2]
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.
No comments:
Post a Comment