Patent GB-572259-A (GB)
Title: Improvements in arrangements for detecting and measuring modulation of carrier oscillations
IPC: G01R29/06
Filed: 1944-03-24
Abstract: 572,259. Measuring and monitoring in radio receivers. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. March 24, 1944, No. 5459. Convention date, March 24, 1943. [Class 40 (v)] A circuit for detecting and measuring the depth of modulation of received waves, whether modulated in amplitude, frequency, or phase, comprises a limiter, which functions only as a coupling device during reception of AM waves, feeding a balanced frequency discriminating detector arranged to give a differential output for FM or PM wave and an additive output for AM waves, the detected output being passed through an attenuation network and a variable gain amplifier to a modulation meter. As shown, the receiver is of the superheterodyne type with a mixer valve MT fed from the aerial through amplifiers RFA and from a local oscillator O through a harmonic generator HG. The IF output is amplified at 15 and passed to a limiter LT comprising two triodes coupled by a cathode resistor 17. When AM waves are being received, a switch S3 disconnects the anode supply of the first triode at the contact AM, thereby removing the limiter action and causing that triode to act as a capacity coupling to the grid of the second triode. The output from LT passes to an amplifier A feeding a balanced discriminator with off-tune circuits 20, 22 across which are connected rectifiers B, C, with resistances 40, 40<1> in their anode leads, and resistances 41, 41<1> in their cathode leads. An additive output across resistances 40<1> and 41 is taken off at the contact AM of switch S1 when AM waves are received, and a differential output across 40, 40<1> at contacts PM, FM when PM or FM waves are received. The selected output is fed to a cathode follower stage D having fixed and variable resistors 56, 57 and a meter 43 in its cathode lead, with a jack 51 connected between cathode and earth for the insertion of an oscilloscope. The output from the stage D is fed through a variable attenuator 60, 62 to an amplifier E, the anode of which is connected through a resistor 65 to a switch arm S2 engaging AM, PM and FM contacts, and to the grid of the next amplifier F. Contacts AM and FM are connected to earth by variable resistors 66, 68 and contact PM by a fixed condenser 69 which co-operates with resistor 65 as an integrating circuit. The output of stage F is taken from a potentiometer 75 to the grid of the next stage G, which feeds a final stage H with its output transformer 73 connected in parallel to jacks 78, meter 76 and switchcontrolled resistor 79. The jacks 78 may be used to plug in telephones or oscilloscopes, the latter being used, if desired, in place of the meter 76 for measurement purposes. The input to stage E includes a switch 59 which is moved to an upper contact when reception is effected, and for calibration purposes to a lower contact which is connected to a tapping on resistance 72, 74 connected between earth and a low A.C. voltage mains supply. When receiving PM or FM waves, correct tuning is indicated by the meter 43 in stage D, giving the same heading as when its grid is earthed at the off-contact of the switch S1. The switches S1, S2, S3 are ganged to operate simultaneously. In order to reduce microphonic variations in the frequency of the local oscillator O, the fixed capacity of its tuned circuit is increased, and a selecting switch provided, while the variable condenser is made of small capacity. The output circuit of the harmonic generator HG comprises a variable condenser C1 and switched inductance coils L1, L1<1>, L1<11>. A similar tuning circuit is used between the aerial and the mixer valve MT. The circuit elements and the wiring of the oscillator are rigidly mounted, and the valve O has low microphonic capacity. The shielding box is mounted on rubber shock-absorbers. The Specification includes detailed description of the manner in which the apparatus is calibrated and subsequently used for the measurement of modulation depths.