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SR1 Operation 53
© 2014 Stanford Research Systems
sweep over the frequency span of the selected analyzer. Because of the synchronous nature of the chirp
signal, a uniform window should be selected in the analyzer when using the log-sine chirp waveform.
Selection of the associated FFT Analyzer is done with the Chirp Source control.
Chirp Source Associated Analyzer
A0: FFT1 Analyzer 0, Single Channel FFT Analyzer
A0: FFT2 Analyzer 0, Dual Channel FFT Analyzer
A1: FFT0 Analyzer 1, Single Channel FFT Analyzer
A1: FFT1 Analyzer 1, Dual Channel FFT Analyzer
If the analyzer span is set to frequencies which are unreachable with the current generator sampling the
"Compliance" led will glow red and no waveform will be output.
Be sure to select the "Uniform" window when using the log-sine chirp. Window functions
attenuate the beginning and the end of the time record which means that some frequencies will
be attenuated more than others by the window.
The advantages using log-sine chirps for impulse response measurements are detailed by Miller and
Massarini ("Transfer Function Measurements with Sweeps",
J. Audio Eng. Soc ., vol. 49, pp. 443-471,
June 2001.) They report better results using log-sine stimulus compared to MLS stimulus for a wide
range of audio DUTs. One particular advantage concerns the unique ability of the log-sine chirp to
differentiate between the linear and non-linear portions of the DUT response. Because of the properties of
the group delay of the log-sine chirp waveform, harmonic responses appear offset in time relative to the
linear response. This can be clearly seen in this impulse response measurement of a consumer stereo
amplifier:
When the purely linear response is gated and transformed back into the frequency domain a much better
measurement of the DUT frequency response is obtained. (This is detailed in the
FFT2 analyzer
discussion. See "Measurement of audio equipment with log-swept sine chirps", by Thomas Kite,
Presented at the 117th AES Convention, San Francisco, CA, October 2004 for a concise explanation of
the unique group-delay properties of the log-sine chirp.)
The log-sine chirp waveform outputs a generator trigger at the beginning of each FFT time
record. Be sure to use generator trigger as the analyzer trigger source when using the log-
sine chirp waveform with the FFT analyzer.