MEASURING MICROWA VE FREQUENCIES 103 



test indicates a lead, whereas the other probe indicates a lag, then the addi- 

 tion of 180° is indicated. 



In microwave circuits it frequently happens that the transfer phase 

 varies quite rapidly with the frequency, particularly if some part of the circuit 

 is at or near resonance. In measuring the phase characteristics of a circuit 

 of this type over a band of frequencies it is necessary, therefore, to take the 

 points of measurement close enough together to avoid phase errors corres- 

 ponding to multiples of 360°. 



When a balance has been established so that the signal is minimized in the 

 detector output, one may observe the presence of the second harmonic of the 

 audio tone. This harmonic is a distortion term generated in the detector. 

 If it is objectionable, it can be eliminated either by a low-pass filter in the 

 audio output, or by using a balanced detector. 



In measuring transfer impedances it is desirable to know the ratio of the 

 magnitudes of an output voltage and an input voltage as well as the phase 

 difference. The equipment described here can be used for measuring ampli- 

 tudes by adjusting the phase shifter for a maximum signal in the audio 

 output. Maximum signal levels can then be compared with the aid of an 

 audio-frequency attenuator and output meter connected as shown in Fig. 1 . 



The apparatus was assembled with standard 4000-megacycle waveguide 

 components. A satisfactory phase shifter was made of an ordinary vane- 

 type variable attenuator by replacing the resistance strip with a vane of 

 quarter-inch thick polystyrene six inches in length. This phase shifter gave 

 a total shift of about 100°. Constructional details of this phase shifter are 

 shown in Fig. 3. Other phase shifters could have been used with equally 

 satisfactory results. It is desirable, however, that the phase shifter be 

 impedance matched to the line in which it is located in order that reaction 

 back on the oscillator shall be a minimum. In the shifter of Fig. 3 the ends 

 of the polystyrene vane have been tapered two inches at each end to accom- 

 plish this result. 



The phase shifter can be readily calibrated by using a standing wave 

 detector fitted with a sliding probe as a standard of phase. The standing 

 wave detector is terminated on one end and connected to the modulated 

 signal source on the other. The signal picked up by the sliding probe is 

 applied to the crystal detector. Knowing the guide wavelength in the 

 standing wave detector, known phase shifts can be introduced by sliding the 

 probe along the guide. By adjusting the phase shifter in the homodyne 

 carrier path for balance, calibration points can be established. 



The measuring procedure described above has been tested experimentally 

 at 4000 megacycles with very satisfactory results. With ordinary care it was 

 possible to measure phase differences with an accuracy of better than half a 

 degree. 



