Distortion Measurement Description
Last updated: May 24, 2006
How is a Distortion measurement made?
Distortion is an audio quality measurement that compares the audio signal level from the mobile at a specified (desired) frequency to the level of signals present at other frequencies. The audio signals from the mobile are typically demodulated tones sent over an analog voice channel (AVC).
The test set makes two measurements to determine distortion. First, the test set measures the total audio signal level into the Audio Analyzer. Next, a precise notch filter is then used to remove the signal at the specified frequency. The remaining signal level indicates the level of unwanted signals (distortion). Distortion is expressed as a percentage of the total audio signal.
Distortion can be measured in the range of 100 Hz to 10 kHz. For information on using the Audio Analyzer, see
Audio Analyzer Measurement Description
.
The distortion measurement can be used to perform the audio harmonic distortion test. In this test, two audio signals are used: a 1004 Hz tone and the 6000 Hz SAT. The 1004 Hz tone is notched out to make the measurement. See IS-137A, section 2.2.2.5 for more information.
Considerations When Making Manual Distortion Measurements
When manually controlling the test set from the front panel, all measurements default to using continuous triggering. If you make digital measurements on a DTC using RF Rise or Protocol triggering, and then hand off to an AVC and select an analog measurement, the analog measurement may not trigger and display a result. This is because the digital measurement is still waiting for a burst transmission to trigger it (which it won't get from an analog signal) and therefore the measurements "freeze". To prevent this from happening when making analog measurements, either turn off the digital measurements or set their trigger source to Immediate.
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