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General notes on setting levels


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

The program assumes that the sound card is close to linear for the majority of its range. It calculates relative power based on the highest input level observed whilst collecting readings. Accuracy of the plot is dependant on the linearity of the whole system, from RF input to the value ultimately presented to PolarPlot by the sound card driver.

When setting RF and AF levels try to avoid a combination that involves either of the controls operating near minimum or maximum levels.  Operating close to these points has a higher chance of working on non-linear parts of the circuit's characteristics and can result in odd looking plots.

Do not forget that you have a significant degree of control over what level the program 'sees' by utilising the software mixer's volume sliders for the particular input chosen.  In particular, do not confuse the Recording controls with the Playback controls - it is Recording that governs the input level.

If you have other inputs connected to the sound card (modems, CD players, TV tuners  etc.) you may, depending on the characteristics of your particular sound card, need to set their volume sliders to zero, and/or de-select them as inputs to the mixer.

Some early sound card drivers incorporate an audio AGC control which is capable of being enabled or disabled.  If you have one of these controls then AGC must be turned off or you will get non-linear readings.  If you have a sound card that has a '20dB Boost' facility for the microphone input, you may have to disable this option.

After the plotting run is completed the readings actually collected are re-scaled by the program to reposition the maximum level actually read to the outer ring at 0 dB on the PolarPlot. However, the highest possible input level without 'flat topping' should be used in order to utilise the maximum number of levels available from the sound card and, if you are using the auto stop feature, avoid the automatic halting of readings by dropping the input level too low.