858,173. Radar. GENERAL ELECTRIC CO. Ltd. May 1, 1957 [May 4, 1956], No. 13955/56. Class 40(7). [Also in Group XXXVI] Each of a group of electrical signals to be tested is converted to a single electrical parameter which is then checked for tolerance limits to indicate whether the original signal is within the predetermined limits. The invention is described with reference to testing the various outputs of a radar installation, -for example voltage and frequency. The various signals to be tested are taken from junction boxes forming part of the radar installation and are fed through attenuators 2 individually adjusted so that all the signals after conversion to the same parameter, will have the same nominal value. The first bank S1 of a six bank selector switch operated by relay 31 selects the signal to be tested while the second bank S2 operates one of relays A-H to direct the signal to the appropriate one of converters 3 to 9 where the signal is converted to a voltage. Alternatively the signal may be routed direct to output line 11. The voltage representing the signal to be tested is then fed to high and low limit circuits 12, 13 which are also fed with check voltages derived from tapped resistors. The centre of these resistors gives a voltage corresponding to the nominal value of the signal and the various tappings represent various percentages above on resistor 21 and below on resistor 22 this value. The appropriate upper and lower limits for the signal are selected by switch banks S3, S4. A timing circuit 27 operates so that at the end of a test period determined by which of relays A-H is operated and therefore which test is being carried out a testing pulse is fed to the two limit circuits 12, 13. If the signal under test is between the appropriate values lead 29 is earthed and switch S6 moves on to conduct the next test. If not a visual indication and/or an alarm is sounded and the step by step switch stops in the position of the faulty test. Bank S6 illuminates lamps 24 in turn to indicate which of the tests is being carried out. An error indicating circuit 26 compares the voltage on line 11 with the mid point of resistors 21, 22. The apparatus may be checked by means of standard signals to switch bank S1 change over switches 121, 122 being closed to apply reduced tolerances to the high and low limit circuits. Peak voltage converter (Fig. 3). The device 3 of Fig. 1 is shown in detail in Fig. 3. The voltage of which the peak is to be determined is applied to cathode follower 43. The signal developed across cathode resistor 44 is rectified by diodes 45, 46 and passed to smoothing network 47 so as to derive at the output terminal 48 a voltage proportional to the peak to peak voltage of the output signal. RMS voltage converter (Fig. 4) (Device 4 of Fig. 1). The input current is passed through transformer 48 and provides a heater current for diode 51 the signal developed across resistor 52 in the diode cathode circuit being a voltage proportional to the RMS value of the input signal. Average voltage converter (Fig. 5) (Device 5 of Fig. 1). The signal, the average value of which is to be determined is fed through transformer 58 to diodes 56, 55 connected to opposite ends of the centre tapped secondary. The diodes have a common cathode resistor the voltage across which is smoothed so as to derive at 60 a D.C. voltage proportional to the average value of the input. Power supply converter, The power supply consists of a direct and two alternating supplies the latter at 400 and 1600 cycles per second. The signal is fed to converter 6 without passing through an attenuator. Device 6 provides two direct current signals which each have a nominal value of 10 volts when the supplies being checked are all present this device is described in Specification 858,172. Frequency converter. The converter 7 of Fig. 1 shown in detail in Fig. 6 converts the frequency of an oscillating signal into a unidirectional voltage which is a measure of the value of the frequency. The signal fed to the appropriate contact of the switch bank S1 has a direct component superimposed thereon which is a measure of the nominal value. As shown, the signal to be tested consisting of the oscillating component and the D.C. component is fed to a triode 65 arranged as an amplitude limiter so that an approximately square waveform passes to a Schmitt trigger circuit. On the negative excursion the right hand portion of the trigger conducts and on the positive the left hand. The anode voltage of triode 68 is differentiated and each positive going pulse selected by diode 70 and fed to the suppressor of a pentode 72 connected in a Miller transitron circuit. This valve is normally cut off by the suppressor grid bias but conducts when the positive peak increases the suppressor grid voltage. After the anode circuit has been switched on by pulsing the suppressor anode current flows for a predetermined period when the valve is again cut off. The screen grid signal developed is rectangular and is arranged to trigger a further Schmitt circuit 73 which produces positive going pulses slashed which last for as long as valve 72 conducts. This period is determined by the value of the D.C. reference voltage applied direct to the control grid of valve 72. The pulse signal from trigger circuit 72 is clamped by diode 75 and is smoothed by network 76 to provide the required output at terminal 77. Peak to peak noise and ripple converter (Device 8 of Fig. 1). This consists of a high gain amplifier followed by a peak to peak detector. Pulse amplitude converter (device 9 of Fig. 1) (Fig. 7). The pulse signal applied to input 81 is passed through a three stage wide band amplifier 82 followed by a peak voltage detector 83. The voltage developed across capacitor 84 is passed through a cathode follower stage 85 to output terminal 86. High limit circuit (Fig. 8). The test signal is applied to the left hand grid and the check signal to the right hand grid of a difference amplifier 88 the total anode current of which is regulated by a triode 89 in the cathode circuit. The difference voltage developed between the two anodes is applied to a modulator 93 fed with a 5 kilocycle per second oscillation from an oscillator 94. The modulated oscillation is amplified at 95 and fed to a phase-sensitive detector 96 comprising pentode 97. The modulated oscillation is applied to the control grid and a reference in phase with the supply to the suppressor grid. The reference oscillation cuts off the pentode on alternate half cycles and the output signal is rectified by diode 98 and used to bias the triode 99 which has the operating winding of relay P in its anode circuit. The arrangement is such that relay P operates only if the difference between the voltages on paths 11, 14 is greater than a predetermined value. Relay P is provided with holding contacts and operates a warning light and stops the sequence of tests. The low limit circuit is supplied by the same oscillator and is similar except that the reference supplied to the suppressor grid is in anti-phase with the modulator oscillation. Error indicating circuit. (Fig. 9). This comprises a pair of triodes to the grids of which are supplied the test and reference voltages the valves are arranged as cathode follower stages and the difference is indicated at 104. A three position switch connects one or other of resistors 107, 109 in circuit to vary the meter range. Timing circuit (Fig. 10). A double triode thermionic valve 111 is connected as a freerunning multi-vibrator. A number of resistors 112 are associated with contacts A1 &c. so that for any test selected by bank S2 the appropriate resistor is connected to allow time for the test to be completed. At the beginning of a timing cycle the left hand triode is cut off. After a period determined by the value of the resistor 112 the right hand triode cuts off. A positive pulse is applied to the suppressor grid of valve 97 (Fig. 8) which is normally non-conducting. Until this pulse is produced no test can be made. When the multi-vibrator resets triode 113 is cut off. The anode voltage is differentiated at 116, 117 and the resulting signal applied to the control grid of pentode 118 to render the valve conducting. When the positive pulse applied to open valve 97 ceases, relay 2 in the anode circuit of 118 is operated this earth lead 29 to operate the step-bystep switch so as to set up the connections for the next test. At the same time test path 11 is earthed. If relay P (Fig. 8) is operated however contacts T3 open so that a negative voltage is fed to the grid of 114. This maintains 114 cut off so that relay 9 cannot operate. Bridging of contacts 100 (Fig. 8) causes the multivibrator to restart. Manual closure of one of switches 33 connected to line 120 causes the multi-vibrator to operate at maximum speed until the step-bystep switch arm reaches the closed switch when triode 114 cuts off and the voltage on lead 115 rises to enable the next test to be carried out. Thus by pressing the appropriate button any test can be selected.