TenKilocycle PoundType Klystron StabilizerH. E. Radford Citation: Review of Scientific Instruments 34, 304 (1963); doi: 10.1063/1.1718344 View online: http://dx.doi.org/10.1063/1.1718344 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/34/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Physical origins of the high structural stability of CLN025 with only ten residues J. Chem. Phys. 141, 105103 (2014); 10.1063/1.4894753 Pound-Drever-Hall-locked, frequency-stabilized cavity ring-down spectrometer Rev. Sci. Instrum. 82, 063107 (2011); 10.1063/1.3595680 An introduction to Pound–Drever–Hall laser frequency stabilization Am. J. Phys. 69, 79 (2001); 10.1119/1.1286663 Stabilization of the output power of a 20MW klystron to within 0.1% Rev. Sci. Instrum. 64, 2306 (1993); 10.1063/1.1143926 Frequency Stabilization Scheme for the PoundWatkins Rf Spectrometer Rev. Sci. Instrum. 32, 27 (1961); 10.1063/1.1717137

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304 NOTES

in Fig. 2. Figure 3 shows a multiple lead thermocouple seal bushing which allows the leads to be brought out of a vacuum enclosure without the junctions, which are fre­quently required with conventional fixed feedthroughs. Because of the inherent lubricating qualities of Teflon, rotary motion can also be transmitted through a modified feed through. To increase the reliability of ground connec­tions made to the metal body of the feedthrough, when connected to coaxial cables, the compression nut can be made to include a sleeve which projects back over the external bushing onto which the outer cable braid can be soldered or clamped.

The bushings used in this laboratory were machined on a lathe from Teflon rod of appropriate diameter, but there appears to be no reason to prevent their being injection molded or made by some more rapid process if required in large numbers.

1 W. E. Bron, R. Mannheimer, and G. Taylor, IBM Research Note NC-127, 14 August 1962.

2 Manufactured by the Crawford Fitting Company, Cleveland Ohio. '

Ten-Kilocycle Pound-Type Klystron Stabilizer*

H. E. RADFORD

National Bureau of Standards, Washington, D. C. (Received 31 December 1962)

THE stabilized, tunable microwave oscillator required for paramagnetic resonance spectroscopy and for

the bench testing of microwave systems usually consists of a reflex klystron locked in frequency to a tunable high-Q resonant cavity. One of the simplest locking methods is the frequency modulation (FM) method, in which the cavity, because of its frequency-dependent response, gen­erates an amplitude-modulated error signal when fed with a sample of the frequency-modulated klystron output. A tuned amplifier and synchronous detector convert the error signal into a dc correction voltage, and the feedback loop is closed by applying this voltage to the klystron reflector electrode. Easy to use, because it requires no tuning of the microwave circuit, the FM stabilizer has found wide use as a component part of the Varian V-4S00 EPR spectrometer, and can also be purchased as a self­contained unit from the Triconix Company. The un­avoidable frequency modulation imparted to the klystron output by these commercial stabilizers is, in most applica­tions, a small price to pay for their very desirable features of ready availability and trouble-free operation. In certain applications, however, frequency modulation of the klys­tron output is intolerable. It is the purpose of this note

MONITOR STABllIZAT10N ON-OFF

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KLYSTRON - CRYSTAl

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FIG. 1. Block ~agra~ of the Triconix KS-ID stabilizer, showing modificatIOns necessary for CW operation.

to point out that, in such cases, the commercial FM unit can be modified very easily to function as a CW stabilizer. The modified circuit is nearly identical to that of the i.f. stabilizer devised several years ago by R. V. PoundJ The major difference is that the operating frequency is 10 kc (the ready-made frequency of the commercial circuits) rather than the 30 Mc used by Pound.

The Triconix version of the FM stabilizer, modified for alternative operation as a Pound-type stabilizer, is shown by Fig. 1. The circuit injects its correction voltage into the reflector voltage lead and, limited only by the high­voltage insulation of the three isolation transformers, can be used with any voltage-tunable oscillator and its power supply. The present modification consists simply of adding a three-pole switch to divert the modulation voltage from the reflector lead to an external connector, where it may be taken off to a crystal modulator in the microwave circuit. The switch should have a voltage rating as high as that of the transformers and, to avoid accidental crystal burnout, must be of the nonshorting type. The reflector terminals are shunted with a resistor so as to maintain reflector voltage on the klystron at all times. Phone jacks are also added, to allow the rectified crystal currents to be monitored.

V-I!5! KLYSTRON

IN23D MODULATOR

CAVITY WAVEMETER

FIG. 2. A complete 10-kc Pound-type klystron stabilizer.

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NOTES 305

The complete CW stabilizer, as constructed in this laboratory for use at X-band frequencies, is diagrammed in Fig. 2. A sample of the klystron output, taken from the the waveguide run by a 20-dB directional coupler, is fed through a variable attenuator to a Pound microwave discriminator, assembled from commercial waveguide components. The incident power divides in the hybrid tee, half going to the MA423A crystal mixer and half going to the cavity wavemeter through a variable phase shifter. Power reflected from the cavity enters the fourth arm of the tee, where it is amplitude modulated at 10 kc by the 1N23D crystal modulator. Sideband power re­flected from the modulator enters the crystal mixer arm, where it mixes with the original carrier power to yield a 10-kc component of rectified crystal current. This 10-kc signal may be given the amplitude and phase character­istics of an error signal by proper adjustment of the phase shifter. Impedance matching is fairly critical, and for operation over a wide frequency range tunable crystal mounts should be used. The crystal mixer mount may be tuned by substituting a matched termination for the cavity and phase shifter, and then tuning for maximum rectified crystal current. The modulator mount is then tuned in the same way, with the cavity and phase shifter back in place and with the 10-kc drive reduced to zero. For best performance the modulator tuning may be trimmed later to yield a maximum lO-kc error signal under actual operating conditions.

The dynamic response of the CW stabilizer described here is much inferior to that of the original Pound circuit, primarily because of the lower operating frequency, and the consequent narrower bandwidth of the signal ampli­fier. The bandwidth of the Triconix amplifier between 90° phase shift points is approximately 400 cps, as compared to bandwidths of several Mc which are common in 30-Mc i.f. amplifiers. To achieve stable operation at this narrow bandwidth, with reasonable loop gain, requires that the circuit be heavily damped. As constructed, the commercial FM stabilizer circuits have a response time in the neigh­borhood of 0.1 sec, and are hence incapable of stabilizing a klystron against the often troublesome frequency ex­cursions, amounting to several kc or tens of kc, caused by blower-fan vibration, reflector voltage ripple, and ac heating of the klystron filament. Rather than go to the trouble of constructing and testing a wide-band stabilizer of the Pound i.f. type, however, it is frequently easier to reduce these frequency excursions by eliminating, as far as possible, their respective sources. The familiar techniques of shock-mounting the klystron, immersing it in a constant-temperature oil bath, and supplying it with well-regulated dc voltages, including dc filament voltage, are all effective, and were applied to the klystron of Fig. 2. These measures confined short-term frequency ex-

cursions within approximately 500 cps to either side of the long-term stabilized frequency, which is adequate for most applications, ours included. Long-term frequency drifts, measured over periods of several minutes, were typically less than 1 kc, and were traceable to imperfect temperature compensation of the stabilization cavity, which for this work was a Demornay-Bonardi secondary standard wavemeter.

* This work was supported in part by the U. S. Office of Naval Research.

1 R. V. Pound, Rev. Sci. Instr. 17, 490 (1946).

Photographic Measurement of Burning Rates in Solid Propellant Rocket Motors*

J. R. OSBORN, J. M. MURPHY, AND S. D. KERSHNER

(Received 26 November 1962; and in final form, 17 December 1962)

THIS note describes a technique for measuring the burning rate of solid propellants under conditions

closely approximating those of an actual solid rocket motor. The technique involves photographing the moving surface of the burning propellant and computing a distance burned from a sequence of the photographs for which the combustion chamber pressure has been measured.

Figure 1 illustrates the essential features of the rocket motor. The rocket motor consists of a two-dimensional combustion chamber equipped with transparent windows made of Plexiglas (Rohm and Haas) so that light may be transmitted through the combustion chamber. Propellant is mounted on the top and bottom surfaces of the com­bustion chamber, and the sides of the upper propellant block are visible through the window.

A 16-mm Fastax (model D163269) high-speed framing camera is employed for photographing the burning block of propellant in the combustion chamber. The camera is equipped with a Wollensak 152-mm 1/4.5 cine telephoto lens and positioned so that the optical axis of the camera is normal to the plane of the transparent window. The camera is positioned with a field of view of approximately 1.5 in. in diameter. The average camera speed is adjusted for each static firing of the rocket motor by regulating the input voltage to the camera motors; a Variac transformer is employed for this purpose. The camera has been op­erated at approximately 425 frames per second with an aperture setting of 1/16. Ansco FPC 132 (color) film is used. A time base is provided on the film by a small argon lamp flashing at 120 cps.

The combustion-chamber pressure is measured by em­ploying the conventional techniques of an electrical trans­ducer (Wiancko P1951) and an oscillograph (CEC type

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