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A microwave frequency standard Scovil, Henry Evelyn Derek


The various stages in the design and construction of a frequency standard for use in microwave spectroscopy is described. Procedures are laid down for the use of the instrument. The standard is of the variable frequency type which gives it several advantages over the fixed frequency machines. It is capable of providing complete coverage at microwave frequencies and is capable of determining the microwave harmonic generated without recourse to a wave-meter. Furthermore it can conveniently be used in conjunction with a calibrated communications receiver to measure frequency separations to a high degree of accuracy. A stable 200 kc crystal controlled oscillator is used as the base frequency. This frequency is multiplied by harmonic generators to 4800 kc and supplied to the oscillator grids of a balanced mixer. The variable 125 to 250 kc output from a BC221 signal generator is supplied to the signal grids of this same mixer. The mixer output is tuned with a band pass circuit passing 4925 to 5050 kc and consequently the two input frequencies are summed. The balanced mixer serves to eliminate several troublesome frequency components. The mixer signal is multiplied successively to 25 mc, 75 mc and 200 mc. An output from one of these VHF stages is chosen and impressed upon a 1N26 silicon crystal rectifier. This rectifier acts as a harmonic generator producing frequency markers in the microwave region. Since both the 200 kc oscillator and BC221 signals have a high degree of stability and can conveniently be calibrated relative to standard frequency broadcasts from the U.S. Bureau of Standards, these microwave frequency markers are known to a high degree of accuracy. This microwave standard signal is fed into the spectrometer waveguide and picked up by a crystal mixer. The spectrometer klystron signal is also detected by this crystal and consequently beats are produced equal to the difference between the standard harmonics and the klystron frequencies. This beat note is supplied to a tuned amplifier whose, output is applied to the Y plates of the spectrometer oscillograph. The time base of the latter is supplied by the sawtooth wave which sweeps the klystron frequency. When the beat note frequency is equal to the frequency to which the amplifier is tuned a marker pip will occur on the screen. Since the frequency of the standard signal and the tuning of the amplifier are known the frequency of the klystron at this point of the time base can be determined. By tuning the variable frequency oscillator, the pip can be moved across the horizontal axis until it is coincident with a spectrum line. Consequently the frequency of this line can be measured with an accuracy of 1 part in 10⁶.

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