A simple circuit is described which enables a local secondary frequency standard to be set quickly to a definite zero beat with a received short‐wave standard frequency signal, without the usual uncertainty due to amplitude fading of the short‐wave signal. In principle, two identical receivers, both fed by the same antenna, are used. One receiver picks up WWV, for example, and a weak signal from the local standard. The other picks up WWV either alone or together with a second weak signal from the local standard which is 180° out of phase with the first. The outputs of the detectors of the two receivers are subtracted, leaving only the beat note, which is recorded on a strip chart. In practice, one unmodified AM receiver is used on a time‐sharing basis by means of a chopper. The band width of the device may be made much less than one cycle per second, and therefore, it is particularly useful when the WWV signal is reasonably stable in phase but far too weak for an audible zero beat to be achieved. Alternatively, the device may be regarded as a simple means of observing fast Doppler shifts of a received standard frequency signal. A schematic diagram and sample recorder charts are included.

Topics
Doppler effect, Frequency standards
1.
A secondary frequency standard is usually a very good quartz crystal oscillator. If cycle counting equipment (a clock) is driven by the same oscillator and its time‐keeping over long intervals is compared with time signals based on astronomical observation, the system is then termed a primary frequency standard.
2.
A tuned radio‐frequency receiver has been described for simplicity. It will be seen that use of a superheterodyne receiver makes no difference here, because the superheterodyne cannot alter the difference in frequency between the two carriers.
3.
A description of this amplifier will be submitted to the Review of Scientific Instruments.
4.
Manufactured by the Bristol Company, Waterbury 20, Connecticut.
5.
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Fleming
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J.
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7.
F. E. Terman, Radio Engineers Handbook (McGraw‐Hill Book Company, Inc., New York, 1943), p. 647 and references thereon.
8.
“Standard Frequencies and Time Signals—WWV and WWVH.” Letter Circular LC 1023 (June, 1956) available on request from Division 84.20, National Bureau of Standards Laboratories, Boulder, Colorado.
9.
Reference 4, p. 724.
10.
WWV transmits 3‐min periods of tone modulation separated by 2‐min periods without tone modulation. The tone is alternately 440 and 600 cps. The tone modulation is transmitted from WWV (except at 25 Mc) as a full carrier together with a single upper sideband of one‐third carrier power. A convenient expression for the resulting rf wave form is given by S. Goldman in Frequency Analysis, Modulation and Noise (McGraw‐Hill Book Company, Inc., New York, 1948), p. 160, Eq. (42). In the present case, the carrier level meter records the average value of the tone‐modulated dc developed across the load of a conventional diode detector, which is fed by this rf wave form. The wave form of the tonemodulated dc is of course the envelope of Goldman’s Eq. (42). A computation of its average value for the WWV case shows that the carrier level meter should read only ∼7.6% higher with the tone side band on than with it off, whereas the changes recorded in Fig. 9 are ∼100%.
11.
J. A.
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J. A.
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13.
NBS “Proposal for standard frequency broadcast at very low frequency,” Natl. Bur. Standards (December 6, 1956) and references therein. Same source as reference 5.
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© 1957 The American Institute of Physics.
1957
The American Institute of Physics
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