Sawtooth Modulation Research Articles

Beam Spread with the Princeton FM Cyclotron

Two methods have been employed on the Princeton 18-meV FM cyclotron to increase the duty cycle of the proton beam. In the first method, the dee oscillator was cut off just before normal beam extraction and turned on again about 50 μsec later. A moderate beam spread is produced. In the second method, an auxiliary accelerating electrode, in the form of a ``C'' subtending an azimuthal angle of 70° and encompassing the outer 20% of the radial orbits, was added. It is placed opposite the single dee in the region near the deflector and exit channel. In order to produce a spread beam, the regular beam is allowed to enter the region of the C without rf drive on the C. The remaining acceleration necessary for extraction is accomplished by driving the C with an rf program and turning off the dee oscillator. Several frequency-modulation programs were tried; noise, sinusoidal, sawtooth, and combinations of these. Sawtooth modulation produced best beam spread. The duty factor (as measured by the ratio of pile-up to single counts from elastically scattered protons using a sodium iodide-scintillation counter) was increased by a factor of about 4 to 5 in the first method and 12 to 15 in the second method. The average current was reduced by a factor of three. However pulsed operation of the dee oscillator makes it possible to use a higher peak dee voltage for the same average power, thereby reducing the loss of current due to stochastic extraction.

The Serrodyne Frequency Translator

A serrodyne frequency translator will translate, or shift, the frequency of a signal in a nearly ideal manner. Linear sawtooth modulation of a transit-time device, such as a traveling-wave tube or klystron, is employed to effect the translation. The power output at the translated frequency is practically equal to the capability of the same device operating as an ordinary amplifier. Furthermore, very little power is produced in undesired intermodulation frequency components. Frequency translations of a microwave signal ranging from subaudio frequencies to 57 megacycles have been accomplished. The translation loss for a translation of about 30 megacycles was less than 1 decibel; moreover, each undesired frequency component was suppressed at least 20 decibels. The serrodyne method can be used at nonmicrowave frequencies and with larger fractional frequency shifts, provided modulable transit-time devices having certain properties can be devised. An analysis is given of serrodyne performance as limited by a number of important practical factors. A general spectrum analysis is included which is in a convenient form for a variety of problems in combined AM and PPM (or combined AM and PM) when arbitrary modulating waveforms are used.

A Method for Calibrating Microwave Wavemeter

A method of calibrating wavemeters in the microwave range is described in which the frequency of a calibrating oscillator having sufficient power to drive a wavemeter is continually checked against the harmonics of a crystal-controlled reference oscillator by visual means. The two frequencies being compared are separately heterodyned with that of a third oscillator, which is frequency-modulated. The modulation products then pass through a narrow-band receiver to an oscilloscope whose sweep is synchronized with the saw-tooth modulation voltage. Whenever the difference between the instaneous frequency of the modulated signal and the output of one of the generators equals the frequency to which the receiver is tuned, a pulse of energy is transmitted through the receiver and appears as a pip on the oscilloscope. Pips will appear corresponding to the moments when the instantaneous frequency is greater or less than that of each of the fixed frequencies. The oscillographic pattern then comprises two fixed pulses due to the reference oscillator, and another pair which can be shifted by changing the frequency of the calibrating oscillator. When the two pairs are superimposed, the frequency of the power oscillator matches that of the crystal harmonic. By superimposing the higher-frequency pulse of one pair on the lower-frequency pulse of the other pair, and vice versa, additional adjustments of the power oscilator can be made to differ from the standard by ± twice the frequency of the receiver.