Skip to content

Up and Down frequency conversion


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

Goals and Requirements

This tutorial aims at familiarizing you with the frequency conversions performed by the SHFLI front-ends and their consequences. The practical examples and exercises are meant to provide better understanding of the technical aspects and introduce the tools that will help you avoid possible pitfalls.

In particular, it will show how the channel center frequency values make the 2 channels independent and, depending on how they are chosen, may prevent one channel from being able to measure the signals generated by the other.

There are no prerequisites for this tutorial, but completing the Simple Loop will make it easier to follow along.


In this tutorial we need to connect Signal Output 2 to Signal Input 1 with a short (10 to 20 cm) SMA coaxial cable. Channel 2 will be used to generate a signal that is then measured with Channel 1. This will highlight the role of the Center Frequency setting. As in the Simple Loop, it is possible to also visualize the signal using a stand-alone oscilloscope by splitting the signal from the output using a T connector. Figure 1 displays a sketch of the hardware setup.

Figure 1: Tutorial single tone, two channels setup

Make sure that the SHFLI is powered and connected to the computer, and start the LabOne user interface. Please refer to the Preparation section in the Simple Loop tutorial for more details on this.

Generate the Test Signal

Perform the following steps in order to generate a 1.6 GHz signal of 0.25 V peak amplitude on Signal Output 2. Please note that these are very similar to the ones in the Simple Loop, but performed on Channel 2

  1. In the Signal Inputs section of the Lock-in tab, make sure that the Frequency Range of Input 2 is set (dark blue) to RF and then set its Center frequency (labeled c2) to 1.5 GHz: enter 1.5G or 1500000000 in the field and press <TAB> or <ENTER> on the keyboard, or click somewhere else in the GUI to activate the setting.

  2. Change the frequency value of oscillator 2 (Lock-in tab, Oscillators section, labeled f2) to 100 MHz: click on the field, enter 100000000 or 100M in short.

  3. (Without SHFLI-MF option) 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 2. The Read-only Frequency field of Output 2 should show 1.6 GHz.

    (With SHFLI-MF option) In the Output 2 section of the Lock-in tab, set Amp to 250 mV for demodulator 8 (8th 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 1.6 GHz. At the bottom of the Output 2 section, set the Range selector to 0.5 V.

  4. By default all physical outputs of the SHFLI 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 2 section. The switch turns dark blue when enabled.

  5. 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 SHFLI-MF Multi-frequency option.

Table 1: Settings: generate the test input signal (without SHFLI-MF Multi-frequency option)
Tab Section # Label Setting / Value / State
Lock-in Signal Inputs 2 Freq Range RF
Lock-in Signal Inputs 2 Center Freq (Hz) 1.5 GHz
Lock-in Oscillators 2 Frequency 100 MHz
Lock-in Signal Outputs 2 Range 0.5 V
Lock-in Signal Outputs 2 Amplitude 0.25 V
Lock-in Signal Outputs 2 On ON
Table 2: Settings: generate the test input signal (with SHFLI-MF Multi-frequency option)
Tab Section # Label Setting / Value / State
Lock-in Signal Inputs 2 Freq Range RF
Lock-in Signal Inputs 2 Center Freq (Hz) 1.5 GHz
Lock-in Oscillators 2 Frequency 100 MHz
Lock-in Output 2 8 Amp (V) 0.25 V
Lock-in Output 2 8 Amp Enable ON
Lock-in Output 2 Range 0.5 V
Lock-in Output 2 On ON

One important aspect to note is that the center frequency is channel-based, i.e., it is the same for both input and output of that channel. Its input field in the LabOne graphical user interface is located in the Signal Inputs section.

Visualize the Signal with the Scope

Next, adjust the parameters of Signal Input 1 (please note that we are now setting up the other channel) as shown in the following table, so that they match the ones of Channel 2.

Table 3: Settings: configure the Signal Input
Tab Section # Label Setting / Value / State
Lock-in Signal Inputs 1 Range 500 mV
Lock-in Signal Inputs 1 Freq Range RF
Lock-in Signal Inputs 1 Center Freq (Hz) 1.5 GHz

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, and matching the center frequency to the one of Channel 2 ensures that the 2 measurement windows overlap completely.

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 2 GSa
Scope Control Horizontal Length 4992
Scope Control Vertical Channel 1 Signal Input 1
Scope Control Vertical Channel 1 On
Scope Run / Stop ON

The Scope now displays single shots of Signal Input 1 after the analog frequency down-mixing. 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.

Click on "Freq FFT" in the Scope’s Control panel, Horizontal section, to display the spectrum of the signal. You should see a peak at 100 MHz on the plot. The Scope, in RF mode, shows the complex signal coming from the analog front-end’s mixer, so the spectrum is centered around 0 Hz with positive and negative frequencies, from -1 GHz to +1 GHz. To visualize the signal’s real frequency, go to the "Advanced" panel in the Scope tab and click on the "Absolute Freq" button.

If you now change the center frequency of channel 1, the signal will move on the screen relatively to the window’s center. For example, try to change channel 1’s center frequency to 1.7 GHz. The signal is now displayed to the left of the window’s center, as this is now located at 1.7 GHz, but its frequency has not changed because you haven't modified any of channel 2’s parameters. Turning off "Absolute Freq" will show the signal’s relative frequency to be -100 MHz now. If you set channel 1’s center frequency higher than 2.6 GHz, the signal will no longer be visible because its measurement window no longer contains the 1.6 GHz frequency.

Measure the Signal with a Demodulator

Let’s now set up a lock-in measurement of the signal coming from Output 2. The following table shows the settings that need to be made in the Lock-in tab, starting with resetting the center frequency of channel 1.

Table 5: Settings: configure the Signal Input
Tab Section # Label Setting / Value / State
Lock-in Signal Inputs 1 Freq Range RF
Lock-in Signal Inputs 1 Center Freq (Hz) 1.5 GHz
Lock-in Oscillators 1 Frequency 100 MHz
Lock-in Demodulators 1 Input Signal Sig In 1
Lock-in Demodulators 1 Osc f1
Lock-in Demodulators 1 n 1
Lock-in Demodulators 1 BW 3 dB 100
Lock-in Demodulators 1 Rate (Sa/s) 1000
Lock-in Demodulators 1 En ON

With these settings, demodulator 1 demodulates at a frequency of 1.6 GHz, equal to the one of the signal at the output. This can be verified in the read-only frequency fields next to demodulator 1 and next to the active frequency component in Output 2.

You can now check the demodulator output in the numerical tab: you should see both amplitude and phase panels showing rather stable readings.

Now let’s play with the frequencies of channel 1 similarly to what we did earlier with the Scope: if we increase the center frequency by 200 MHz, to 1.7 GHz and change the frequency of oscillator 1 (f1) to -100 MHz, we end up at the same demodulator frequency, so we should see a similar readout in the numerical tab. The two readings are likely different: the amplitude may be slightly different because of slight variations in the analog path response with frequency, while the phase measurement, although stable, is likely very different because, differently from the Simple Loop tutorial, we are using 2 independent numerical oscillators and changing the frequency of one modifies the relative phase offset between them.

Finally, if we changed the center frequency of channel 1 to 2.5 GHz, the measurement windows of channel 2, generating the signal, and of channel 1, measuring it, would overlap only at 2 GHz, so in order to be able to measure the signal generated by channel 2 using channel 1, we need to change the frequency of oscillator 2 (f2) to +500 MHz and that of oscillator 1 (f1) to -500 MHz. Increasing the gap between the center frequencies further will completely separate the windows and signals generated in one would no longer be measurable by the other.