Get additional insights into your setup with the MF-DIG option

Lock-in amplifiers are at their best when rejecting the noise and increasing the signal-to-noise ratio. When the setup is well characterized the measurements are carried out at a single frequency and measured against external parameters. Throughout the measurement, detailed noise spectra reveal critical changes in the physics of the experiment but an external oscilloscope is likely to interfere with the measurements. When we make new devices, explore new materials, and research a new topic we get a chance to learn from the noise figures and use the scope as an integral part of the research. In this blog, we see how this is possible and cover several important advantages of having the Digitizer Option that makes the Scope an invaluable part of your measurements.

As all Zurich Instruments lock-ins come with an oscilloscope that utilizes the digitized signal at the input, the raw signal in the time and frequency domain is always available. When we enable MF-DIG Digitizer option on an MFLI we get a chance to perform a higher resolution noise spectra and connect the lock-in measurement with that of the scope by advanced triggering capabilities. This has been proved crucial for performing the cross-correlation measurements to lower the noise floor using the two MFLIs or a single UHFLI. Let’s see how this is done by looking at what MFLI Scope with MF-DIG option offers for measurements in the time and frequency domain.

Display Options

With a basic MFLI unit, the Scope comes with the possibility to monitor one input channel at at time with a variable sampling rate from 60 MSa/s down to 916 Sa/s and maximum length of 16 k points. With the MF-DIG option installed several advanced measurement opportunities open up.

The Scope can now serve as a two channel time/frequency domain tool where you can monitor current (I) and voltage (V) inputs at the same time, and also any other signal port such as Aux (In/Out)s, Trigger (In/Out)s, etc. However you also get an access to the Demodulator data which you can depict as a signal input for the Scope channel. The Input Node Tree for an MFLI unit with MF-DIG option is shown in the right red box of Figure 1. By adding more options, such as the MF-MD option, the Node tree is growing accordingly.

 

Fig 1 A screen shot of the Scope time domain trace showing simultaneously  the Voltage (blue) and the Current (red) Input signals. Red framed box on the right depicts the Node Tree selection available for the MFLI with MF-DIG option.

For a given sample length (L) and Sample Rate (SR) the scope shot length of time (LT) can be calculated  as: LT = L/SR, so for a SR= 234 Sa/s and the maximum L=2,56 MSa we get LT= 10,94 s. In the Frequency domain, on the other hand, you get an increased frequency resolution (FR) that you simply get by dividing the chosen Sample Rate with the given sample length, FR = SR/L, which for the above example amounts to 91 mHz.

Continuous Streaming

Continuous Scope streaming allows you to stream any of the two scope channels directly on the Plotter with the adjustable streaming rate of 1.83 kHz up to 3.75 MHz. A simple example is shown in Figure 2 where a 1 kHz signal with the amplitude of 10 mV is shown both in the Scope tool and streamed via Plotter. Note how the Scope is updated through the Scope shots whereas the Plotter continuously streams the signal with 1.8 MHz rate.

 

Figure 2 (Click to run) Continuous Streaming mode of the Scope via Plotter while the Scope represents the signal in Scope shots.

Cross-Domain Triggering

Having the MF-DIG option unleashes additional lock-in functionality that truly makes different device domains work together. A simple example is shown in Figure 3 where a noise spectra acquizition is triggered by an event in the lock-in signal amplitude (R). We show this by sweeping the frequency across a resonator circuit, where an fft from the scope is taken every time the amplitude reaches certain value. Captured spectra can be seen in the history tab and recorded in segments when the measurement is done.

 

Figure 3 A screen shot of a Sweeper and Scope tool showing the frequency domain trace taken after triggering on the set Demodulator R amplitude. This an example of the cross-domain triggering that uses segmented recording to collect the scope data with the memory maximum depth of 2.56 MSa per channel.

 

The trigger engine allows cross domain triggering between the software and hardware units of the with a complete schematics shown in Figure 4.

 

Figure 4 Full Digitizer option connectivity across the MFLI device depicting cross domain triggering capabilities.

X/Y Display

With the X/Y display capability you can plot any two Scope channels against one another. In the example below we show Aux In 1 and Aux In 2 of the MFLI each with a periodic signal applied from an external device to analyze the phase and frequency relationship between the two signals. The signals are chosen such that we can plot Lissajous figures of the high order (5, 6).

 

Figure 5 The X/Y Display allows you to plot against each other, any two signals monitored on the scope channels. This is also available on the Aux In channels that can be used independently of the lock-in measurements.

More importantly you can use this feature to visualize the phase offset between the two signals provided by the same reference frequency.

Summary and Conclusion

The MF-DIG option is a simple addition to your MFLI Lock-in Amplifier that allows you to perform advanced scope measurements with high resolution and perform cross-domain triggering taking the scope shots only when your experimental conditions are met. All this can be done without the need to add additional cable connections that can compromise the quality of your experiment.