Patent GB-1191256-A (GB)
Title: Apparatus for measuring a characteristic of a repetitively varying electrical signal
IPC: G01R19/04
Filed: 1968-10-04
Abstract: 1,191,256. Phase and voltage measurement. TEKTRONIX Inc. Oct. 4, 1968 [Oct. 5, 1967], No. 47296/68. Heading G1U. An apparatus for measuring a characteristic of a repetitively varying input signal at a selected point on the waveform representing the characteristic e.g. phase or voltage, operates by generating a pair of opposite phase signals, (i.e. inverted with respect to one another), each having an amplitude variation corresponding to the characteristic of the input signal which is of interest, and presenting these opposite phase signals on a cathode ray tube display. Arrangements in the apparatus enable the two signal displays to be positioned so that corresponding reference levels in each coincide, and enable the displays to be then relatively adjusted until selected corresponding parts in the two are superimposed. The amount of adjustment represents the value of the characteristic at the corresponding selected part of the input waveform. Measuring Phase Distortion in the luminance signal in an NTSC type colour television system. The stairstep luminance signal, which is non- linearly mixed with a colour signal so that the distortion at each stairstep is different, is applied at 16, Fig. 1 to a pre-amplifier 14 and thence to a bandpass filter 12 which separates the colour subcarrier signal and applies it to a phase detector 10. A reference oscillator 20 when triggered by the input provides a reference signal at the colour subcarrier frequency, and this is passed via a calibrated phase adjuster 22 and uncalibrated adjuster 24 to a phase switch 26 which may be switched at 30 to provide two alternative outputs 180 degrees apart. The phase adjuster 24 will be set so that these will be at 90 degrees and 270 degrees to the undistorted colour subcarrier. The output from the switch 26 is applied to the phase detector 10, which provides a voltage output signal corresponding to the phase distortion in the luminance signal. The switch 30 is controlled from a bi-stable 34, controlled by the oscilloscope sweep generator, so that two alternative waveforms are derived from the detector 10, one being the inverse of the other. These are observed on the oscilloscope and the adjuster 24 viewed until their zero reference levels coincide. The phase shifter 22 is then adjusted to bring into coincidence any selected step in the waveforms, the adjustment representing the phase shift at that step. Voltage measurement. An input waveform is applied at 16, Fig. 3 to a pre-amplifier 14 and thence to a paraphase amplifier 58. The zero level may be adjusted at 60 and an adjustable D. C. voltage 62 is applied to the other amplifier input. The two antiphase outputs from the amplifier are switched alternately by the switcher 26' to the oscilloscope. The switch system incorporates a trigger 64 and multi-vibrator 34 operated from the oscilloscope sweep generator. The zero level of the two complemenary traces are superimposed by adjusting 60 and then the amplitude at any selected point determined by adjusting 62 until the corresponding points in the two waveforms coincide.