This invention relates to a phase measurement apparatus with automatic calibration, and more particularly, to a phase measurement apparatus for measuring the phase difference between optical signals and electrical signals derived from electric waves and the phase characteristics of an object to be measured.
A typical conventional phase measurement apparatus is illustrated in FIG. 1. Referring to FIG. 1, reference numeral 1 denotes an object to be measured. An output signal from variable frequency signal generator 2 is applied to object 1, and the frequency signal passed through object 1 is supplied to mixer 3. The output signal from generator 2 is directly supplied to mixer 4. Mixers 3 and 4 mix a local frequency signal from local oscillator 5 with the output signals supplied through object 1 and directly from generator 2, and generate low-frequency signals. These low-frequency signals are filtered through filters 6 and 7, respectively. The filtered signals are supplied as IF (Intermediate Frequency) signals to phase detector 8. Detector 8 calculates a difference between these IF signals and generates a phase difference signal. The phase difference signal is amplified by variable gain amplifier 9 and the amplified signal is converted by A/D (Analog/Digital) converter 10 to a digital signal. The digital signal is then displayed on display 12. In other words, the phase difference caused by object 1, that is, the phase characteristics of the object, is displayed. It should be noted that frequency setter 11 sets frequencies required for control of generator 2 and oscillator 5, and display 12.
Since the gain (output gain) of phase detector 8 varies particularly in the phase measurement apparatus due to changes in ambient conditions such as changes in temperatures, calibration is performed to correct such variations. In a conventional phase measurement apparatus, two calibration methods (i) and (ii) are selectively used:
(i) The phase measurement apparatus is connected to a reference phase setter in place of object 1. The phase characteristics of the phase setter under predetermined ambient conditions are already known. The output frequency of variable frequency signal generator 2 is changed by frequency setter 11 within any relatively narrow frequency band of f1 to f2 (=.DELTA.f1). Variation .DELTA..phi.1 representing a difference between the phase difference signals from detector 8 at frequencies f1 and f2 is calculated. The amplitude of variable gain amplifier 9 is manually adjusted such that variation .DELTA..phi.1 as the phase difference corresponding to frequency variation .DELTA.f1 is equal to a predetermined phase angle (a frequency vs. phase angle).
(ii) As shown in FIG. 3, the output frequency of variable frequency generator 2 is sequentially changed to measure difference .DELTA..phi.2 between actual folded phases corresponding to theoretical folded difference values 360.degree. and 0.degree., that is, a difference between maximum and minimum values of the folded phase differences. The gain of variable gain amplifier 9 is manually adjusted according to the measurement results. According to this method, the reference phase setter in place of object 1 need not be used.
The reference phase setter is used in method (i). The setter is expensive and its phase characteristics are unstable in relation to changes in ambient conditions such as changes in temperatures. For this reason, the calibration ambient conditions must be maintained to be the designated constant conditions. However, this requirement is extremely difficult to satisfy in practice. According to method (ii), the influences of changes in ambient conditions can be neglected. However, method (ii) poses the following problem. In method (ii), the phase measurement apparatus is calibrated by difference .DELTA..phi.2 between the actual phase differences corresponding to theoretical folded phase difference values 360.degree. and 0.degree.. However, in practice, the phase difference signals from phase detector 8 at the folded phase differences are unstable, and accurate calibration is difficult. More specifically, the actual folded phase differences can be almost the same as the theoretical folded phase difference values 360.degree. and 0.degree., but cannot coincide therewith in a strict sense. Therefore, even if difference .phi.a between the phase differences at given frequencies f3 and f4 is measured, a reference value for difference .phi.a is indefinite, and accurate calibration cannot be performed. Therefore, accurate calibration is difficult to perform by the two conventional methods described above.
In addition, the reference phase setter used in method (i) has a narrow frequency band representing the limited frequency characteristics. Therefore, in order to calibrate the phase measurement apparatus in a wide frequency range, several expensive phase setters must be used.