Imaging

This tutorial is applicable to all MF Instruments.

Goals and Requirements

This tutorial explains how to capture and display an imaging signal, i.e., a signal structured in lines and frames that can be built up to a 2-dimensional data set. To follow this tutorial, one will require a 3rd-party programmable arbitrary waveform generator to generate a realistic imaging signal with line triggers, or access to a real imaging signal including line triggers or EOL triggers e.g. from an atomic force microscope.

Preparation

Connect the cables as shown in the figure below. Make sure that the MF Instrument 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 . After starting LabOne the default web browser opens with the LabOne graphical user interface.

fig tutorial imaging setup
Figure 1. Setup for the imaging tutorial with MF Instrument

The tutorial can be started with the default instrument configuration (e.g. after a power cycle) and the default user interface settings (e.g. as is after pressing F5 in the browser).

Imaging Signal Properties

In this section we discuss the properties of the external signal used in this tutorial. It is most illustrative to discuss the imaging functionality based on a realistic signal generated by an arbitrary waveform generator (AWG), or even a real imaging signal. The imaging functionality of the instrument can also be tried out without external equipment, but it’s not easily possible to generate a nicely structured imaging signal with the MF instrument alone. In order to facilitate the phase-locking between AWG and lock-in, an AWG with digital modulation capability and a possibility to output the phase reference signal separately from the AWG signal is helpful. Examples are the UHFAWG and the HDAWG from Zurich Instruments.

We will assume the following scanning parameters: a line scanning frequency of about 200 Hz and a line number of 256. We will furthermore assume that the imaging signal on the AWG signal output is an amplitude-modulated signal at a fixed carrier frequency of 300 kHz. This signal is wired to the Signal Input +V connector of the MF instrument. The carrier phase reference, a square wave at 300 kHz with about 1 Vpk amplitude, is generated on the AWG marker output 1 and is connected to the lock-in reference input Aux In 1. At the start of each line, the AWG generates a rising edge of a TTL signal generated on its marker output 2. This line trigger signal is connected to the Trigger In 1 connector of the MF instrument. The minimum trigger signal width required to correctly trigger the data acquisition is equal to the inverse demodulator sample rate used. The reason is that the state of the is Trigger In 1 connector is transferred to the host computer together with the demodulator data which limits the time resolution and therefore the minimum trigger pulse width.

Measure the Imaging Signal

For this example, we programmed the AWG to generate a signal with an amplitude varying between 0 and about 0.6 Vrms which builds up to an image of the Zurich Instruments logo. We let the AWG run continuously, which means it will permanently generate this signal, the line trigger, and the phase reference signal. Here we will set up the lock-in amplifier with sufficiently high demodulator bandwidth and sampling rate in order to faithfully measure the imaging signal in external reference mode.

For locking to the external reference lock-in reference input, we need to select the reference input signal and change the lock-in amplifier to external reference mode. You can check in the DIO Tab whether the corresponding input connector shows a toggling signal. It may be necessary to adjust the threshold level of the input to match the TTL signal level generated by the AWG. Setting up a measurement in external reference mode is more generally described in External Reference .

Table 1. Settings: enable external reference mode
Tab Sub-tab Section # Label Setting / Value / State

Lock-in

All

Demodulators

2

Signal

Aux In 1

Lock-in

All

Demodulators

2

Mode

ExtRef

We choose demodulator filter settings and sampling rate sufficiently high to measure the fast components in the signal up to several 10 kHz. You can find a more general description on selecting filter constants in Simple Loop. The table below shows the settings to be made. Here we use a sampling rate of 200 kSa/s, which requires the 1GbE connection rather than USB due to its higher bandwidth.

Table 2. Settings: configure the demodulator
Tab Sub-tab Section # Label Setting / Value / State

Lock-in

All

Signal Input

1

Range

1.2 V

Lock-in

All

Low-pass Filters

1

BW 3 dB

30 kHz

Lock-in

All

Low-pass Filters

1

Order

8

Lock-in

All

Data Transfer

1

Rate

220 kSa/s

Lock-in

All

Data Transfer

1

Enable

ON

Now we can monitor the imaging signal as well as the line triggers in the Plotter Tab . Open the Plotter tab and add the demodulator R signal as well as Trigger In 1 to the plot.

Table 3. Settings: measure imaging signal and line trigger in the Plotter
Tab Sub-tab Section # Label Setting / Value / State

Plotter

Control

Vertical Axis Groups

Tree Selector

Demodulators/1/ Sample/R

Plotter

Control

Vertical Axis Groups

Tree Selector

Demodulators/1/ Sample/Trig In 1

Plotter

Control

Run / Stop

ON

The Plotter should now display the continuously streamed imaging data. The figure below shows in blue the demodulator R signal, and in green the line trigger signal marking the beginning of each line. The cursors indicate a line repetition period of about 1.66 ms, and In the following, instead of displaying these data in a continuous stream in the Plotter, we would like to capture a full image frame.

btn mnu plotter um
fig tutorial imaging plotter
Figure 2. Imaging signal (blue) and line trigger (green) displayed in the LabOne Plotter

Set up the Grid Mode

The Data Acquisition Tab with its Grid Mode is the suitable tool to capture images. In this section we go through the configuration of this tool.

The Data Acquisition tool in grid mode acquires 2-dimensional data sets with pre-defined rows and columns that are defined by a trigger timing for each line, a well-defined line number, and a well-defined line duration. The acquired data stream can be linearly interpolated to a well-defined number of data points (e.g. pixels) for each line, or it can be acquired exactly with the transfer rate of the demodulator in exact mode. It furthermore supports averaging over multiple frames.

Here we select the Trigger In 1 signal as trigger source in the Settings sub-tab. We set the hold-off time to 0 s to ensure that no triggers are lost in between successive lines. By changing the delay, we can compensate for a possible misalignment between trigger timing and line start, or to configure the Data Acquisition tool for a line end trigger, rather than line start trigger.

In the Grid sub-tab, we select a number of rows corresponding to what we have programmed on the AWG. In Exact (on-grid) mode, we select the number of columns such that the duration is sufficiently long to capture one line, but shorter than 1 line trigger period so the DAQ tool can re-arm for every new line. Here we select N = 300 columns, corresponding to a duration of T = 1.36 ms. The two numbers are related to the demodulator sampling rate fs = 220 kSa/s by T = N/fs.

Finally, we select the 2D display in the Control sub-tab, and we make sure to add demodulator 1 R as a displayed signal in the Vertical Axis Groups section. The DAQ tab also supports multi-channel acquisition as more signals, e.g. the phase or other demodulators can be recorded. The table below summarizes the settings.

Table 4. Settings: set up the grid mode
Tab Sub-tab Section # Label Setting / Value / State

DAQ

Settings

Trigger Settings

Trigger Signal

Demod 1 Trig In 1

DAQ

Settings

Horizontal

Hold off time

0 s

DAQ

Settings

Horizontal

Delay

0 s

DAQ

Grid

Grid Settings

Mode

Exact (on-grid)

DAQ

Grid

Grid Settings

Columns

300

DAQ

Grid

Grid Settings

Duration

1.36 ms (read-only in Exact mode)

DAQ

Grid

Grid Settings

Rows

256

DAQ

Control

Time Domain

Plot Type

2D

In order to capture one fresh frame, we shortly disable the AWG. We arm the Data Acquisition tool by clicking on btn uielements single um to acquire a single frame with the exact number of rows specified before, and then restart the AWG. The figure below shows the captured image. The acquired data appear as an entry in the History sub-tab and can easily be saved from there.

btn mnu daq um
fig tutorial imaging 2d
Figure 3. 2D image captured by the LabOne Data Acquisition tool in Grid mode