Created on 2012-11-09 00:48:00

The APx Continuous Sweep is a patented implementation of impulse response measurement methods using log-swept sines. The Continuous Sweep method is used in the APx500 Continuous Sweep, Frequency Response and Acoustic Response measurements.

A continuous sweep provides a brief broadband stimulus signal to the DUT. This stimulus consists of a log-swept sine wave (often called a log chirp or a chirp) that moves continuously across a specified range of frequencies. The DUT output is acquired by the analyzer and is mathematically processed to provide a number of results.

Advantages

Continuous sweep is very fast and can provide many high-resolution results from a single acquisition. A typical continuous sweep measurement can return 15 separate results, each made up of many thousands of data points across multiple channels through a range of 20 Hz to 20 kHz, in about 4 seconds.

Limitations

1. When Crosstalk Type is set to High Speed, the Continuous Sweep measurement generates log chirps that are delayed from channel to channel, with channel 2 slightly later than 1, channel 3 slightly later than 2, and so on. These staggered sweeps enable the processing algorithms to determine the interchannel crosstalk for frequencies above 1 kHz, all from the same acquisition. In some circumstances, the staggered sweeps can produce unexpected results. See Continuous Sweep Crosstalk types and  Solutions for Unexpected Continuous Sweep Crosstalk Results…
2. Frequency Response and Acoustic Response measurements do not provide crosstalk results, and do not use staggered sweeps.
3. Continuous sweep based measurements must be used in closed loop configuration. Open loop (External Source) configurations are not supported.
4. Total delay through device or system under test must be less than 3 seconds.
5. Sweeps with Stop frequencies less than 1 kHz are not supported. See Sweep range, below.
6. Harmonic Distortion is measured using the THD method. This method may produce different results from the traditional THD+N method. See More about THD+N and THD.

Sweep range

In the patented Audio Precision implementation, the sine can be swept from a minimum of 5.0 Hz to a maximum of 80 kHz. The default range is 20 Hz to 20 kHz.

Start Frequency can be set to any frequency within the range of 5.0 Hz to a frequency equal to 1/2 the current Stop Frequency.

Stop Frequency can be set to any frequency within the range of 1 kHz to 80 kHz (depending upon system bandwidth setting).

Sweep duration

The duration of the sweep can be adjusted from a minimum of 50 ms to a maximum of 2.5 s, with a default of 350 ms. A shorter length, of course, provides for a faster test: stimulus, acquisition and processing time are all reduced. Longer sweeps provide greater resolution. The 350 ms default length is a good compromise between speed and resolution and provides excellent results.

Sweep duration is set on the Advanced Settings panel.

Pre-sweep duration

The total length of the stimulus includes a pre-sweep time in addition to the sweep time. The pre-sweep is an extension of the sweep that begins slightly below the specified start frequency, reaching the start frequency at the end of the pre-sweep time. The amplitude of the pre-sweep is modulated from zero to the sweep level to avoid an initial transient “pop.” Pre-sweep times can be set from 0 s to 1 s, with a default of 100 ms.

Pre-sweep length is set on the Advanced Settings panel.

Extend Acquisition

The DUT output is acquired, digitized and processed in DSP. The length of the acquisition is usually somewhat longer than the stimulus sweep to allow for time-delayed artifacts created in the DUT. In APx500, the acquisition can be extended up to 3.00 s after end of stimulus. The default is 50 ms.

Extend Acquisition is set on the Advanced Settings panel.

Input Range

The analyzer inputs must be ranged to provide an optimal acquisition level for a continuous sweep. The APx500 implementation uses autoranging, with the first range set to the lowest level for the best noise performance. If this range is too low, data from the out-of-range sweep are discarded. The input range is moved up and the sweep is repeated. This process may result in several sweep attempts before the correct range is determined; however, since the sweeps are very fast (typically one second) the total acquisition time is short.

If your sweep is a step in an automated sequence that you would like to run as fast as possible, you can optimize the speed of the ranging process by setting the range floor to the correct range for the measurement.

Ranging is set on the Advanced Settings panel.