Simple Loop¶

Note

This lock-in amplifier tutorial is applicable to all UHFLI instruments and to UHFAWG instruments with the UHF-LI option installed. No other options are required. Some settings depend on whether the UHF-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 Zurich Instruments lock-in amplifiers. By using a very basic measurement setup, this tutorial shows the most fundamental working principles of a UHF Instrument and the LabOne UI in a hands-on approach.

There are no special requirements for this tutorial.

Preparation¶

In this tutorial, you are asked to generate a signal with the UHF Instrument and measure that generated signal with the same instrument. This is done by connecting Signal Output 1 to Signal Input 1 with a short BNC cable (ideally < 30 cm). Alternatively, it is possible to connect the generated signal at Signal Output 1 to an oscilloscope by using a T-piece and an additional BNC cable. Figure 1 displays a sketch of the hardware setup.

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

Connect the cables as described above. Make sure that the UHF unit is powered on 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 from the Windows start menu. 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 30 MHz signal of 0.5 V peak amplitude on Signal Output 1.

Change the frequency value of oscillator 1 (Lock-in tab, Oscillators section) to 30 MHz: click on the field, enter 30000000 or 30 M in short and press either <TAB> or <ENTER> on your keyboard to activate the setting.
(Without UHF-MF option) In the Signal Outputs section of the Lock-in tab, set the Range pull-down to 1.5 V, the Offset to 0 V and the amplitude to 500 mV for Output 1.

(With UHF-MF option) In the Output Amplitudes section of the Lock-in tab, set Amp 1 to 500 mV for demodulator 4 (4^th row and 1^st column of the Output Amplitudes section) and enable the button next to this field. In the Signal Outputs 1 section set the Range selector to 1.5 V and the Offset to 0 V.
By default all physical outputs of the UHF instrument are inactive to prevent damage to connected circuits. Turn on the main output switch by clicking on the button labeled "On" in the Signal Outputs 1 section. The switch turns to dark blue indicating it’s 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 quickly summarize the instrument settings to be made without and with installed UHF-MF Multi-frequency option.

Table 1: Settings: generate the test input signal (without UHF-MF Multi-frequency option)
Tab	Sub-tab	Section	#	Label	Setting / Value / State
Lock-in	All	Oscillators	1	Frequency	30 MHz
Lock-in	All	Signal Outputs	1	Amplitude	500 mVpk
Lock-in	All	Signal Outputs	1	Offset	0 V
Lock-in	All	Signal Outputs	1	On	ON

Table 2: Settings: generate the test input signal (with UHF-MF Multi-frequency option)
Tab	Sub-tab	Section	#	Label	Setting / Value / State
Lock-in	All	Oscillators	1	Frequency	30 MHz
Lock-in	All	Output Amplitudes	4	Amp 1	500 mVpk
Lock-in	All	Output Amplitudes	4	Amp 1 Enable	ON
Lock-in	All	Signal Outputs	1	Offset	0 V
Lock-in	All	Signal Outputs	1	On	ON

Oscillators and Demodulators are both represented as rows in the Lock-in tab (parameter table, or "All" side tab), but need to be distinguished for a good understanding of the user interface. This is particularly important for users of the UHF-MF Multi-frequency. 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 Amplitudes section, the frequency of this signal depends on row 1 of the Oscillators section (and not row 2) by default.

Check the Test Input Signal¶

Next, adjust the input parameters range, impedance and coupling to match the values in the following table.

Table 3: Settings: configure the Signal Input
Tab	Sub-tab	Section	#	Label	Setting / Value / State
Lock-in	All	Signal Inputs	1	Range	1 V
Lock-in	All	Signal Inputs	1	Scaling	1 V / V
Lock-in	All	Signal Inputs	1	AC	ON
Lock-in	All	Signal Inputs	1	50 Ω	ON

The range setting ensures that the analog amplification on the 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 graphical range indicator next to the numerical range setting shows about 50% usage of the possible dynamic range.

The incoming signal can now be observed over time in the Scope tab. A Scope view can be placed in the web browser by clicking on the icon in the left sidebar or by dragging the Scope icon 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	1.8 GHz
Scope	Control	Horizontal	Length	4 k
Scope	Control	Vertical	Channel 1	Signal Input 1
Scope	Trig	Trigger	Enable	ON
Scope	Trig	Trigger	Level	0 V
Scope			Run / Stop	ON

The Scope now displays single shots of Signal Input 1 with a temporal distance given by the Hold off Time. The scale on top of the graphs indicates the 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 zooming 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 1 V ensures that no signal clipping occurs. If you set the Input Range to 0.2 V, 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 BNC 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 1 V.

The Scope is a handy tool for checking quickly the properties of the input signal in the time and frequency domain. The Scope window can display up to 64 kSa in the basic version, or up to 128 MSa with installed UHF-DIG option. 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 UHF instrument to demodulate the input signal and measure its amplitude and phase. You will use two tools of the LabOne User Interface: 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	Sub-tab	Section	#	Label	Setting / Value / State
Lock-in	All	Frequencies	1	Harm	1
Lock-in	All	Frequencies	1	Phase	0
Lock-in	All	Input	1	Signal	Sig In 1
Lock-in	All	Low-Pass Filters	1	Sinc	OFF
Lock-in	All	Low-Pass Filters	1	Order	3 (18 dB/Oct)
Lock-in	All	Low-Pass Filters	1	TC / BW 3dB	9.3 ms / 8.7 Hz
Lock-in	All	Data Transfer	1	Rate	100 Sample/s (automatically adjusted to 107 Sample/s)
Lock-in	All	Data Transfer	1	Trigger	Continuous
Lock-in	All	Data Transfer	1	Enable	ON

These above settings configure the demodulation filter to the third-order low-pass operation with a 9 ms integration time constant. Alternatively, the corresponding bandwidths BW NEP or BW 3 dB can be displayed and entered. The output of the demodulator filter is read out at a rate of 107 Hz, implying that 107 data samples are sent to the host PC per second with equidistant spacing. These samples can be viewed in the Numerical and the Plotter tool which we will examine now.

The Numerical tool provides the space for 16 or more measurement panels. Each of the panels has the option to display the samples in the Cartesian (X,Y) or in the polar format (R, Θ) plus other quantities such as the Demodulation Frequencies and Auxiliary Inputs. The unit of the (X,Y,R) values are by default given in V_RMS. The scaling and the displayed unit can be altered in the Signal Input section of the Lock-in tab. The numerical values are supported by graphical bar scale indicators to achieve better readability, e.g. for alignment procedures. Display zoom is also available by holding the control key pressed while scrolling with the mouse wheel. Certain users may observe rapidly changing digits. This is due to the fact that you are measuring thermal noise that maybe in the μV or even nV range depending on the filter settings. This provides a first glimpse of the level of precision of your UHF instrument.

If you wish to play around with the settings, you can now change the amplitude of the generated signal, and observe the effect on the demodulator output.

Next, we will have a look at the Plotter tool that allows users to observe the demodulator signals as a function of time. It is possible to adjust the scaling of the graph in both directions, or make detailed measurements with 2 cursors for each direction. Signals of the same signal property 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. While zooming in, the mode in which the data are displayed will change from a min-max envelope plot to linear point interpolation depending on the density of points along the x axis as compared to the number of pixels available on the screen.

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 using the Config Tab (Settings section).

Different Filter Settings¶

As next step in this tutorial you will learn to change the filter settings and see their effect on the measurement results. For this exercise, use the second demodulator with the same settings as the first except in changing the time constant of the integration to 1 ms which corresponds to a 3 dB bandwidth of 83 Hz.

Table 6: Settings: change the demodulator filter settings
Tab	Sub-tab	Section	#	Label	Setting / Value / State
Lock-in	All	Low-Pass Filters	1	Order	3 (18 dB/Oct)
Lock-in	All	Low-Pass Filters	1	TC / BW 3dB	1 ms / 83 Hz

Increasing the time constant increases the filter integration time of the demodulators. This will in turn "smooth out" the demodulator outputs and hence decrease 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. For example, typing 1 k in the Rate field will result in 1.7 kSa/s 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 0.5 V to 0.7 V and vice-versa. The green plot will 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 change slower 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.