Simple Loop¶

Note

This lock-in amplifier tutorial is applicable to all GHFLI instruments as no option is required. Some settings depend on whether or not the GHF-MF Multi-frequency option is installed, and the differences are pointed out where necessary.

Goals and Requirements¶

This tutorial is for people with no or little prior experience with the Zurich Instruments GHFLI Lock-in Amplifier. By using a very basic measurement setup, it shows the most fundamental working principles of the GHFLI and the LabOne UI using a hands-on approach.

There are no special requirements to complete the tutorial.

Preparation¶

In this exercise, you are asked to generate a signal with the GHFLI and measure that signal with the same instrument. This is done by first connecting Signal Output 1 to Signal Input 1 with a short SMA cable (ideally 10 to 20 cm). Optionally, it is possible to connect the generated signal at Signal Output 1 to an oscilloscope by using a T-piece and an additional cable. Figure 1 displays a sketch of the hardware setup.

Figure 1: Tutorial simple loop setup (LAN connection shown)

Make sure that the GHFLI unit is powered and connected by USB to your host computer or by Ethernet to your local area network (LAN) where the host computer resides. Start the LabOne User Interface as explained in Connecting to the Instrument. The LabOne Data Server and the LabOne Web Server are automatically started and run in the background.

Generate the Test Signal¶

Perform the following steps in order to generate a 200 MHz signal of 0.25 V peak amplitude on Signal Output 1.

Change the frequency value of oscillator 1 (Lock-in tab, Oscillators section) to 200 MHz: click on the field, enter 200000000 or 200 M in short and press either <TAB> or <ENTER> on your keyboard to activate the setting.
(Without GHF-MF Multi-frequency) In the Signal Outputs section of the Lock-in tab, set the Range pull-down to 0.5 V and the amplitude to 250 mV for Output 1.

(With GHF-MF Multi-frequency) In the Output 1 section of the Lock-in tab, set Amplitude to 250 mV for demodulator 4 (4^th row) and enable the button next to this field, if it’s not enabled yet (dark blue). The read-only Frequency field of this component should show 200 MHz. At the bottom of the Output 1 section, set the Range selector to 0.5 V.
By default all physical outputs of the GHFLI are inactive to prevent damage to connected circuits. Turn on the main output switch by clicking on the On/Off button at the top right of the Output 1 section. The switch turns to dark blue when enabled.
If you have an oscilloscope connected to the setup, you should now be able to see the generated signal.

Table 1 and Table 2 summarize the instrument settings to be made without and with GHF-MF Multi-frequency option.

Table 1: Settings: generate the test input signal (without GHF-MF Multi-frequency option)
Tab	Section	#	Label	Setting / Value / State
Lock-in	Oscillators	1	Frequency	200 MHz
Lock-in	Signal Outputs	1	Range	0.5 V
Lock-in	Signal Outputs	1	Amplitude	0.25 V
Lock-in	Signal Outputs	1	On	ON

Table 2: Settings: generate the test input signal (with GHF-MF Multi-frequency option)
Tab	Section	#	Label	Setting / Value / State
Lock-in	Oscillators	1	Frequency	200 MHz
Lock-in	Output 1	4	Amp (V)	0.25 V
Lock-in	Output 1	4	Amp Enable	ON
Lock-in	Output 1		Range	0.5 V
Lock-in	Output 1		On	ON

Oscillators and Demodulators are both represented as rows in the Lock-in tab, but need to be distinguished for a good understanding of the user interface. This is particularly important for users of the GHF-MF Multi-frequency option. By default, oscillator 1 is assigned to demodulators 1-4, and oscillator 2 is assigned to demodulators 5-8. This means, for example that when generating a signal using row 2 of the Output 1 section, the frequency of this signal depends on row 1 of the Oscillators section (and not row 2) by default.

Hovering over the read-only frequency field of each output component shows a tool-tip that describes what elements compose that frequency.

Check the Test Input Signal¶

Next, set the input range to 500 mV as shown in the following table.

Table 3: Settings: configure the Signal Input
Tab	Section	#	Label	Setting / Value / State
Lock-in	Signal Inputs	1	Range	500 mV

The range setting ensures that the analog amplification on Signal Input 1 is set such that the dynamic range of the input high-speed analog-digital converter is used optimally without clipping the signal.

The incoming signal can now be observed in the Scope tab. The Scope can be opened by clicking on its icon in the left sidebar or by dragging it to one of the open tab rows. Choose the following settings on the Scope tab to display the signal entering Signal Input 1:

Table 4: Settings: configure the Scope
Tab	Sub-tab	Section	Label	Setting / Value / State
Scope	Control	Horizontal	Sampling Rate	4 GSa
Scope	Control	Horizontal	Length	4096
Scope	Control	Vertical	Channel 1	Signal Input 1
Scope			Run / Stop	ON

The Scope now displays single shots of Signal Input 1. The scale on top of the graphs indicates the time-axis zoom level for orientation. The icons on the left and below the figure give access to the main scaling properties and allow one to store the measurement data as a SVG image file or plain data text file. Moreover, the view can be panned by clicking and holding the left mouse button inside the graph while moving the mouse.

Note

The mouse wheel can be used to zoom in and out horizontally. To zoom vertically, the shift key needs to be pressed while using the mouse wheel.

Having set the Input Range to 500 mV ensures that no signal clipping occurs. If you set the Input Range to 100 mV, clipping can be seen immediately on the scope window accompanied by a red error flag on the status bar in the lower right corner of the LabOne User Interface. At the same time, the LED next to the Signal Input 1 SMA connector on the instrument’s front panel will turn red. The error flag can be cleared by pressing the clear button marked with the letter C on the right side of the status bar after setting the Input Range back to 500 mV.

The Scope is a useful tool for checking quickly the properties of the input signal in the time and frequency domain. For the full description of the Scope tool please refer to the functional description in Scope Tab.

Measure the Test Input Signal¶

Now, you are ready to use the GHFLI Lock-in Amplifier to demodulate the input signal and measure its amplitude and phase. You will use two tools of the LabOne User Interface: the Numerical and the Plotter.

First, adjust the following parameters on the Lock-in tab for demodulator 1 (or choose another demodulator if desired):

Table 5: Settings: measure the test input signal
Tab	Section	#	Label	Setting / Value / State
Lock-in	Frequencies	1	n	1
Lock-in	Frequencies	1	Phase	0
Lock-in	Input	1	Signal	Sig In 1
Lock-in	Low-Pass Filters	1	Order	3 (18 dB/Oct)
Lock-in	Low-Pass Filters	1	TC / BW 3dB	9.3 ms / 8.7 Hz
Lock-in	Data Transfer	1	Rate	100 Sample/s
Lock-in	Data Transfer	1	Enable	ON

These settings configure the demodulation filter to the third-order low-pass operation with a 9 ms integration time constant. Alternatively, the corresponding 3 dB bandwidth can be displayed and entered. The output of the demodulator filter is read out at a rate of 100 Hz: 100 data samples are sent to the host PC each second with equidistant spacing. These samples can be viewed in the Numerical and the Plotter tools which we will examine next.

The Numerical tool provides the space for 16 or more measurement panels. Each of the panels has the option to display the demodulation samples in Cartesian (X,Y) or in polar (R, Θ) representation, plus other quantities such as the Demodulation Frequencies. The unit of the (X,Y,R) values are by default given in V_RMS. The numerical values are accompanied by graphical bar scale indicators that provide better readability, e.g. for alignment procedures. Display zoom is also available by holding the control key pressed while scrolling with the mouse wheel. You may observe rapidly changing digits in the Numerical panels. This is due to the fact that you are measuring thermal noise that may be in the μV or even nV range depending on the filter settings.

To better familiarize yourself with the settings, you can now change some of the values entered before, such as the amplitude of the generated signal, and observe the effect on the demodulator output.

Next, we will have a look at the Plotter tool, which allows users to observe the demodulator signals as a function of time. It is possible to adjust the scaling of the graph in both axes, or make detailed measurements with 2 cursors for each axis. Signals with same properties, e.g. amplitude from different demodulators, are automatically added to the same default y-axis group. This ensures that the axis scaling is identical. Signals can be moved between groups. More information on y-axis groups can be found in the section called "Plot Area Elements".

Try zooming in along the time dimension using the mouse wheel or the icons below the graph to display about one second of the data stream.

Figure 2: LabOne User Interface Plotter displaying demodulator results continuously over time (roll mode)

Data displayed in the Plotter can also be saved continuously to the computer memory. Please have a look at User Interface Overview for a detailed description of the data saving and recording functionality. Instrument and user interface settings can be saved and loaded in the Settings section (Config Tab).

Different Filter Settings¶

Next you will learn to change the filter settings and see their effect on the measurement results. For this exercise, configure the second demodulator with the same settings as the first one, except for the time constant that you set to 1 ms, corresponding to a 3 dB bandwidth of 83 Hz.

Table 6: Settings: change the demodulator filter settings
Tab	Section	#	Label	Setting / Value / State
Lock-in	Low-Pass Filters	2	Order	3 (18 dB/Oct)
Lock-in	Low-Pass Filters	2	TC / BW 3dB	1 ms / 77.38 Hz

A higher time constant increases the filter integration time of the demodulators. This, in turn, "smooths out" the demodulator outputs and hence decreases available time resolution. It is recommended to keep the sample rate 7 to 10 times the filter 3 dB bandwidth. The sample rate will be rounded off to the next available sampling frequency. In this example, type 1k in the Rate field, which is sufficient to not only properly resolve the signal, but also to avoid aliasing effects. Figure 3 shows data samples displayed for the two demodulators with different filter settings described above.

Figure 3: LabOne User Interface Plotter: Demodulator 1 (TC = 9.3 ms, blue), Demodulator 2 (TC = 1 ms, green)

Moreover, you may for instance "disturb" the demodulator with a change of test signal amplitude, for example from 0.25 V to 0.4 V and vice-versa. The green plot may go out of the display range which can be re-adjusted by clicking the Auto Scale button , cf. Plot Functionality. With a large time constant, the demodulated data changes more slowly in reaction to the change in the input signal compared to a small time constant. In addition, the number of stable significant digits in the Numerical tab will also be higher with a high time constant.