Frequency marker display system

In a frequency marker display system having a display device and means for indicating on said display device a marker indicative of a center frequency and a plurality of markers indicative of side frequencies which are lower and higher than said center frequency, said indicating means including a sweep repetition oscillator for producing a sweep repetition signal, a sweep signal oscillator for producing a sweep signal whose frequency varies about said center frequency in a periodically repetitive manner, a first standard frequency oscillator for producing an output signal at said center frequency, a frequency mixer coupled to receive said output signals from said sweep signal oscillator and said first standard frequency oscillator for producing an output signal at zero frequency when said sweep signal oscillator output signal attains said center frequency, a first low-pass filter coupled to receive an output from said first low-pass filter to produce an output signal when said center frequency is attained by said sweep signal oscillator, a second standard frequency oscillator, a harmonic frequency mixer coupled to receive an output signal from said standard frequency oscillator and said frequency mixer for producing an output signal containing frequency components comprising the sum and difference frequencies of said output signal from second standard frequency oscillator and a plurality of harmonics thereof with said output signal from said frequency mixer, a second low-pass filter coupled to receive said output from said harmonic frequency mixer to produce an output signal to indicate one of said side frequencies when the frequency of said output signal from said frequency mixer coincides with the frequency of said output signal from said second standard frequency oscillator and with said harmonics thereof, means for producing a signal which is the sum of said signals from said first and second low-pass filters, an output from said summing means being applied to said display device to cause said frequency markers to be displayed, the improvement comprising:

• side marker voltage selector switch means operable to selectively produce an output signal at each of a plurality of voltage levels, each of said voltage levels corresponding to a level of said output signal from said sweep repetition oscillator at which a frequency corresponding to one of said side frequencies is generated by said sweep oscillator;
• level comparator circuit means for comparing the amplitude of said output signal from said side marker voltage selector switch means with the amplitude of said output signal from said sweep repetition oscillator and for generating an output signal when coincidence between said amplitudes is detected;
• and
• control gate means connected in the circuit path between said second standard frequency oscillator, said harmonic frequency mixer, said second low-pass filter and said signal summing means, and responsive to said output signal from said level comparator circuit means for enabling the application of an output signal from said second low-pass filter to said signal summing means;
• whereby at least one selected side frequency marker corresponding to the output signal from said side marker voltage selector switch means is indicated on said display device, while all other ones of said side frequency markers are inhibited from being so indicated.