Method of calibrating interleaved digitizer channels

An improved method of calibrating interleaved digitizer channels for a multi-channel test and measurement instrument of the type where each channel of the oscilloscope has a preamplifier for receiving an input signal coupled in series via a switch to a digitizer for producing data samples representing a waveform, the outputs from each digitizer being calibrated using manufacturer calibration constants to produce a bandwidth enhance filter for each channel that correct the magnitude and phase characteristics over the bandwidth of each channel, the improved method comprising the steps of:

• configuring the switch to an interleaved mode such that the input signal applied through the preamplifier for one channel is applied to the digitizer for that channel to produce a reference digitizer channel path and to the digitizer of another channel to produce an interleaved digitizer channel path;
• computing a match filter for the interleaved digitizer channel path based upon fast Fourier transforms performed on the outputs of the corresponding waveforms for the reference and interleaved digitizer channel paths so the output of the interleaved digitizer channel path matches the output of the reference digitizer channel path in phase and magnitude at all frequencies when the compensation filter is applied to the output of the interleaved digitizer channel path;
• performing a fast Fourier transform on the outputs of the reference digitizer channel path and the interleaved digitizer channel path after application of the respective bandwidth enhance filters to determine a phase angle between the reference and interleaved digitizer channel paths at a largest harmonic greater than a predetermined frequency value;
• and
• converting the phase angle into a time value for delaying a sample clock for the digitizer for the reference digitizer channel path to become a delayed sample clock for the digitizer for the interleaved digitizer channel path.