Patent GB-556727-A (GB)
Title: Improvements relating to valve voltmeters
IPC: G01R15/08
Filed: 1942-07-21
Abstract: 556,727. Voltage and inductance measurements. SMITH, S. J. July 21, 1942, No. 10149. [Class 37] A multi-range valve voltmeter, particularly for use in a Q-meter, comprises a rectifying valve, an indicating instrument, and a range-changing switch operating to change in fixed steps a bias applied to the rectifying valve such that a signal that gives a maximum reading on a lower range will give a zero reading on the next higher range. In the Q-meter shown, current from a source 10 of radio-frequency current of variable frequency is passed through resistors 11, 12 to the earthedline 13. The resistor 12 is also included in a series circuit comprising the inductance coil 14 whose Q-factor is to be determined and a variable condenser 15. The potential difference set up across the resistor 12 is thus injected into the tuned circuit 14, 15 and causes a large current to flow therein at resonance. The ratio of the voltage across the condenser 15 to the injected voltage represents the Q factor. The voltage across the resistor 12 is maintained constant, so that the voltage across the condenser 15 is a direct measure of the Q factor of the coil 14. This voltage is measured by means of a valve voltmeter comprising a diode rectifier 16, a D.C. amplifier 17, and an indicating instrument 18. The anode of the diode 16 is connected to the high potential terminal of the condenser 15 through a condenser 19 and the cathode is connected to earth through a condenser 20. The anode is also connected to earth through a load resistor 21 and with the grid of the amplifier 17 through a low pass filter comprising a resistor 22 and condenser 23. The amplifier 17 is arranged in a balanced bridge circuit with the indicating instrument 18 connected between the junction of resistors 24, 25, connected between the cathode of the valve 17 and earth, and a variable tapping on a potential divider 26 in series with resistors 27, 28 between the high tension terminal and earth. The cathode of the diode 16 is connected through a filter comprising a resistor 31 and condenser 32 with the arm of a switch 33 moving over contacts connected to tapping points on potential dividers 34 ... 38, so that increasing positive potential may be applied to the cathode of the diode 16. A backing-off bias is. thus applied to the diode 16 in increments each of which is equal to the peak value of the input that gives a full scale, deflection on the lowest range. The knob of the switch 33 may be calibrated to indicate the amount to be added to the meter reading. The current flowing through the resistor 12 is measured by measuring the voltage developed across the resistor 11, the value of which is at least 100 times that of the resistor 12. This measurement is effected by a diode 40 of which the anode is connected to the high potential end of the resistor 11 and the cathode is connected through a low-pass filter comprising a resistor 41, condenser 42, and the windings of the instrument 18 to earth. When the desired P.D. is set up across the resistor 12 the pointer of the instrument 18 reaches a definite mark on the scale. The meter 18 is connected either to the diode 41 or to the amplifier 17 by a switch 44. This switch is first thrown to the "Test" position and the output of the source 10 is adjusted until the pointer reaches the setting mark. The switch 44 is then changed over to the " measure "- position with the switch 33 in the earthed position. The frequency of the source 10 is adjusted until the circuit 14, 15 is in resonance and the deflection of the meter 18 noted. If this deflection is off-scale, the switch 33 is adjusted to bring the pointer on the scale and the number indicated on the switch is added to the scale reading to give the Q factor of the coil. The resistor 12 may be of the kind described in Specification 556,726.