Step 5: Calibrating the Voltage Settings
Some people won’t care whether the voltages are completely accurate; seeing the waveforms and frequency spectra is enough. They can stop here. Otherwise, you will need to fine-tune the trimpot with a source of calibrated low voltage sine waves around 1000 Hz. You can provide this signal with almost any computer or a second smartphone, using the free SourceForge audio program Audacity or the free android app Signal Generator from Radon Soft. Take the signal from the earphone jack using a standard 3.5 mm stereo plug. Ideally, you would have access to another, calibrated scope to match yours against it. Otherwise, as in the picture above, a VOM will get within 5-10 %. First, generate a 1 KHz sine wave tone at high output so you can read it accurately on the lowest AC setting of the VOM. It should be around 0.5 V RMS or somewhat higher. You can then use that signal to carefully adjust the input trimpot on your preamp until your scope value agrees. If you added the extended higher voltage range, calibrate with the SPDT switch connected to the trimpot (lower voltage range), not to the resistance bridge. Remember that an RMS signal of 1.0 volt on the VOM corresponds to a waveform of 2.8 volts peak-to-peak on the scope.
With two voltage scales, a software range setting on the NFX Oscilloscope Pro of “5” is a good compromise; this will provide full-scale ranges of 5 V and 50 V (±25 V), although you could go all the way up to ±100 V. Remember that when using the extended range, 0.1 V on the scope software will correspond to 1.0 V at the alligator clips, 1 V on the scope will correspond to 10 V at the input, etc.