Circuit arrangement for attenuating a broadband background noise level and interfering signals superimposed thereupon

Claim 2

A circuit for attenuating broadband background noise signals in a frequency region between the circuit frequency limits and superimposed interfering signals in a limited frequency range within and narrower than the frequency region, in a system for detecting useful signals of shorter duration than the interfering signals, said circuit comprising, in combination; means defining an input line connected to receive those background noise signals, interfering signals and useful signals lying within the frequency region; means defining a first processing channel connected to said input line for selectively processing those signal components on said line which are within the limited frequency range; means defining a second processing channel connected for influencing those signal components on said line which are outside of the limited frequency range and within the frequency region; control amplifier means connected in both processing channels for amplifying the signals in the channels and connected to have its gain controlled in a manner to produce a constant output voltage level, the gain-adjusting control time constant of said amplifier means being long compared to the duration of the useful signals and short compared to the duration of the background noise and interfering signals so that useful signals do not substantially influence the gain of said amplifier; and output means connected to the signal output of said amplifier means for producing an output signal composed of the useful signals within the frequency region superimposed on a substantially constant amplitude background derived from the broadband background noise and the interfering signals.

Claim 3

An arrangement as defined in claim 1, wherein: said channels are connected in parallel; said control amplifier means includes a first control amplifier in said first channel and a second control amplifier in said second channel; said first channel comprises a first bandpass filter connected between said input line and the signal input of said first amplifier and having a pass band corresponding to the limited frequency range, and a second bandpass filter connected between the signal output and control input of said first amplifier and having a pass band within, and narrower than, the limited frequency range; said second channel comprises a band-rejection filter connected between said input means and the signal input of said second amplifier and having a stop band corresponding to the limited frequency range, and a third bandpass filter connected between the signal output and control input of said second amplifier and having a pass band outside of the limited frequency range but within the frequency region; and said output means comprise adder means having inputs connected to the signal outputs of said first and second amplifiers and an output at which the output signal appears.

Claim 4

A circuit for attenuating broadband background noise signals in a frequency region between the circuit frequency limits and superimposed interfering signals in a limited frequency range within and narrower than the frequency regiOn, in a system for detecting useful signals of shorter duration than the interfering signals, said circuit comprising, in combination: means defining an input line connected to receive those background noise signals, interfering signals and useful signals lying within the frequency region; means defining a first processing channel connected to said input line for selectively processing those signal components on said line which are within the limited frequency range; means defining a second processing channel connected for influencing those signal components on said line which are outside of the limited frequency range and within the frequency region; a control amplifier connected to both of said processing channels for amplifying the signals in its associated channel and arranged to have its gain controlled in a manner to produce a constant output voltage level, the gain-adjusting control time constant of said amplifier being long compared to the duration of the useful signals and short compared to the duration of the interfering signals so that useful signals do not substantially influence the gain of said amplifier; output means connected to the signal output of said amplifier for producing an output signal composed of the useful signals within the frequency region superimposed on a substantially constant amplitude background derived from the broadband background noise and the interfering signals; a signal level reference member having a comparison signal input, a second signal input and an output at which appears a signal representing tbe amount by which a signal at said second signal input exceeds a signal at said comparison signal input; said second channel includes comparison means connected to said comparison signal input and providing a comparison signal having a permanently settable level representing the anticipated level of the background noise signals outside the limited frequency range; and said first channel includes a controllable band-rejection filter having a stop band corresponding to the limited frequency range and having a control input, the stop band attenuation of said band-rejection filter being proportional to the value of the signal at its said control imput, said filter having its control input connected to said reference member output, its signal input connected to said input line, and its signal output connected to the signal input of said control amplifier, and a bandpass filter having a pass band corresponding to the limited frequency range and connected between said amplifier signal output and said reference member second signal input.

Claim 5

A circuit for attenuating broadband background noise signals in a frequency region between the circuit frequency limits and superimposed interfering signals in a limited frequency range within and narrower than the frequency regiOn, in a system for detecting useful signals of shorter duration than the interfering signals, said circuit comprising, in combination: means defining an input line connected to receive those background noise signals, interfering signals and useful signals lying within the frequency region; means defining a first processing channel connected to said input line for selectively processing those signal components on said line which are within the limited frequency range; means defining a second processing channel connected for influencing those signal components on said line which are outside of the limited frequency range and within the frequency region; a control amplifier connected to both of said processing channels for amplifying the signals in its associated channel and arranged to have its gain controlled in a manner to produce a constant output voltage level, the gain-adjusting control time constant of said amplifier being long compared to the duration of the useful signals and short compared to the duration of the interfering signals so that useful signals do not substantially influence the gain of said amplifier; output means connected to the signal output of said amplifier for producing an output signal composed of the useful signals within the frequency region superimposed on a substantially constant amplitude background derived from the broadband background noise and the interfering signals; a signal level reference member having a comparison signal input, a second signal input and an output at which appears a signal representing tbe amount by which a signal at said second signal input exceeds a signal at said comparison signal input; said second channel includes comparison means connected to said comparison signal input and providing a comparison signal having a permanently settable level representing the anticipated level of the background noise signals outside the limited frequency range; and said first channel includes a controllable band-rejection filter having a stop band corresponding to the limited frequency range and having a control input, the stop band attenuation of said band-rejection filter being proportional to the value of the signal at its said control imput, said filter having its control input connected to said reference member output, its signal input connected to said input line, and its signal output connected to the signal input of said control amplifier, and a bandpass filter having a pass band corresponding to the limited frequency range and connected between said amplifier signal output and said reference member second signal input.

Claim 6

An arrangement as defined in claim 5 wherein said controllable band-rejection filter comprises: a voltage divider having an input connected to said input line and two outputs and including means for varying the division of the signal at its input between its two outputs; a further band-rejection filter connected to one output of said divider and having a stop band corresponding to the limited frequency range; and an adder having one input connected to the other output of said divider, another input connected to the output of said further band-rejection filter, and an output at which appears a signal corresponding to the sum of the signals at its inputs.