| 摘要 |
1,159,886. Automatic frequency control systems; phase sensitive detectors. VARIAN ASSOCIATES. 24 Nov., 1967 [9 Dec., 1966], No. 53662/67. Heading H3A. An atomic frequency standard of the type wherein the standard frequency is frequency modulated at a low frequency (107 Hz) and applied to a gas cell 2 illuminated by a source 5, is characterized in that the output of the photodetector 8 is passed in series to a phase-sensitive detector 13 supplied with the second harmonic (214 Hz) of the modulation and to a second phase-sensitive detector 19 supplied with the modulation frequency, the output of the latter providing an error signal for controlling the oscillator 7 which generates the standard frequency. The output of the first detector 13 preferably passes through an amplifier 16 to an RC high-pass filter 17 which separates the fundamental component and to an RC low-pass filter 18 which passes the detected second harmonic D.C. component to an alarm gate 23 wherein it trips an alarm if its amplitude falls below a predetermined value, thereby indicating that frequency lock has been lost. The fundamental and second harmonic signals are preferably generated from an oscillator 14 via flip-flops 15 and 21 acting as frequency dividers so that these signals contain no even harmonics. Phase-sensitive detectors, Fig. 2.-On one half-cycle of the reference signal applied to terminals 36 and 37 the FETs 25 and 31 conduct to allow the input from terminal 34 to pass in one direction through the load resistance 29; on the next half-cycle the FETs 26 and 32 conduct whereby the input passes through resistance 29 in the opposite direction. |
