Patent GB-1371387-A (GB)
Title: Microwave spectrometer apparatus
IPC: G01R33/60
Filed: 1971-11-02
Abstract: 1371387 Micro-wave resonance spectrometers VARIAN ASSOCIATES 2 Nov 1971 [2 Nov 1970] 50950/71 Heading G1N In an electron spin resonance spectrometer, Fig.1, micro-wave energy from Klystron 2 is coupled via a circulator 3 to a resonance cavity 5 containing the sample under investigation, the signal reflected from the cavity is phasemodulated in a reflection modulator 8 comprising a P.I.N. diode 24 spaced an odd integral number # wavelengths from a reflective wall 25 and is detected at detector 9 which also receives a reference signal via directional couplers 13 and 14 and a variable phase shifter 16. Phase modulator 8 receives pulses from a programmer 27 which also controls gates 28 and 29 to direct an absorption signal (selected for about 10% of the time, when diode 24 is transmissive) to an A.F.C. loop 23, and a dispersion signal (selected for about 90% of the time, when diode 24 is reflective) to a recorder 35 via a phase-sensitive detector 34 synchronized with a field modulation signal from an oscillator 21. In a modification, Fig.5 (not shown), the programmer (27) switches diode (24) at twice the frequency of the field modulation oscillator (21). In another arrangement, Fig.3 (not shown), the phase modulator (8) includes a second P.I.N. diode (38) an odd integral number of “ wavelengths from the reflective wall (25). The diodes (24, 38) and A.F.C. gate (28) are switched under the control of programmer (27) to provide an absorption signal for the A.F.C. loop twice per cycle of an audio frequency oscillator (26). No gate is needed for the record channel as phasesensitive detector (42) which detects a dispersion signal for recording, can reject the second harmonic absorption signal. The recording channel also includes a high audio frequency phasesensitive detector (34) synchronized with the field modulation signal from oscillator (21). In a modification, Fig. 7 (not shown), the magnetic field is modulated at a low audio frequency, 35H z , and diode (38) is switched at a high audio frequency 100kH z , during the record intervals to overcome low frequency detector noise. ENDOR spectrometer, Fig.9 shows an electron nuclear double resonance spectrometer. Nuclear resonance is excited by a frequency swept R.F.O. 62 connected to coil 64 via an amplifier 63 which is pulsed at 6 kH z . The output of detector 9 is gated at 73 to phase detector 75 which detects the absorption signal and passes it to a low frequency phase detector 77 for detection against a 35H z  signal supplied to diode 24 in the phase modulator 8. Diode 24 is an odd integral number of “ wavelengths from reflective wall 25. The output of phase detector 77 is recorded on a recorder 35 whose X axis is synchronized with the frequency sweep of the R.F.O. 62. A servoloop 82, 83, 81 automatically adjusts a screw in the iris 7 to maintain matching between the cavity 5 and circulator 3. The A.F.C. loop will properly lock for either phase of the phase modulated reflected cavity signal is the power incident on the cavity 5 is > 4 times the power in reference channel 11; otherwise the A.F.C. must be gated for one of the phases or the reference phase of the A.F.C. phase-sensitive detector (not shown) may be switched by 180 degrees at the 35H z  phase modulation frequency.