Lambda SR1, Руководство по эксплуатации, Страница: 65 / 317

SR1 Operation 65
© 2014 Stanford Research Systems
Highpass The white or pink noise is filtered with a 4th order Butterworth hipass filter at the
frequency specified in the filter frequency control.
1/3 Octave The white or pink noise is filtered with a 4th order 1/3 octave bandwidth Butterworth
bandpass filter at the frequency specified in the filter frequency control.
MLS Noise*
The MLS noise waveform outputs a Maximum Length Sequence with a specified repetition interval and
frequency profile. MLS waveforms are typically used as a stimulus in impulse response measurements
where several special properties of the MLS (the autocorrelation of an MLS sequence is a delta function)
simplify the calculation of the impulse response. Because SR1 uses a full 2-channel FFT analyzer to
make impulse response measurements it is not necessary to use MLS noise as a stimulus— any
broadband signal will work. Nevertheless SR1 includes the MLS waveform because of its historical
association with impulse response measurements.
Amplitude controls, as with all SR1 waveforms, the peak value of the noise output. The crest factor of
the MLS waveform, which is essentially a square wave, is close to 1.
The Length selection controls the length of the MLS sequence. Selecting "13," for example, chooses an
MLS sequence whose repetition interval is 2
13
-1 samples.
The Pink checkbox specifies that the noise output should be filtered by a 3 dB/octave "pinking" filter.
While the power contained in a white noise signal is linearly proportional to the measurement bandwidth,
pink noise will have equal power in equal logarithmic frequency intervals, e.g. the power contained in the
100 Hz to 200 Hz interval will be the same as the power contained in the 10 kHz to 20 kHz interval.
USASI Noise*
USASI noise is a special type of filtered noise designed to mimic the content of audio program material.
USASI noise is typically used in testing broadcast transmitters to measure compliance with
transmission bandwidth requirements. Because the frequency content of USASI noise is fixed, only an