Created on 2013-02-01 00:18:00
Regulation in Audio Precision analyzers refers to automatic control of one parameter by another. APx500 uses regulation to automatically adjust the generator level so that a specified target value is produced at the DUT output. Target values can be the measured DUT output THD+N ratio (non-linear regulation) or the measured DUT output level (linear regulation).
Measurements using Regulation in APx500
The Maximum Output and Maximum Output (Burst) measurements use regulation to a target THD+N ratio.
The Auto Gen Level feature in Reference Levels and the Regulated Frequency Sweep measurement use regulation to either a target THD+N ratio, or to a target level.
Regulating to a target THD+N ratio (non-linear regulation)
For audio testing, it is often desirable to perform certain tests just below the clipping point of the DUT. As the level rises, a harmonic distortion value exceeding 1.0 % is commonly agreed to indicate the onset of soft clipping, establishing a device’s maximum operating level, or MOL.
The amount of distortion in the output signal does not follow the generator level linearly, requiring more iterations to satisfy the regulation algorithm. For practical considerations, the regulation algorithm operates on one selected Measured Channel.
APx500 non-linear regulation algorithm
The generator is turned on at a certain level (see Generator Starting Level, below). A measurement is taken. If the initial result is below the target, the generator level is increased by a factor Dgen, and another measurement is taken; if the initial result is above the target, the generator level is decreased by Dgen for the next measurement. When the measured result crosses the target value, Dgen is halved and its sign is changed, so that for the next measurement the generator level moves in the opposite direction by a smaller amount. When Dgen becomes small enough that the change in the generator level would be smaller than the value set in the Generator Tolerance field, regulation ends.
Generator starting level for non-linear regulation
The starting level of the generator depends on the context:
1. In the Auto Gen Level function in Reference Levels, the generator starts at its current level, displayed in the Level field.
2. In the other measurements, the generator starts at half the value set in the Maximum Level field.
3. The first point in a Regulated Frequency Sweep follows rule #2, starting at half the value set of Maximum Level. For each subsequent point, the starting generator level is set to the final regulated generator level for the previous point.
Regulating to a target level (linear regulation)
It is often desirable to make an audio measurement at a specific DUT output level (1 dBu, for example), regardless of the gain or loss of the DUT channel. Within the operating range of the DUT, the output level follows the generator level linearly, requiring only one or two iterations of to satisfy the regulation algorithm. Linear regulation operates on all channels or on a selected channel.
APx500 linear regulation algorithm
1. The generator is turned on at the level specified in the Level field in the Reference Levels measurement. A measurement is taken. The gain of the DUT is inferred from the measured level.
2. The generator level is set as the Target Level divided by the DUT gain. If this level is greater than the Maximum Level, the process aborts with an error.
3. If the resulting measured level is not within 1% of the specified target level, steps 1 and 2 are performed again using the current generator level.
4. If the resulting measured level is still not within 1% of the specified target level, the process aborts with an error.
Maximum Level setting
A maximum level is set to protect the DUT. If the generator reaches the value set in Maximum Level, or the minimum level (fixed internally), regulation ends and a failure message is displayed.
Troubleshooting Regulation Issues
Regulation can fail under three common conditions:
1. If the Maximum Level setting (set to protect the DUT from overload) is set too low for a particular DUT, the target value will not be achieved and regulation will fail.
Solution: set the Maximum Level higher.
2. If cables between instrument and DUT are accidentally crossed, the regulation will fail. This is particularly interesting for the “Regulate Measured Level” setting in the Regulated Frequency Sweep, where crossed cables can cause regulation to simultaneously drive one channel up and another channel down.
Solution: uncross the cables.
3. For regulation to measured THD+N Ratio, if the initial generator level is set too low, the regulation algorithm may direct the generator to a lower level rather than a higher level, and regulation will fail. Explanation: If the measured distortion is below the target distortion, the algorithm responds by increasing the generator level. If the distortion is above the target, the algorithm decreases the level. For example, a target distortion of 1% is equivalent to –40 dB. If the initial generator level is at least 40 dB above the noise floor, the measured THD+N will be less than 1% and regulation will correctly proceed by increasing the generator level. If the If the initial generator level is less than 40 dB above the noise floor, the measured THD+N will be more than the target (again, using 1% as an example) and regulation will attempt to reduce the distortion by reducing the generator level. This moves the measurement closer to the noise floor, and the THD+N ratio will increase. This process repeats, ultimately resulting in a failure of regulation.
Solution: set the initial generator level higher. As explained above, in Auto Gen Level initial level is directly set; in the other regulation features initial level is one-half of the Maximum Level value.
4. The DUT is characterized by high distortion, high noise, or level instability.