Top 10 Ways to Improve Your Lock-in Amplifier Measurement

Meanwhile I have helped so many people working with lock-in amplifiers with a handful of generally applicable Tricks, that I’d like to share them in a compact compilation. Certainly they apply to the Zurich Instruments HF2LI Lock-in Amplifier and to the Zurich Instruments UHFLI Lock-in Amplifier. Hope you enjoy it.

1. Use about 50% of the input range

Sacrifice 1 bit of your ADC for protection against clipping. Better feel save to not run into clipping than to worry about it all the time. Also, the input amplifiers work best in this range and show smallest distortion. Using the “Auto Range” function of Zurich Instruments will automatically set up a 50% input range use.

2. Remove Ground loops: also use the Lock-in Amplifier as generator

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

3. Use AC-coupling only on input ranges smaller than 0.5 V

The main purpose of AC coupling is to be able to apply amplification to signals with a significant offset voltage. If applying AC coupling does not allow you to reduce the input range to below 0.5 V, then it is probably not worth doing it.

4. Use a high filter order when noise suppression is more important than the dynamics of the 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.

5. Avoid the use of sampling-commensurable frequencies

For example, instead if working at precisely 180.00 MHz on a lock-in with 1.8 GHz sampling rate (frequency is precisely 1/10th of the sampling rate), work, e.g., at 180.11 MHz.

6. Use 50-ohm termination when possible

Especially when cable lengths L exceed the quotient of 10 MHz divided by the signal frequency F. That is when L > 10MHz 1m/F.

7. If unsure about optimal timeconstant, sweep the timeconstant setting

In a modern Lock-in Amplifier you can sweep the timeconstant in a sweeper and see how the timeconstant setting influences the measured signal.

8. If optimal timeconstant cannot be determined, use multiple timeconstants simultaneously

Modern lock-in amplifiers, like all Zurich Instruments Lock-in Amplifiers, have many demodulators. You can use several demodulators running on the same reference and signal but use different timeconstants. Later, you can 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 of more than 30MB/s, the UHFLI even can be above 100MB/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. You might much rather be limited by 1/f fluctuations of a laser intensity or the gain fluctuation of some signal generator. Often there is the possibility to have a reference sensor, which will just measure the baseline without the DUT. You can use the second Signal Input of Zurich Instruments Lock-in Amplifiers to capture that signal and correct your measurements by the reference measurement.