Stanford Research Systems SR785, Руководство по эксплуатации и программированию

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2-52 Swept Sine Measurements
SR785 Dynamic Signal Analyzer
Auto Resolution examines the measurements of successive frequency points. If the
newest measurement is within the Faster Threshold of the previous measurement (for
BOTH channels), then the sweep will take larger steps, skipping frequency points. Each
successive time this threshold is met, the step size is increased until the Maximum Step
Size is reached. This speeds up the sweep in regions where the response is flat (varies
less than the Faster Threshold).
If a measurement differs from the previous measurement by more than the Slower
Threshold (for EITHER channel), then the sweep returns to the previously measured
point and moves to the very next frequency point in the sweep (with no skipping). The
sweep continues from this point, speeding up if allowed and slowing down when
required. This ‘fills’ in skips in the sweep which vary by more than the Slower
Threshold.
Measurements which differ by more than the Faster Threshold (on EITHER channel) but
less than the Slower Threshold (on BOTH channels), maintain the present sweep speed.
The number of points skipped remains the same in this case.
In order to adjust these parameters appropriately, some knowledge of the frequency
response is required. In order to save time, the Faster Threshold must be set to allow
some speeding up. Set the Faster Threshold to the desired amplitude resolution
remembering that some sacrifice is required to save time. In order to avoid missing the
major features in the response, set the Slower Threshold to less than half of the feature
size. If the feature is narrow, set the Maximum Step Size to less than half of the feature
width (in sweep points).
It is good practice to start by taking a sweep with Auto Resolution Off. This ensures that
the span and resolution of the sweep are correct as well as the averaging times. Once
these have been established, turn Auto Resolution On to save time on successive
measurements. Start by setting the Faster Threshold to something greater than the
variations of the flat regions of little interest. Set the Slower Threshold to twice the
Faster Threshold and the Maximum Step Size to no more than 5% of the Sweep Number
Of Points. Adjust the parameters as necessary to optimize the sweeps where needed.
A well specified Auto Resolution sweep can take 80% off of the sweep time while
preserving the essential features of the frequency response. This is very useful in
repetitive measurements.
Input Auto Ranging
Input auto ranging has the largest impact on extending the dynamic range of a swept sine
measurement. It is also the simplest optimization to setup and use. Simply press the
[Auto Range] keys to turn on Auto Ranging. Auto Ranging always tracks the input
signals during a swept sine measurement. At each frequency point, each input range is
adjusted to keep the signal within -6 dB of full scale (if possible). If the input is
overloaded, then the range is increased until the overload is removed.
Auto Ranging increases the measurement times whenever an input range is changed.
This is especially true when the signal drops below -6 dBfs. In order to detect this under-
range condition, the measurement must be made for the entire integration time. At low