Lock-in amplifier

Claim 1

A lock-in amplifier for measuring a particular component of an input signal at a frequency of interest, comprising: (a) a first mixer having a first input for receiving an input reference signal having a frequency F R equal to the frequency of interest in said input signal to be measured and a second input for receiving a signal of a fixed predetermined frequency F 0 , said first mixer generating an output having a frequency F 0 +F R equal to the sum of said fixed predetermined frequency F 0 and said input reference frequency F R ; (b) a second mixer having a first input for receiving the output of said first mixer having a frequency F 0 +F R and a second input for receiving a signal having a frequency F 0 x, where x<1, said second mixer generating an output having a frequency F 0 +F R -F 0 x; (c) a third mixer having a first input for receiving said input signal having said component at said frequency of interest F R and a second input for receiving the output of said second mixer having a frequency F 0 +F R -F 0 x, said third mixer generating an output having a frequency equal to F 0 -F 0 x; (d) a narrow bandpass filter which receives the output generated by said third mixer and filters out signals at frequencies other than F 0 -F 0 x. (e) means for generating a second signal having a frequency F 0 -F 0 x from said signal Fox applied to said second mixer; (f) an adjustable phase shifting means for shifting the phase of said second signal having a frequency F 0 -F 0 x to coincide with the phase of the signal F 0 -F 0 x output from said narrow bandpass filter; (g) a phase sensitive detector having a first input for receiving the signal F 0 -F 0 x output from said narrow bandpass filter and a second input for receiving the phase shifted signal F 0 -F 0 x output from said phase shifter means, said phase sensitive detector generating an output having a dc component corresponding to the magnitude of the input signal at the frequency of interest, said dc component being maximum when the phase of the two signals F 0 -F 0 x at its inputs are aligned; and (h) low pass filter means for removing any ac signal output from said phase sensitive detector corresponding to frequencies in said input signal other than said frequency of interest.