Patent GB-1021868-A (GB)
Title: Signal generator for testing stereophonic radio receivers
IPC: H04H1/00
Filed: 1963-11-15
Abstract: 1,021,868. Testing radio receivers. RADIO CORPORATION OF AMERICA. Nov. 15, 1963 [Nov. 29, 1962], No. 45287/63. Heading H4L. In a signal generator for testing stereophonic radio receivers a single audio-frequency signal is used to represent both the L+R and L - R signals and means are provided for changing the phase of a pilot signal forming part of the composite signal generated to simulate different operating conditions of the receiver. The receivers are for use in the system wherein the L - R signal modulates a 38 kc/s. subcarrier to provide a double-sideband amplitude modulated suppressed subcarrier signal which is combined with the L+R signal and a 19 kc/s. pilot signal to form a composite signal which frequency modulates a main carrier. As shown in Fig. 1, the output of a 19 kc/s. oscillator 10 is supplied via a frequency-doubler 12 and amplifier 16 to a balanced modulator 18 which also receives via a buffer amplifier 26 a selected frequency from an audio-frequency oscillator 20, which may be switched to provide a plurality of audio frequencies and also frequencies of 19 kc/s. and 38 kc/s. The output of the modulator 18 consists of a suppressed carrier amplitude modulated signal together with the audiofrequency signal from the oscillator 20, and is supplied via a low-pass filter to suppress unwanted harmonics, to contacts 3, 4, 5 of a switch 52. The output of the 19 kc/s. oscillator 10 is also supplied via a buffer 14 to a terminal 34 supplying three branch circuits which supply oscillations via appropriate networks to terminals 3, 4, 5 of switch 48 which are respectively in phase, leading by 45 degrees, and lagging by 45 degrees, with respect to the oscillations at terminal 34. The signal selected by switch 48 is combined via amplifier 60 with the signal from filter 30 and with the ganged switches 48, 50, 52, 54 in the position shown the resulting composite signal is fed via amplifier 62 and potentiometers 63, 65 to frequencymodulate a 100 Mc/s. carrier generated at 66. The frequency-modulated signal is fed via an attenuator 68 to an outlet 70 which may be connected to the aerial connection of the receiver, or alternatively a test signal may be taken from outlet 74 to test the de-multiplexing portion. The two outputs of the receiver are connected through respective cables 80, 81 and diodes 76, 79, to a meter 75 controlled by a potentiometer 77 to be at midscale on position 3 of the switches when a correctly adjusted receiver is connected. In the switch position shown the pilot signal has a phase relationship such that there will be no output due to the subcarrier if a receiver is correctly adjusted, and the L+R signal will be equally divided between the two outputs of the receiver. On position 4 the phase of the pilot signal is advanced by 45 degrees and the resultant signal will be equivalent to that in which L-R is in phase with and equals L + R so that there will be an output L indicated by the meter 75. Similarly, on position 5 there will be an output R but no output L. At position 1 of the switches a 5À35 Mc/s. oscillator 64 is switched on and with its harmonics provides at outlet 73 check points for the tuning dial calibration. Also 6-3 volts A.C. is supplied via switch 52 to frequency modulate oscillator 66 and provide a signal for checking I.F. tuning &c. At position 2 a selected signal from oscillator 20 is supplied via amplifier 62 to modulator 66 and also to outlet 74. These signals may be used to check monophonic F.M. receivers.