Patent GB-682116-A (GB)
Title: Improvements in apparatus for effecting measurements by the aid of high frequency electromagnetic waves
IPC: G01R25/00
Filed: 1950-08-22
Abstract: 682,116. High-frequency measurements. WHITEHEAD, E. A. N. Aug. 21, 1951 [Aug. 22, 1950], No. 20802/50. Class 37. In apparatus for effecting measurements by the aid of high-frequency waves, of the kind in which waves generated in an oscillator are caused to travel through a path including a device of which some characteristic is to be measured, a proportion of the high frequency wave output of the oscillator is caused to travel through a second path leading to the input of the detector and the phase of the signal in one of the two paths is continuously and cyclically shifted at a predetermined frequency so that the two signals arriving at the input to the detector will beat with each other. As shown in Fig. 1 the output power from a microwave oscillator 1 is distributed between two branches 2, 3 of a wave-guide 4 in a predetermined ratio. A variable-phase changer 5 is disposed in the branch 3 and is driven by a synchronous motor 6 supplied from A.C. mains so as to produce a continuous and cyclical change in the phase of that proportion of the microwave power which is transmitted through the branch 3. The power transmitted through the branch 2 is supplied as a test signal to a device 7, such as a variable attenuator, of which some characteristic is to be measured, and the modified signal leaving this device is supplied to a mixer 8, which also receives the signal transmitted through the branch 3. The two signals beat with each other to produce an audio frequency which is supplied to an amplifier 9 having a narrow band width. The output from the amplifier 9 is supplied to a measuring instrument 10. In order to measure the phase of the attenuator output the circuit may be modified as shown in Fig. 2. The phase changer 5 is driven by a motor 11 which also drives an A.C. generator 12 to produce a current having the same frequency as the phase change produced by the phase changer 5. The signals transmitted through the branches 2, 3 of the waveguide 4 are fed to the mixer 8 and the resulting heterodyned signal is amplified by the amplifier 9. The outputs from the amplifier 9 and the generator 12 are fed to a phase-measuring circuit 13 which supplies a signal to a phase indicator 14. Fig. 3 shows an apparatus for determining the proportion of the microwave energy transmitted along a wave-guide which is reflected from a terminal thereof. The output power from a microwave oscillator 15 is fed into a wave-guide 16 which is fitted with directional couplers 17, 18 so that energy fed into the branch 19 of the coupler 17 is proportional to the total energy fed into the main waveguide 16, and the energy flowing into the branch 20 of the coupler 18 is proportional to the energy reflected from the terminal 21 of the wave-guide 16. The branches 22, 23 of the couplers 17, 18 are provided with matched terminations 24, 25 which absorb any power entering the branch. The branches 19, 20 are connected to a mixer 26, the branch 20 including a rotary phase changer 27 operated by a synchronous motor 28. The heterodyned output from the mixer 26 is fed through an amplifier 29 to a measuring instrument 30.