Patent GB-601385-A (GB)
Title: Improvements in frequency measuring devices
IPC: G01S13/34
Filed: 1945-04-06
Abstract: 601,385. Frequency measurements. MARCONI'S WIRELESS TELEGRAPH CO., Ltd April 6, 1945, No. 8621. Convention date, Feb. 27, 1941. [Class 37] [Also in Group XL (c)] Frequency is measured by deriving from the current whose frequency is to be determined a current proportional to the frequency, applying this current to a circuit including a meter and having a predetermined time constant, and varying this time constant as a function of the amplitude of the current whose frequency is to be determined. The frequency of a current subject to excessive downward variations, e.g. reflected waves in radio altimeters, may thus be measured notwithstanding such variations. As shown in Fig. 1, a signal source 1 is connected through a biassing battery 3 to the input of a thermionic valve limiter 5, the output circuit of which includes a resistor 7 and a source 9 of constant voltage. A byepass capacitor 11 and a limiter 13 are connected across the source 9. The upper terminal of the resistor 7 is connected through a capacitor 15 to one cathode 16 and anode 18 of a pair of rectifiers 17, the other anode 20 being connected through a resistor 19, which is shunted by a capacitor 21 to a potentiometer 23 shunting the source 9. The anode 20 is also connected through an isolating rectifier 25 and a limiting resistor 27 to the grid of a D.C. negative feedback amplifier 29. A capacitor 31 is connected between the anode of the isolating rectifier 25 and earth. The cathode 24 of the amplifier 29 is connected to the second cathode 22 of the pair of rectifiers 17 and through a resistor 33 and meter 35 to earth. The resistor 33 may be byepassed by a capacitor 37. A signalcontrolled discharge path is provided by means of a second pair of rectifiers 39, of which the anode 26 is earthed, and the other anode 28 and cathode 30 are connected through a capacitor 41 and resistor 43 to' the upper terminal of the resistor 7. The junction of the resistor 43 and capacitor 41 is connected to earth through a capacitor 45. The second cathode 32 is connected to the anode of the isolating rectifier 25. Currents of the frequency to be determined are applied to the limiter 5, the bias on which prevents response to currents of less than a predetermined amplitude. Currents above this amplitude are limited by the anode resistor 7 and alternately charge and discharge the capacitor 15 and pair of rectifiers 17. The rectified currents charge the network including the resistor 19 and capacitor 21, so that the voltage across the network is proportional to the frequency of the applied signal currents. The time constant of the network is made longer than the lowest frequency to be measured. The charge on the capacitor 21 is of such polarity that the terminal 47 is negative with respect to the point 48. The potentiometer 23 provides an initial positive bias. The capacitor 31 is charged through the rectifier 25 to approximately the potential of the point 47 and establishes a potential on the grid of the amplifier 29. Owing to the degenerative feedback, the output voltage between cathode 24 and earth is slightly greater than that between grid and earth. The output voltage is indicated by the meter 35, which may be calibrated to indicate frequency, altitude or drift. In the presence of applied signals the circuit including the rectifiers 39 provides a constant current for the capacitor 31 opposite in polarity to the main charging current. The network 43, 45 has an attenuation characteristic increasing with frequency, while the capacitor 41 and rectifier 39 develops a voltage proportional to the frequency and the applied. voltage. The resulting combination has a. characteristic independent of the frequency and amplitude of the signals applied to the limiting amplifier 5, so that the current from the rectifiers 39 is constant with respect to frequency and amplitude. In the absence of a signal, no current flows through the rectifiers 39 and therefore the capacitor 31, isolated by the rectifier 25, has no discharge path except the infinitely high resistance input of the amplifier 29, so that the meter 35 continues to indicate the frequency of the signal currents last charging the capacitor 31. On again applying the signal, increase or decrease of frequency causes rapid corresponding increase or decrease of the charge on the capacitor 31, so that the meter 35 continues or indicate correctly the frequency of the last applied current and responds immediately to indicate any change in frequency independently of amplitude fluctuations of the signal. The cathode 22 of the rectifiers 17 is biassed somewhat positive with respect to the rectifier anode so as to overcome contact potential or lack of linearity in the rectifier. The effect on the meter 35 of voltages developed in the rectifier 25 and the grid-cathode of the amplifier 29 by their contact potentials may be minimized by applying frequency meter voltages which are many times greater than the undesired contact potentials. In a modification, Fig. 2, the isolating rectifier 25 and a resistor 49 are shunted across the capacitor 31 in which the charge is proportional to the frequency of the signal currents. The discharge path includes the rectifier 25 and the resistor 49, which connects the anode 51 to a source of positive bias. The output circuit of a pentode 53 is connected across the resistor 49, the input circuit being connected through a capacitor 55 to the upper terminal of the resistor 7. In the presence of applied signals the anode circuit resistance of the pentode alternately rises to infinity and falls to a finite value, the shunt path across the capacitor 31 alternately being finite and infinite, so that the shunt path is effective only half the time. During abnormal operation, i.e. when the signal approaches zero, the shunt path resistance becomes infinite and the capacitor 31 is isolated, this condition being determined by the bias applied to the input of the pentode. Fig. 3 shows a modification suitable for use in an altimeter employing the constant modulation band system, in which the derived frequency meter voltage may be increasing positive with increasing frequency. The signal source 1 is connected to the input of the limiting amplifier 5, which is biassed to cut-off by the bias battery 3. The output includes the resistor 7 which is connected through the capacitor 15 to diodes 17, connected to a network including the capacitor 21 shunted by the resistor 19. The upper terminal of this network is connected through the isolating diode 25 to the input grid of the degenerative amplifier 29 and to the capacitor 31. The cathode circuit of the amplifier 29 includes the self-bias resistor 33, meter 35, and byepass capacitor 37. The discharge path, which is controlled by signal current, is obtained by connecting the cathode of the diode 25 through a resistor 71 to the suppressor grid 73 of the limiting amplifier 5. The operation differs from that of the circuit of Fig. 1 in the following ways. There is no steady bias voltage in series with the frequency meter voltage derived from the signal currents in diodes 17. The frequency meter voltage becomes increasingly positive as the applied frequency increases. The signal controlled discharge path for the capacitor 31 includes the resistor 71 and grid 73, which acts as an anode to provide a constant discharge path effective only in the presence of an applied input signal.