Patent GB-656103-A (GB)
Title: Improvements in tuning indicator systems
IPC: H03J3/14
Filed: 1948-09-09
Abstract: 656,103. Frequency modulation. RADIO CORPORATION OF AMERICA. Sept. 9, 1948, No. 23748. Convention date, Nov. 7, 1947. [Class 40 (v)] [Also in Group XL (b)] Means for indicating the relationship between the average peak values of two voltages comprises a discharge tube having a control grid and cathode, a pair of capacitors each having one terminal connected to the cathode, an impedance wherein the peak potential variations are set up, a charging path for one capacitor including a rectifier connecting the said capacitor with the impedance, a discharging path for the other capacitor including a rectifier connecting that capacitor with the impedance, and means for applying a voltage which represents the mean of the charges of the capacitors to said control grid. The embodiment described is a tuning indicator for a frequency-shift telegraph system wherein marks and spaces are represented by frequencies equally spaced above and below a carrier frequency. A switch S<SP>2</SP> is first thrown to the set zero position so that valve V<SP>1</SP> conducts and condensers 10 and 11 charge up equally over a diode V<SP>3</SP>. The potential of these condensers is applied to the grid of V' which is in a bridge circuit and the bias resistor 18 is adjusted until the pointer of a meter 20 is in its centre position. The switch S<SP>2</SP> is then turned to one of its TUNE positions in which the grid of V<SP>1</SP> is connected to a point X of one channel of a diversity receiving system. When the incoming two-tone signals are properly tuned their frequencies are centred on that of the carrier and marks and spaces produce at the point X positive or negative potentials of the same amplitude. If a mark frequency is received the cathode current of V<SP>1</SP> is increased and the anode potential of the diode V<SP>3</SP> increases. The condenser 10 immediately increases its charge by a predetermined amount but the condenser 11 is substantially unaffected because of its long time constant due to resistors 13 and 14. Additional marks merely maintain this condition but when a space is received the anode potential of V<SP>3</SP> and the cathode potential of V<SP>2</SP> drop and the condenser 11 discharges by the same predetermined amount as the increase of condenser 10, which remains at a substantially steady potential since it cannot discharge over V<SP>3</SP> or, because of the time constant, over resistors 13 and 14. Further signals whether of mark or space frequency maintain the potentials of the condensers 10, 11 at these new values, the centre point of the equal resistors 13, 14 remaining at the same potential as during the setting up process. The bias of V<SP>4</SP> therefore remains constant and the meter 20 maintains its pointer in the centre. If the carrier frequency drifts and the frequencies received are both higher, or both lower, than their correct values then the potentials set up at the point X will not be of equal magnitude and this will result in one condenser receiving an additional charge during signals which is different from the charge lost by the other condenser during signals. The potential at the grid of V<SP>4</SP> will therefore alter and the meter pointer will move off centre. During idle periods when the signalling condition is continuous marking (or spacing), both condensers 10, 11 will in time become charged equally at a potential dependent upon the signalling condition. The meter pointer will then move off centre to a point which may be suitably marked on the scale.