Skip to content

External Reference


In this tutorial we generate a test signal with the HF2 and use it as a reference signal for demodulation in the same way as we would do it with a reference signal coming from an external source.

This is done by connecting the Output 2 connector with the Input 2+ connector with a BNC cable. This tutorial shows a single-ended operation, meaning that there is no signal going into the Input 2- connector. Optionally, it is possible to connect the generated signal from Output 2 to an oscilloscope by using a T-piece and an additional BNC cable. The Output 1 connector is to be connected to the Input 1+ connector. This allows you to check the generated reference signal. The measurement setup is shown in the following figure.

Figure 1: Tutorial external reference setup

Make sure the HF2 unit is powered on, and then connect the HF2 to your computer with a USB 2.0 cable. Finally, launch LabOne (Start Menu/Programs/Zurich Instruments/LabOne User Interface HF2).

Generate the Reference Signal

In this section you generate a 1.0 MHz signal with a 1 V amplitude on Output 2 for use as the external reference. The settings for generating the reference signal are shown in the following table.

Table 1: Settings: generate the reference signal
Output 2 range 1 V
Demodulator 8 Amplitude 2 1.0 V / ON
Oscillator 2 frequency 1 MHz
Signal input 2 range / AC / Diff / 50 1.2 V / ON / OFF / OFF

When connecting an oscilloscope to the Output 2 connector, you should be able to observe the sinusoid. Alternatively, you can look at the signal in the LabOne UI Scope with the following settings.

Table 2: Settings: acquire the reference signal
Scope Signal Input Signal Input 2
Scope Trigger Signal Signal Input 2
Scope Sampling rate 53 MSa, 39 us


In the Scope tab set the Horizontal Mode to Freq Domain FFT in order to see the frequency spectrum of the signal. This will also set a logarithmic Y scale by default. An Exponential Moving Average can be enabled in the Avg Filter field to reduce the noise floor in the display.

Activate the External Reference Mode

In this section we activate the external reference mode. Based on the external reference, we demodulate a separate signal of interest.

Table 3: Settings: generate the signal of interest
Output 1 range 1 V
Demodulator 7 Amp 1 1.0 V / ON
Oscillator 1 frequency 1 MHz
Demodulator 1 En/Rate ON / 100 Hz
Signal Input 1 range / AC / 50 / Diff 1.3 V / ON / OFF / OFF

The external reference mode makes use of demodulators 7 and 8. These two demodulators can not be used for measurement. They serve as phase sensitive detectors to set up phase-locked loops locking an external reference and an internal oscillator. Demodulator 7 is assigned to oscillator 1, and demodulator 8 is assigned to oscillator 2. Previously we have chosen oscillator 2 as the source of the reference signal on Signal Output 2. Now we want lock oscillator 1 to this reference. In order to achieve that, we set the Mode field of demodulator 7 to ExtRef. In the drop-down selector in the Input column of demodulator 7, select Signal Input 2.

Figure 2: LabOne UI enabling external reference mode

Once the external reference mode has been enabled, the frequency of oscillator 1 changes continuously, adapting to the frequency of the external reference signal. This can be verified by changing the frequency of oscillator 2 and noting how the frequency of oscillator 1 follows. A green light next to the oscillator frequency field indicates that the instrument has locked to an external reference.

In the demodulation process, the measurement signal is not multiplied directly with the external reference signal. Instead, the measurement signal is multiplied with newly generated reference signal from the internal oscillator, using only the frequency and phase information of the external reference. The continuous toggling of the oscillator frequency shows that the newly generated reference is continuously adjusted to the external reference.

Change External Reference Input

In this section you will modify the setup to use DIO 0 as the external reference instead of Signal Input 2. This is useful in practice since it means that the two sensitive Signal Inputs of the Instrument remain available for actual measurements. The modified setup is shown on Figure 3. Note that the DIO 0 connector is located on the back panel of the HF2 Instrument.

Figure 3: Tutorial external reference using DIO 0 setup

It is important to make sure that DIO 0 is configured as an input before connecting anything to it. This can done using the DIO tab in LabOne. Note that the button to the right of bits 7...​0 should be off.

When using the DIO 0 as the external reference signal, it should be taken into account that this is a digital I/O, which should be operated at TTL levels. Therefore the Aux 1 output is connected to the Add connector of Output 2, to provide a DC shift of the test signal and thus make it TTL compatible.

The settings used for generating the test signal are shown in the following tables. The resulting signal will have a DC offset of 1.5 V and an amplitude of 1 V and will thus oscillate between 0.5 V and 2.5 V, which is TTL compatible.

Table 4: Settings: generate the test signal
Signal Output 1 range 1 V
Signal Output 1 On / Add ON / ON
Demodulator 8 Amp 2 1.0 V / ON
Oscillator 2 Frequency 1 MHz
Table 5: Settings: generate the DC shift
Aux 1 Signal Manual
Aux 1 Offset 1.5 V

Figure 4: Configuring DIO 0 as reference input

Finally use the Input drop-down selector for demodulator 7 in the Lock-in tab and set it to "DIO 0". This makes oscillator 1 lock on DIO 0. The frequency of oscillator 1 should start updating similarly to what was described in Activate the External Reference Mode .