A periodic signal to be detected plus associated noise components are ampied, filtered and applied as an input to a pair of chopper switches gated synchronously with the signal to switchably chop the signal into phase-locked, complementary half wave portions. In effect, each of the chopper outputs has a direct current waveform with one of the outputs being opposite in sign to the other. The chopper outputs then are applied as separate inputs to the inverting and non-inverting inputs of a differential, low-pass, narrowband filter to eliminate random noise as well as any common errors present on both of half-wave inputs. The output of the low-pass filter is a single-polarity dc voltage which being proportional to the amplitude of the signal to be detected, provides the means for detecting the signal. A reference signal is used to gate the choppers and preferably it is processed by an integrated logic circuit to provide a pair of complementary gating signals one of which is with the input signal and the other 180.degree. out of phase. The chopper switches may be MOSFET components interconnected in a series-shunt configuration which permits very rapid switching to provide distortion-free operation over a wide range of signal frequencies (1-5MHz).
Disclosed is an electronic circuit for determining the phase difference between an input signal and a reference signal where both signals are of the same frequency. Furthermore, the circuit provides an amplitude output indicative of the input signal even when that signal is obscured by noise. A chopper is supplied with the obscured signal input and a reference signal input and provides an output to an integrator which provides a control voltage for a voltage controlled phase shifter which supplies an out of phase input to the chopper. The input to the voltage controlled phase shifter is of the same frequency as the signal obscured by noise. A further output of the voltage controlled phase shifter is 90.degree. out of phase with respect to the reference supplied to the chopper and thus is in phase with the signal obscured by noise and provides the desired marker output. This marker output is also supplied as a reference to a second chopper which is also supplied with the obscured signal as an input. The output of the second chopper after suitable amplification, is proportional to the amplitude of the signal that is buried in noise. Thus, as long as the frequency of the signal buried in noise is known and supplied in the form of a reference signal, the phase and amplitude of the signal can be determined with the synchronous phase marker and amplitude detector.
An improved detection arrangement for detecting the presence of a given tone in a received input signal. A reference generator, in the form of a reference clock and a programmable divider, produces a first reference frequency, equal to the frequency of the tone, and at an arbitrary phase angle thereto. A second generator output is in quadrature to the first. Each reference signal is product detected with the input signal via exclusive OR gates. A unique digital low pass filter, operating in a time multiplex mode, receives the gate outputs, removes all high frequency components therefrom, and produces pairs of output binary number encoded signals, each of which is representative of a vector component of a detected tone. Threshold detector circuitry monitors the outputs indicating an above threshold condition at its output in response to predetermined combinations of the binary numbers. The combinations represent all input vectors whose resultant absolute magnitude exceeds a chosen threshold. A second detector samples the threshold detector output, recording the number of samples over threshold during a given interval. If the number exceeds a selected minimum an output signal is produced which indicates the presence of a received tone.
