Vackář oscillator

Schematic of what is commonly called the Vackář oscillator. Vackář credited Radioslava with developing this circuit in 1945.[1]

A Vackář oscillator is a wide range variable frequency oscillator that strives for a near constant output amplitude over its frequency range. It is similar to a Colpitts oscillator or a Clapp oscillator, but those designs do not have a constant output amplitude when tuned.

Invention

In 1949, the Czech engineer Jiří Vackář published a paper on the design of stable variable-frequency oscillators (VFO).[2] The paper discussed many stability issues such as variations with temperature, atmospheric pressure, component aging, and microphonics. For example, Vackář describes making inductors by first heating the wire and then winding the wire on a stable ceramic coil form. The resulting inductor has a temperature coefficient of 6 to 8 parts per million per degree Celsius.[3] Vackář points out that common air variable capacitors have a stability of 2 parts per thousand; to build a VFO with a stability of 50 parts per million requires that the variable capacitor is only 1/40 of the tuning capacity (.002/40 = 50ppm). The stability requirement also implies the variable capacitor may only tune a limited range of 1:1.025.[3] Larger tuning ranges require switching stable fixed capacitors or inductors.[4]

Vackář was interested in high stability designs, so he wanted the highest Q for his circuits. It is possible to make wide range VFOs with stable output amplitude by heavily damping (loading) the tuned circuit. However, that tactic substantially reduces the Q.[5]

Vackář was also concerned with the amplitude variations of the variable-frequency oscillator as it is tuned through its range. Vackář assumes the tuned circuit has a constant Q over the VFO's frequency range. Vackář reviewed several existing circuits for their amplitude stability.[1] The Clapp oscillator's transconductance requirement is proportional to ω3. If the Clapp transconductance is set to just oscillate at the lower frequency range, then the oscillator will be overdriven at its highest frequency. The Seiler and Lampkin oscillators have a transconductance requirement that is proportional to ω−1,.

Vackář then describes an oscillator circuit due to Radioslava in 1945 that maintained "a comparatively constant amplitude over a wide frequency range."[6] Vackář reports that VFO circuit being used by the Czechoslovak Post Office since 1946. Vackář does not claim the circuit, but he analyzes the circuit and explains how to get an approximately constant amplitude response. This circuit has become known as the Vackář VFO.[7] Vackář does refer to the circuit as "our circuit" and states that O. Landini independently discovered the circuit and published it (without an analysis) in Radio Rivista in 1948.[8] Vackář describes a VFO design using this circuit that covers a range of 1:1.17.[8]

Vackář then describes a variation of the Radioslavia circuit that can cover a range of 1:2.5.[9] The circuit need not assume the tuned circuit has a constant Q. Vackář patented this new circuit and two variations of it.[10]

Circuit operation

The schematic is the equivalent of Fig. 5 in his paper (Radioslavia design), 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[11] and Lampkin[12] 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:

C_v \ll C_0 ,
C_g \gg C_v ,
C_a \gg C_0 ,

and the Q of the resonator increases in proportion to ω,[2] which is often approximated by real-world inductors.

References

  1. ↑ 1.0 1.1 Vackář 1949, p. 5
  2. ↑ 2.0 2.1 Vackář, Jiří (December 1949), LC Oscillators and their Frequency Stability, Tesla Technial Reports, Prague, Czechoslovakia: Tesla National Corporation, UDC 621.396.615.12, archived from the original on 2012-02-19
  3. ↑ 3.0 3.1 Vackář 1949, p. 2
  4. ↑ Modern low phase noise voltage controlled oscillators use bank switching.
  5. ↑ There is an insertion loss issue that Vackář ignores.
  6. ↑ Vackář 1949, p. 6
  7. ↑ Schetgen, Robert, ed. (1996), "The G3PDM Vackar VFO", The ARRL Handbook for Radio Amateurs (seventy-third ed.), pp. 14.17–14.18, ISBN 0-87259-173-5
  8. ↑ 8.0 8.1 Vackář 1949, p. 7 citing Landini, O. (November 1948), Radio Rivista Missing or empty |title= (help)
  9. ↑ Vackář 1949, p. 7
  10. ↑ US 2706249, Vackář, Jiří, "Stabilization of resonant circuits", published 10 February 1950, issued 12 April 1955
  11. ↑ Seiler, E. O. (November 1941), "Variable Frequency Oscillator", QST
  12. ↑ Lampkin, G. F. (March 1939), "An Improvement in Constant Frequency Oscillators", Proceedings of the IRE

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