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13 June 2022

Oscilloscope Basics: Cal Out and Aux Out

Fig. 1: Cal Out and Aux Out 
provide many useful outputs.
Oscilloscopes are generally thought of in terms of the signals that are input to them, but even oscilloscopes that are not equipped with function/signal generators can usually output some useful test signals.

Nearly all oscilloscopes have a Cal Out (calibration output) terminal on the front. Most Teledyne LeCroy oscilloscopes also have an Aux Out (auxiliary output) connector on either the front or back, depending on  model. Both outputs provide configurable signals that can assist you to compensate probes and attenuators, test frequency response, trigger waveform acquisition and coordinate multiple test instruments.

Cal Out

Most oscilloscope users are familiar with using the calibration output (Cal or Cal Out) for probe compensation. By default, the Cal Out supplies a 1 kHz, 1 V square wave used to adjust the low-frequency compensation of high-impedance passive probes.  Probes are hooked to the Cal terminal while the probe ground is connected to the adjacent ground clip. The probe’s low-frequency compensation is then adjusted to obtain the “squarest” square wave possible on the oscilloscope’s display. Probe compensation instructions can be found in the probe user manual. 

However, on many oscilloscope models, a different Cal Out signal can be configured on the Aux Output tab of the Utilities setup dialog (Figure 2). The frequency of the square wave is user programmable from 500 Hz to 1 MHz, and the amplitude can be set for 50 mV to 1 V into 1 MΩ. Additionally, the Cal Out signal can be a DC Level instead of a square wave, with an amplitude of 50 mV to 1 V into 1 MΩ.  These outputs are useful for calibrating non-standard, high-impedance probes and attenuators. 

Fig. 2: The Aux and Cal signals are configured using the Aux Output tab of the Utilities dialog.

Aux Out

The settings on the left side of this same dialog are used to configure the Aux Out connector, which can deliver different types of output to other instruments via a simple BNC connection. The number of possible outputs is oscilloscope model dependent. With the exception of the Fast Edge signal, the Aux Out signal amplitude is settable from 50 mV to 1 V into 1 MΩ, with different frequencies and durations, depending on the output conditions. 

Square Wave 

Fig. 3: Typical amplifier responses to the Aux Out square wave.
Round leading edges indicate poor high-frequency response.
Vertical edges indicate poor low-frequency response.
The first possibility is to output a square wave with a frequency between 500 Hz and 5 MHz. The default is the standard 1 kHz with an amplitude of 1 V, which can be quickly recalled using a button beneath the settings readout.

The square wave Aux Out signal, with a nominal transition time of 700 ps, can be used to adjust the high-frequency compensation of passive probes. 

The Aux Out square wave can also be used for a qualitative evaluation of an amplifier’s frequency response. The square wave is applied to the amplifier input and the shape of the output waveform provides information about the amplifier’s frequency response, as show in Figure 3.

DC Level

Like the Cal Out, the Aux Out can be set to output a DC Level in the range of 50 mV to 1 V into 1 MΩ instead of a square wave, which is useful in calibrating high-impedance probes and attenuators used at the oscilloscopes input. 


On WaveRunner 8000HD oscilloscopes with the OscilloSYNC option, the Aux Out signal path is used to synchronize the two instruments that are connected. During a SYNC, it cannot be used for any other type of output.

Trigger Enabled

Trigger Enabled sends a pulse from the Aux Out when the oscilloscope is armed and ready to trigger (the Ready indicator on the front panel is lit), but not necessarily triggered. This signal is useful when the oscilloscope is part of an automated test system where it indicates that the oscilloscope trigger is armed and ready to acquire a signal, alerting the other equipment to “wake up” and take whatever actions they must. It can be used to initiate a test signal source and begin a test. It can also be used to gate a signal being applied to the oscilloscope to assure that the oscilloscope is available for an acquisition.

Trigger Out

Trigger Out sends a pulse when the oscilloscope trigger has fired (TRIG'D Indicator lit). The pulse duration is adjustable from 400 ns to 400 s. This signal can be used by an automated test system to indicate that the oscilloscope has begun an acquisition. The duration can be set to match the acquisition time so that the trailing edge indicates the end of the acquisition. 

Pass/Fail Pulse

Pass/Fail testing is standard on all contemporary Teledyne LeCroy oscilloscopes, enabling the user asses if a signal “passes” or “fails” based on the results of a mask test or the comparison of different measurement parameters. One possible action that can occur with either a Pass or Fail is to output a pulse from the Aux Out connector. The Pass/Fail pulse duration is adjustable from 1 µs to 131,072 ms. This pulse can be sent to any other test equipment that is capable of responding to an input pulse, perhaps as part of an automation sequence, enabling you to set up test dependencies based on complex conditions besides the state of the oscilloscope trigger. Be sure to also select Pulse Aux Out on the Pass/Fail Actions dialog to enable this output while testing.

Fast Edge Signal

Fig. 4: Three BNC tee adaptors are used to split
the Fast Edge signal to four probes.
Only available on higher-bandwidth oscilloscopes, the Fast Edge signal is a 5 MHz square wave with an amplitude of 450 mV into 50 Ω. The rise time of the Fast Edge signal is nominally 350 ps or faster, depending on the oscilloscope model, providing the necessary time resolution to deskew high-bandwidth probes, for which it is most often used (Figure 4). 

The Fast Edge pulse has a flat frequency response and can be used as a signal source for measuring the frequency response of a device. Finally, it can be used as the oscilloscope trigger source signal.

See also:

Oscilloscope Basics: External, Line and Fast Edge "Triggers"

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