Patent GB-953477-A (GB)
Title: Apparatus for the automatic display of complex electrical characteristics of networks
IPC: G01R27/32
Filed: 1962-02-26
Abstract: 953,477. Plotting complex characteristics of electrical networks. L. ROHDE, and H. SCHWARZ, [trading as ROHDE & SCHWARZ]. Feb. 26,1962 [March 3, 1961], No. 7372/62. Heading G1U. A circuit for automatically indicating the complex electrical characteristics (e.g. impedance, admittance or propagation constants) of an electrical network, in a polar co-ordinate system or its conform equivalent, includes an arrangement for feeding a test signal of varying frequency to the network under test, and two signal channels, through one of which is fed a signal derived from the signal reflected or transmitted by the network under test, whilst through the other is fed a signal derived from the signal fed into the network under test or emanating from a network of known characteristics. According to the invention the signals in the two channels are converted into constant frequency form and the constant frequency signal in the first channel has derived from it a further signal shifted in phase through 90 degrees, the unshifted and phase shifted signals from the first channel being fed to the mutally perpendicular deflection systems of a two coordinate recorder, whilst the signal from the second channel is used to control the intensity (or presence) of the trace produced by the recorder. The trace thus produced on the recorder is in a position representing in polar coordinates the modulus and phase angle of the complex characteristics of the network, under test, or their values relative to the characteristics of the known network. In the described embodiment a wobbulating oscillator so produces a signal fo, which is passed along the decoupled paths EL 1  and EL 2  and mixed with the outputs f1, f2 from fixed frequency oscillators S1 and S2 respectively. The mixed signals are then passed through filters to produce signals (fof1) and (fo-f2) respectively at the points C and'D. The signal (fo-f1) at C is used as the test signal, and is directed to the test terminals, to one of which (E) is connected the circuit under test, whilst to the other (F) is connected a reference circuit. Circuit elements D I , D II  serve to isolate the source C from reflected signals, whilst directional couplers RK I , RK II  direct the reflected signals to the mixers M I , M II , respectively. In these the reflected signals are mixed with the frequency (fo-f2) from the point D so that subsequent operations are performed at a constant I.F. of (f1 + f2). The signals, after amplification in the I.F. amplifiers V I  and V II , may subsequently be frequency changed in the mixers U I , U II  with a signal from the oscillator S3. This signal is frequency varied under the control of any frequency drift in the reference channel, through an amplifier RV 3 , a limiter discriminator B and an amplifier RV 4  so that any such drift is compensated for. The output from the amplifier V II <SP>1</SP> in the reference channel is differentiated in the differentiator DN and the pulses so produced are applied to the control grid of a cathode ray tube KR. The signal from the mixer U I  in the test channel is passed via an amplifier V I <SP>1</SP> to a phase shifting circuit PS, alter which it passes to the horizontal deflecting system of the cathode ray tube, and also directly to the vertical deflecting system, via a resistor R. With this system it is possible to read on the face of the tube KR the complex reflection coefficient, or the impedance, or the transfer characteristic, of a network connected between the terminals E and F. The circuit described also includes feedback channels RV 1 , RV 2  for controlling the operating currents of the mixers M I , M II  and the voltage at the point G respectively. The Specification also describes an arrangement which avoids the need to apply the differentiated reference signal to the grid of the tube, by using gating arrangements between this signal and the test channel signal (Fig. 7 not shown). This allows operation and recording to be carried out at very slow wobble rates.