Oscillator calibration for AE modular

The Oscillators in AE modular - 2OSC and VCO - are true analog oscillators; therefore, their frequency characteristic is not as rock-solid as a digital oscillator.
The 2OSC and VCO modules work with a 1V per octave characteristic, that means, increasing the CV input by 1 volt results in an increase of the output frequency by one octave; this characteristic is widely standard in the modular synth world. But this behaviour depends on several factors like temperature, operating time and others. In the consequence,  the 1V per octave characteristic may change, so increasing the input by 1V changes the frequency either by more or by less than one octave. But this can be adjusted for each oscillator, so they work again as expected.
So if you encounter a behaviour that the notes don't sound as they should (especially when controlling the oscillators via MIDI), it's time for an oscillator calibration! 

Basic principle:
The calibration is done by trimmer potentiometers that are accessible at the bottom of AE modular (see picture below); they work as follows:

• Turning the trimmer clockwise stretches the scale, meaning the frequency for higher notes (= higher CV input voltage) is INCREASED
• Turning the trimmer counter-clockwise compresses the scale, meaning the frequency for higher notes (= higher CV input voltage) is DECREASED

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Below two different ways are shown step-by-step:

Option 1: External MIDI synth or Plugin as reference 
(Suitable if all oscillators in the AE modular seem to be off)
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1. Power up AE modular and wait for at least 20 minutes to stabilize the oscillators (they are temperature dependent and need some time to reach a stable frequency)
2. Connect a MIDI keyboard, -controller, DAW to the MIDI input of the MASTER module and select a simple basic sound at the external synth / plugin, preferably a square or sawtooth wave without any modulation
3. connect the oscillator output of the module being calibrated (e.g. the square wave output) to your monitoring / headphones
4. Connect the CV input of the oscillator with the BUS CV signal with a short patchwire
5. Play a C0 note (MIDI note number 24) on the MIDI synth (this means that the CV is set to 1V) - this is our LOWER reference
6. Adjust the FREQ pot of the oscillator module so that both frequencies are equal
7. Play a C2 note (MIDI note number 48) on the MIDI synth (this means that the CV is set to 3V) - this is our UPPER reference
8. Adjust the trimmer pot at the bottom of the AE case (see picture to the right):
   - If the oscillator frequency is lower than the sound of the reference synth / plugin, turn the trimmer pot clockwise until both frequencies are equal again
   - If the oscillator frequency is higher than the sound of the reference synth / plugin, turn the trimmer pot counter-clockwise until both frequencies are equal again
9. Repeat with Step 5. This process has to be iterated a few times, until the difference at step 7 is near zero.

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Patching 2OSC module for calibration

location of the trimmer pots for the oscillators

Option 2: Match two oscillators within AE modular
(This procedure is suitable if one oscillator is a bit "off" compared to another one.)

1. Here, we assume that the lower one of the two oscillators in a 2OSC module has to be calibrated
2. Connect a MIDI source to the MASTER module
3. Connect the inputs CV1 and CV2 of the 2OSC module to the same CV (either from the BUS CV socket on the module itself or from the BUS CV socket at the master module). By this, both oscillators play "parallel".
4. Play a C0 note on the MIDI synth (this means that the CV is set to 1V)
5. Adjust FREQ 1 and FREQ 2 pots so that both frequencies are equal
6. Play a C2 note on the MIDI synth (this means that the CV is set to 3V)
7. Adjust the trimmer pot at the bottom of the AE case (see picture to the right):
   - If the oscillator frequency is lower than the sound of the reference synth / plugin, turn the trimmer pot clockwise until both frequencies are equal again
   - If the oscillator frequency is higher than the sound of the reference synth / plugin, turn the trimmer pot counter-clockwise until both frequencies are equal again
8. Repeat with Step 4. This process has to be iterated a few times, until the difference at step 7 is near zero.