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Checking sound card linearity

**SC**  None of this section is applicable to using a wattmeter

Some sound cards have input overload or AGC circuitry built into the card.  This can result in non-linear readings being taken and 'flat-topping' of the plot. 

It is recommended that you perform the procedure below at least once whenever you install the program onto a PC or whenever you change the sound card.

Use the settings screen to set the Peak Input Level from the sound card. To determine what the value should be you need an AF oscillator and a stepped AF attenuator with a range of at least 60dB.

a) Start PolarPlot and double-click on the plot area. On the Settings screen in the Sound Card Characteristics section click the Set Default Values button. This makes the Peak Input Level value 1000. Click OK to save and close the screen

b) Set the AF oscillator to  a frequency of around 800 Hz and connect its output leads via the attenuator to line-in, left channel

c) Set the AF attenuator to -10dB and set the oscillator output level to minimum

d) Click the Calibrate Input button to start the calibration sweep

e) Turn on the AF oscillator and adjust the output level so that the plotted points line up with the -10dB ring on the plot

f) Vary the attenuator by 10dB steps up and down and observe where the points are plotted

g) If your sound card is linear the points will be on the respective rings of the plot. 

h) If the values plotted with 0dB of attenuation do not line up within 1dB of the outer ring of the plot then some input level protection is being experienced. Restart this sequence with the Peak Input Level at a value of 100 lower until you get within 1dB of the outer ring with 0dB attenuation.