Stanford Research Systems SR785, Руководство по эксплуатации и программированию

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Swept Sine Measurements 2-47
SR785 Dynamic Signal Analyzer
Swept Sine Measurements
A swept sine measurement is basically a sine sweep which steps through a specified
sequence of frequency points. At each point, the source maintains a constant frequency
and the inputs measure signals only at this frequency. After each measurement point is
complete, the source then moves to the next point in the sequence. Unlike the FFT which
measures many frequencies at the same time, a swept sine source measures the frequency
response a single frequency at a time.
Why Use Swept Sine?
Frequency response can be measured using the FFT. However, if the frequency response
has a large variation within the measurement span, then the FFT may not be the best
measurement technique. It’s limitation comes from the nature of the chirp source that
must be used. The FFT measures the response at all frequencies within the span
simultaneously, thus the source must contain energy at all of the measured frequencies.
In the time record, the frequency components in the source add up and the peak source
amplitude within the time record generally exceeds the amplitude of each frequency
component by about 30 dB. Since the input range must be set to accommodate the
amplitude peak, each component is measured at -30 dB relative to full scale. This
effectively reduces the dynamic range of the measurement by about 30 dB! If the
frequency response has a variation from 0 to -100 dB within the measurement span, then
each bin of the FFT must measure signals from -30 dBfs to -130 dBfs. Even with a large
number of vector averages, this proves difficult, especially with large measurement
spans.
Swept sine measurements, on the other hand, can optimize the measurement at each
frequency point. Since the source is a sine wave, all of the source energy is concentrated
at a single frequency, eliminating the 30 dB chirp dynamic range penalty. In addition, if
the transfer response drops to -100 dBV, the input range of Channel 2 can auto range to -
50 dBV and maintain almost 100 dB of signal to noise. In fact, simply optimizing the
input range at each frequency can extend the dynamic range of the measurement to
beyond 140 dB!
For frequency responses with both gain and attenuation, the source amplitude can be
optimized at each frequency. Reducing the source level at frequencies where there is
gain prevents overloads and increasing the amplitude where there is attenuation
preserves signal to noise. To optimize the measurement time of sweeps covering orders
of magnitude in frequency, the detection bandwidth can be set as a function of
frequency. More time can be spent at lower frequencies and less time at higher
frequencies. In addition, frequency points can be skipped in regions where the response
does not change significantly from point to point. This speeds measurements of narrow
response functions.
The figure below illustrates the difference between FFT and swept sine when measuring
the frequency response of an elliptic low-pass filter. This filter has a stop band of -80 dB
and a zero of about -100 dB. The 400 point FFT measurement was made with 2500
vector averages taking about 10 seconds to complete. The swept sine also took 400
points. Each point was averaged for 16 ms or 10 cycles, whichever was longer. The
entire sweep also takes about 12 seconds.