Top 10 Tips for Your Lock-in Measurement

Nowadays, thanks to their digital nature, modern Lock-in Amplifiers are so packed with features and tools that, occasionally, it might be unclear a priori which specific setting to use to conduct an optimized measurement.

With this blog post, we would like to share a compact compilation of simple yet effective tricks to improve your measurements with Zurich Instruments devices, regardless of your specific application. Some of these tips are also reviewed in this video.

1. Use about 50% of the input range

Sacrifice 1 bit of your ADC for protection against clipping. It's better to feel safe to not run into signal clipping than worry about it all the time. Also, the input amplifiers work best in this range where they show the smallest distortion. The "Auto Range" function of Zurich Instruments will automatically set up a 50% input range use.

2. Remove the DC offset from the input signal

A large DC component at the input can use up most of the lock-in's dynamic reserve and dynamic range, resulting in reduced measurement performance and limited resolution. Setting the input to AC coupling adds a high-pass filter on the input path that removes the DC offset. The cutoff frequency of this filter is different for each lock-in model, so ensure that your signal frequency is high enough to avoid attenuation.

3. Use a high filter order when noise suppression is more important than the dynamics of the device under test (DUT)

Generally, high filter orders are always a good thing when your DUT is not dynamic. Zurich Instruments Lock-in Amplifiers offer up to 8th order, 48 dB/oct filtering.

4. Avoid ground loops: use the Lock-in Amplifier also as a generator

Zurich Instruments Lock-in Amplifiers comprise high-performance signal generators. Using them instead of an external function generator, ground loops are significantly reduced.

5. Employ a stable and clean reference signal 

A sound lock-in measurement is based on an optimal reference signal. When using an external reference, ensure its frequency is well-defined and free from jitter, frequency, or phase noise. Amplitude noise is less critical, but only to a certain extent; TTLs or pure sine waves meet the above requirements perfectly.
Alternatively, using one of the internal oscillators for demodulation (and modulation) makes life even easier.

6. Choose the right input impedance: 50 Ohm or high impedance

This depends on the frequency involved, the interface to the test sample, cable lengths, and other components. As a rule of thumb, the cables must be treated as transmission lines if their length is approximately one-tenth of the wavelength or more. In this case, selecting a 50 Ohm impedance prevents reflections in the circuit which otherwise could result in increased noise and less power into the lock-in input, all leading to a drop in signal-to-noise ratio. On the other hand, if you need to measure the voltage drop at the sample very accurately, it is essential to keep the influence of the measurement instrument to a minimum by switching the input to high impedance. The drawback may be higher Johnson noise which could, in some cases, become dominant and prevent you from achieving maximum signal-to-noise ratio.

7. If you are unsure about an optimal parameter of your measurement, sweep it

Thanks to the Sweeper tool present in all Zurich Instruments Lock-in Amplifiers, you can sweep many parameters of the instrument (frequency, phase, amplitudes, signal output offset, and many others) while measuring the response of your DUT. This will help shorten the optimization procedure and debugging of your experiment.

8. If the optimal time constant cannot be determined, use multiple ones simultaneously

Zurich Instruments lock-in amplifiers are equipped with many demodulators. You can use multiple demodulators running on the same reference and signal but use different time constants. By looking at the SNR and dynamics of your measurement results in the Plotter, you can later decide which data to use.

9. Directly stream the lock-in output to the PC

This removes the need for an additional signal acquisition system, reduces your complexity, and makes your setup more robust. Plus you can record many channels. All Zurich Instruments products allow for sustained data rates, for the GHFLI and SHFLI even up to 50 MSa/s.

10. Fully differential measurement using the second lock-in "Signal Input"

In many cases, the lock-in is nowhere near the limiting factor for SNR. Rather, you might more likely be limited by 1/f fluctuations of the laser intensity or the gain fluctuation of some signal generator. In some experiments, there is the possibility of having a reference sensor, which will measure the baseline without the DUT. You can use the differential voltage Inputs or the second Signal Input of Zurich Instruments Lock-in Amplifiers to capture that signal and correct your measurements by the reference measurement.

Acknowledgments

I want to thank our former CEO, Sadik Hafizovic, for sharing his multi-year experience with lock-in measurements, and for providing valuable guidance in the writing.