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THD+N vs Frequency

The THD+N versus Frequency utility will generate a stepped frequency signal using the selected output device and compute and display a THD+N vs Frequency plot.  This utility allows you to quickly and conveniently measure the distortion characteristics of your device over a range of frequencies.

To start the utility click on the <Utilities><THD+N vs Frequency> menu or click on the "THD Freq" button on the toolbar.  The utility will produce a stepped frequency sweep which provides the stimulus for the measurement.  It must be connected from the output of the sound card to the device under test (DUT).  The output of the device is then connected to the input of the sound card.   During the test, no operator intervention is required.  The computer performs all the ncessary steps and plots the results across the audio band. 

Once the results have been analyzed, the plot can be viewed, printed and stored to disk.   Right clicking on the plot will also popup a menu of options allowing you to place the data or plot image on the clipboard for use in other applications.  
The THD+N vs Frequency options dialog box can be activated by clicking on the "Options" button along the right side of the plot area.  This dialog box allows you to change the frequency span, display color scheme and a number of other useful options:

Start and Stop Frequencies - this sets the frequency limits of the measurement.  The stop frequency must be less than 1/2 of the current sampling rate used on the main analyzer settings.  

Channels - select left only or both left and right.

THD Type - Choose the distortion measurement type (THD or THD+Noise)

Signal Output Level - sets the test signal output level in dB full scale.  0.0 dB is the signal level produced when a 100% full scale sine wave is generated.  Because most sound cards have the ability to control the output  volume,  "full scale" is not necessarily the same thing as the "maximum possible" signal level unless the volume control is set to its maximum value. 

Frequency Step size - select a specific linear step size in Hz or octave scaling (1/1 thru 1/96).  The stepped sine takes between 1 and 2 seconds per frequency step - decreasing the step size will increase the time required to perform the test.

Spectral Weighting - sets the spectral weighting used for the computations.  

Plot Top - this sets the uppermost point for the amplitude axis.  The value is entered as a percentage.

Plot Bottom - this sets the lowermost point for the amplitude axis.  The value is entered as a percentage.

Frequency Axis Scaling - this sets the scaling format for the frequency axis.
Amplitude Axis Scaling - this sets the amplitude axis scaling format.  

Test Limits - this lets you specify a file containing limits for each channel.  The limit data will show as a separate trace.  The limit file format is the same as the overlay and mic files and is a simple ASCII text file with two tab delimited columns of numbers; the first column contains the frequency and the second column contains the THD limit.  The values must be in increasing order by frequency.  

Prompt to Save - controls whether or not you will be prompted to save the data before starting a new test.

a) You must use the internally generated stepped frequency signal as the source.  
b) The only main analyzer settings that are used for this utility are as follows:
  • Device selection 
  • Sampling Rate 
  • Sampling Precision