How to check the input noise level of MFLI?

The Zurich Instruments MFLI is a low noise mid-frequency lock-in amplifier. Its measurement frequency ranges from DC to 5 MHz, and its frequency resolution is 1 μHz. Lock-in amplifiers are widely used in applications where small signals are comparable to the noise floor of measurement instrumentation such as in low-temperature transportation, Hall effect measurements, etc. With a lower noise floor, it is possible to measure smaller signals. To accomplish those measurements, our users care about the input noise level of the device. In the user manual (User Manual MFLI, Figure 5.3 MFLI Voltage Input voltage noise density), the input noise density spectrum for available input ranges is shown in Figure 1 . The lowest noise floor is achieved with the lowest input range of 1 mV, and increases with increasing input range up to 3 V.

Fig. 1

One of the most common questions we are asked is: how can we check the input noise level of the MFLI in our own laboratory?

The answer is that this can be easily checked by using the LabOne® user interface. LabOne® is Zurich Instruments’ instrument control software, and is provided with the instrument  (download page) LabOne features a number of tools including the Sweeper function, which allows users to sweep different parameters (such as frequency, amplitude, offset, etc.). In particular, the Sweeper tool can measure the noise spectral density.

Let us take a look at how to perform this measurement.

1. Short the voltage input: Prepare a short cap (ref PN: Amphenol RF 202114, Connector Cap Shorting for BNC Jacks) to short the input channel of the device. Caution: If you leave it open or short with a long cable the result will not be correct.

Fig. 2

Fig. 3

2. Set the input channel parameters: Set the input channel range to 1 Use DC coupling and single-ended input.

Fig. 4

3. Set the lock-in amplifier: Osc=1, Input Signal=Sig In 1, EN=enable, Rate=1.674k

Fig. 5

4. Open the Sweeper tab:

a. In “Settings”, set “Application” to “Noise Amplitude Sweep” as we want to measure the noise density spectrum.

Fig. 6

b. In the “Control” tab, note that the name of the signal curve became “Demod 1 Sample R Noise 1Hz BW”, so that we will see the value of the noise density. But, in order to evaluate the input noise level, we have to add the result of “Demod 1 Sample X Noise 1Hz BW” and “Demod 1 Sample Y Noise 1Hz BW”.

Fig. 7

(Attention: If we had missed step(a), the name would stay as the default “Demod 1 Sample R”, which is not correct.)

c. Set a frequency range f: Start – Stop, sweep points “Length(pts)”, then start the sweep. According to the chosen frequency range and sweep points, the measurement time can go from several minutes to several hours.

Fig. 8

d. The test result as performed at Zurich Instruments is as follows:

Fig. 9

Compare this result with the diagram in the MFLI user manual: as you can see, this MFLI Lock-in Amplifier meets the specifications. For more details of the differences between X(or Y) and R, please refer to my colleague Dr. Mehdi Alem’s blog: Noise Analysis of Signal Components .