Patent ID: 7724848

Claim:
Apparatus for synthesizing a first analog signal for use in the analysis of a second analog signal combined with said first signal, said second signal being a higher frequency than said first signal, comprising: a unit for receiving said combined signals; first and second output lines coupled to an output of said receiving unit; a high pass filter coupled to said first output line for passing said composite signals and filtering out harmonics of said second signal; a low pass filter coupled to said second output line for filtering out the second signal; a summing circuit having first and second inputs; a delay circuit coupling an output of said high pass filter to said first input of said summing circuit; a zero-crossing detector coupled to an output of said low pass filter for detecting zero crossings in a positive and negative going direction of a signal passed by said low pass filter; a generator responsive to non-inverted and inverted outputs of the zero-crossing detector for generating a digital representation of a version of said first signal passing said low pass filter; a unit for adjusting the digital representation to compensate for a timing skew between said signals in said first and second lines so that the signals in said first and second lines are in time synchronism when they reach said summing circuit; a D/A converter for converting the digital representation of said first signal to an analog signal; and said summing circuit canceling the analog signal generated by the D/A converter from the output of said high pass filter.