Patent GB-1023318-A (GB)
Title: A frequency-selective signal system
IPC: H03J7/04
Filed: 1963-07-25
Abstract: 1,023,318. Frequency selective systems. HEWLETT-PACKARD CO. July 25, 1963 [Aug. 20, 1962], No. 29593/63. Heading H3A. In a frequency selective system in which a signal to be examined is mixed with a local oscillator signal to provide an intermediate signal of approximately 3 Mc/s. which is applied to a filter having a pass band 200 c/s. wide centered at 3 Mc/s. and which has a null point in the centre of its transmission characteristic, means are provided for controlling the local oscillator frequency whereby the intermediate signal falls on one or other side of the null point. The filter comprises a first mixer 19 to which the input signal and a 3 Mc/s. signal from oscillator 31 are applied, a first low pass filter 21 coupled via a capacitor to the mixer, a second mixer 25 to which the input signal and the osillator signal after 90 degrees phase shift in unit 33 are applied, a second low pass filter 27 coupled via a capacitor to the mixer 25, a pair of mixers 23, 29 coupled to the oscillator 31 and a summation circuit 30 which reconstitutes the input signal. The output from low pass filter 21 is applied to a rectifying circuit 35, 37 which derives the positive half cycle of this signal and the output from low pass filter 27, which leads or lags by 90 degrees the signal from filter 21 in dependence on whether the applied signal is above (forward) or below (reverse) the 3 Mc/s. signal from oscillator 31, is applied to rectifying circuit 39, 41 which drives the negative half cycles of this signal. The two half cycles are combined by resistors 43, 45 to provide " forward " and " reverse " signals shown. The " forward " signals when applied to the Schmitt trigger circuit 47 produce output pulse signals of constant amplitude and short duration and the " reverse " signals produce similar signals but of long duration. The output of the trigger circuit is connected to a parallel arrangement which includes a frequency to D.C. converter in one branch, a low pass filter in the other branch and means 53, 55 for combining the outputs of both branches. The output signal from the parallel combination comprises a voltage which has a different minimum level in dependence on whether the input signal is forward or reverse and which increases as the input frequency to the filter increases above or decreases below the frequency of oscillator 31. This signal is applied to a transistor amplifier including Zener diode 63 which acts as a common emitter stage for input signals having an amplitude below the largest voltage produced in the " forward " condition and as a common collector stage for input voltages above the lowest voltage produced in the " reverse " condition. The amplifier output which is applied to control the local oscillator 11 thus increases from a low valve through zero as the frequency of the applied signal increases above the frequency of oscillator 31 and decreases from a high valve through zero as the frequency of the applied signal decreases below that of oscillator 31. The local oscillator is controlled to produce an input signal to the filter having a frequency corresponding to one or other of the zero points of the characteristic hence ensuring the frequency does not correspond with the null point of the filter.