Lambda SR1, Operation Manual, Page: 165 / 317

SR1 Operation 165
© 2014 Stanford Research Systems
2.4.7 Multitone Analyzer
Multitone analysis is a technique whereby a device under test is subject to a stimulus signal containing
a number of discrete tones whose frequencies are adjusted to fall exactly on fft bin frequencies of the
multitone analyzer. If the analyzer has sufficient resolution relative to the number of tones in the stimulus
signal even a single FFT can yield a wealth of audio information by examining the amplitudes in three
categories of FFT bins: bins in which a generator tone is present, the amplitudes of which can be used
to derive the frequency response of the DUT, bins in which harmonic and intermodulation distortion
products of the tones in the stimulus signal are present, which can be used to compute the THD and
IMD characteristics of the DUT, and bins in which their are no signal tone or distortion products, whose
amplitude represents the noise of the DUT. By carefully examining the amplitudes of these three types
of FFT bins it is possible to simultaneously measure Frequency Response, THD+N, THD, IMD, and
noise vs. frequency for a device based on a single captured FFT record, measurements that would
otherwise take several separate swept measurements.
Multitone Analysis is typically a synchronous measurement: tones are generated exactly on bin
frequencies and the multitone FFT analyzer is run without a window (uniform window) ensuring that each
tone will occupy a single bin in the analyzer spectrum. For this to be true, the device under test cannot
shift the frequencies in the stimulus signals. When this condition is met, the multitone analyzer (MTA)
can use an interesting technique to measure the noise of the DUT even close to tone and distortion
frequencies. When "Noise Analysis" is enabled, the MTA sets the number of lines in the analyzer FFT to
twice the length of the stimulus signal. By making the frequency resolution of the received spectrum
twice the resolution of the stimulus it is ensured that all tones, harmonics of tones, and IMD products of
tones will fall on even bins in the received spectrum while the odd bins will contain only noise.
For some test situations, the signal chain does shift the frequencies. For instance, the multitone
stimulus signal can be played back on a tape player with a speed error. In this situation the exact bin
frequencies of the stimulus will be smeared over many bins in the received spectra, and the MTA should
be operated in
windowed mode. In windowed mode a window is applied to the received signal to limit
the smearing of the tone frequencies and the noise analysis feature described above for synchronous
mode is not available. In windowed mode a parameter can be entered which describes the maximum
extent of the frequency shift and assists the analyzer in locating regions of tones, distortion, and noise.
Multitone measurements with SR1 first require the configuration of the multitone generator using the
Multitone Configuration Panel . The Multitone Configuration Panel contains options for setting the number
of tones, tone frequency and phase, and the length of the stimulus signal. Once the generator is
configured, the Multitone Analyzer (MTA) can be selected.
Summary of Multitone Analyzer Outputs
The multitone analyzer produces both scalar and vector output measurements which are described
below
Measurement Description
Vector Measurements
Time Record (A/B)
This is the raw time record received by the MTA. While not useful in and of itself,
viewing the time record can be useful in diagnosing measurement setup issues.
Linear Magnitude
(A/B)
This vector is the complete FFT of the input signal to the analyzer including all
bins.
THD+Noise Bins,
(A/B)
This measurement is the vector of all bins that that do not contain a tone in the
generator signal. The amplitude in these bins represents the sum of noise and
distortion products.