Slow Sweep at Low Frequency
By Ford Mays - January 14, 2025
Recently, a Venable Frequency Response Analyzer user experienced an unusual phenomenon. The FRA was taking a measurement at a very low frequency (100mHz), and took many minutes to complete the measurement. In this blog we will discuss why this happens and what can be done to alleviate the problem.
Venable’s FRAs feature auto-ranging measurement channels that simplify operation of the instrument. The user does not have to adjust the input range of the instrument prior to making measurements. However, when starting a sweep at a low frequency, the auto ranging feature can take a long time to settle, as it will have to ‘see’ several cycles of the generator output to optimize the range.
The easiest way to alleviate this problem is to start the sweep at the highest frequency to be measured, and sweep down to the lowest. This will allow the auto-ranging inputs to settle in quickly. During the sweep, the measured voltages are not likely to change much, unless there is a resonant point.
Another way to resolve this issue is to turn off auto-ranging. This can be done in the Analyzer Control pane of the Venable Stability Analysis Software, under ‘Options’ (see Figure 1.)
Figure 1. Auto-ranging Control
However, by turning off the auto-ranging feature, one does have to have a pretty good idea what voltages are going to be measured. Since the measured voltages are a function of the generator output, one may have to do some experimenting at various frequencies to determine the appropriate generator output level for a clean plot, and thusly the expected voltages to be measured.
Normally, auto-ranging is of great benefit, making the Venable FRA easy to use. However, when starting a sweep at a very low frequency, it can result in very slow sweep times. The easiest way to address this is to sweep from high to low over the frequencies of interest.
For additional information on making loop gain measurements, please see these useful blogs:
Did You Know - Adjusting Injection Levels During a Sweep
Injection Signal Location and Levels in Loop Gain Measurements
Using the Servo to Set Feedback Loop Injection Levels for Measuring Loop Gain