Two modern cryogenic variants of the Pound circuit have been devised to increase the frequency stability of microwave oscillators that include cryogenic sapphire-filled cavity resonators. Invented in the 1940s and named after its inventor (R. V. Pound), the original Pound circuit is a microwave frequency discriminator that provides feedback to stabilize a voltage-controlled microwave oscillator with respect to an associated cavity resonator. Heretofore, Pound circuits used in conjunction with cryogenic resonators have included room-temperature electronic components coupled to the resonators via such inter-connections as coaxial cables. The thermo mechanical instabilities of these inter-connections give rise to frequency instabilities. In a cryogenic Pound circuit of the present improved type, all of the active electronic components, the inter-connections among them, and the inter-connections between them and the resonator reside in the cryogenic environment along with the resonator and, hence, are thermo-mechanically stabilized to a large degree. Hence, further, frequency instabilities are correspondingly reduced.
The active microwave devices required in a Pound circuit are two amplitude detectors and a phase modulator. A Pound circuit generates a frequency-error signal by converting a phase modulation (PM) to an amplitude modulation (AM). The AM in question is generated when a microwave signal that is reflected from a resonator has a high value of the resonance quality factor (Q) and the signal frequency differs from the resonance frequency. A pure PM signal is required because any AM at the input terminal of the resonator would generate a frequency error.
In the present cryogenic Pound circuits (see figure), the active microwave devices are implemented by use of state-of-the-art commercially available tunnel diodes that exhibit low flicker noise (required for high frequency stability) and function well at low temperatures and at frequencies up to several tens of gigahertz. While tunnel diodes are inherently operable as amplitude detectors and amplitude modulators, they cannot, by themselves, induce significant phase modulation. Therefore, each of the present cryogenic Pound circuits includes passive circuitry that transforms the AM into the required PM. Each circuit also contains an AM detector that is used to sample the microwave signal at the input terminal of the high-Q resonator for the purpose of verifying the desired AM null at this point. Finally, each circuit contains a Pound signal detector that puts out a signal, at the modulation frequency, having an amplitude proportional to the frequency error in the input signal. High frequency stability is obtained by processing this output signal into feedback to a voltage-controlled oscillator to continuously correct the frequency error in the input signal.
Each of these circuits first generates a carrier-suppressed AM signal and then transforms that signal into PM by use of a bypass from the radio-frequency input that injects a carrier with ≈90° phase shift from the carrier of the AM signal. (If AM carrier suppression is complete, this phase shift is exactly 90°; if incomplete, a phase shift that deviates somewhat from 90° is required.) An approximation of pure PM is obtained via a coarse adjustment of the phase of this bypassed signal, this adjustment being made by use of a mechanical phase shifter. A fine adjustment to increase the accuracy of the approximation is made by varying the DC voltage applied to the modulation diode(s). Once the mechanical phase shifter is adjusted properly, the variation in DC voltage suffices to maintain pure PM.
The two circuits differ in how they generate the carrier-suppressed AM signal. In the first circuit, this involves combining the outputs from two tunnel diodes that are operated as amplitude modulators configured so that both amplitude modulations and carriers are oppositely phased. This functionality is implemented by mounting the diodes in an anti symmetric arrangement that affords the additional benefit of enabling the use of a single modulation signal, superimposed on a single DC bias, as input to both modulator diodes. If the diodes are perfectly matched, then the carrier can be suppressed completely.
The second circuit was developed after extensive tests and modeling of the behaviors of tunnel diodes showed that a nearly-suppressed-carrier AM signal could be generated by use of only one tunnel diode. The obvious advantages of the second circuit, relative to the first one, are fewer components and, consequently, smaller dimensions.
This work was done by G. John Dick and Rabi Wang of Caltech for NASA's Jet Propulsion Laboratory. NPO-42172
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