Stanford Research Systems SR780, Operating Manual And Programming Reference

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Swept Sine Measurements 2-45
SR780 Network Signal Analyzer
Sweep Frequency and Auto Resolution
The span of a swept sine sweep is determined by the Start and Stop frequencies. The
entire 102.4 kHz frequency range of the SR780 is available for swept sine measurements.
Note that starting a sweep at DC is not possible. In fact, beware of starting at any
frequency much less than 1 Hz since the Settle and Integration times are always a
minimum of 1 cycle. If the start frequency is very low, the first point can take longer to
measure than the entire remainder of the sweep!
The Sweep Number Of Points, or resolution, can be set from 10 to 2047. The points can
be in a linear or a logarithmic progression. In many cases it is desirable to sweep over a
wide frequency range while still detecting narrow features in the response function. An
example might be a filter with many zeroes or a narrow notch. In order to resolve the
narrow features, a large number of points must be used in the sweep to improve the
frequency resolution. However, a large amount of the sweep time will be spent measuring
points between the features of interest. This is where Auto Resolution can save
measurement time while preserving resolution.
Auto Resolution is specified by three parameters, the Faster Threshold, the Slower
Threshold and the Maximum Step Size. As with all frequency parameters, these are set
within the [Frequency] menu.
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 transfer
function 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