Function Generation Via Auxiliary Outputs on Your Instruments

November 26, 2020 by Meng Li

Introduction

Quite often one wants to generate an electrical waveform to assist his or her lock-in or impedance measurements. Such a function can be generated by a dedicated function generator, but ideally should be produced by the same instrument, not only to save lab space but also to minimize the latency in transmission between different instruments, which is critical in many applications. Typical examples include tunable diode laser absorption spectroscopy (TDLAS, using a sawtooth wave), single-cell sorting (with a sine wave), and deep level transient spectroscopy (DLTS, using a square pulse).

This short post recaps previous results on function generation and also explains how to generate another wave format, a triangle wave, through auxiliary outputs. The MFLI Lock-in Amplifier (or the MFIA Impedance Analyzer) is used here as a reference. The configuration differs slightly on the HF2LI or the UHFLI.

Sawtooth Wave

Working principle: demodulation of a constant gives a sine wave at the oscillator frequency, ω. The resulting Demod θ, the ωt term, changes linearly with time and is wrapped within -π to π. Required upgrade option: multi-demodulator (MD) option on MFLI (IA), or multi-frequency (MF) option on HF2LI and UHFLI.

Sine Wave

This blog post shows how to output a sine wave. Working principle: Similar to the steps in sawtooth wave generation, but 'unfiltered' Demod X, the sin(ωt) term, is exact a sine wave. The low-pass filter bandwidth should be set higher than ω to allow pass of the signal. If needed, the phase can be adjusted in the lock-in tab. Required upgrade option: multi-demodulator (MD) option on MFLI (IA), or multi-frequency (MF) option on HF2LI and UHFLI.

Figure 1: LabOne^® screenshot showing a sawtooth wave (in blue) generated on Aux Output 1 and a sine wave (in orange) generated on Aux Output 2. In the lock-in tab, the 1 kHz frequency is set by oscillator 1, and for the sine wave, a low-pass filter bandwidth of 4 kHz is used. Note that the signal output is switched off, meaning that these functions can be generated independently of the main signal.

Square Wave (Pulse)

This blog post shows how to set up a square wave using the threshold unit (TU). Working principle: The period and duty cycle of a square wave is defined by logic gates (0 and 1) with customizable durations. To center the wave at 0 V, an offset voltage is also applied at the auxiliary output. Required upgrade option: none. But TU is only available on the MFLI and the MFIA. On other instruments, DIO or trigger outputs may be used in a similar fashion, yet without amplitude control.

Figure 2: LabOne screenshot showing a 50% duty cycle square wave (in blue) generated on Aux Output 3 and a triangle wave (in orange) generated on Aux Output 4. Note that in the Aux tab, all 4 channels can be used together, each giving a different wave format (1 for sawtooth and 2 for sine, as above).

Triangle Wave

Working principle: Open-loop integration (I) of the above square wave converts it into a triangle wave in the same frequency. The value of I only affects the amplitude. P and D are not needed in this operation. Required upgrade option: PID.

Figure 3: LabOne screenshot showing the threshold unit (TU) tab and the PID/PLL tab. TU 3 is set with a duration of 0.5 ms in both 0 and 1 states, creating a 1 kHz square wave in 50% duty cycle. PID 1 takes the square wave as input, and outputs the integrated voltage via Aux out 4.

Conclusions

This post summarizes the generation of different functions via auxiliary outputs. The settings file is available here and you are welcome to give it a try on your own MFLI (or MFIA). Howeve,r please note that these outputs are not designed with a high bandwidth (as opposed to signal outputs) and so it is better to use them primarily at low frequencies. If you have questions, please get in touch with us.