Understanding the Sweep Duration in the LabOne Sweeper Module

A fast sweep capability is often desired by researchers working on fast physical phenomena or short-lived samples, and may also be beneficial purely from a time-saving perspective. This blog post will elucidate the factors on which the sweep rate depends and guide you along several possible ways to make the sweep process faster.

A detailed look at the LabOne Sweeper module

The easiest way to start a sweep is to use the LabOne® Sweeper module, available on all lock-in amplifiers and impedance analyzers from Zurich Instruments. At the first glance (Figure 1(a)), you can find the range and length settings. For the purpose of this blog post, let's restrict ourselves to the frequency sweep, as sweeping at low frequencies is usually time-consuming. However, it should be noted that other parameters, such as amplitude, DC bias, PID setpoint, etc, can also be swept by selecting them in the node-tree selector of the 'Sweep Param'.

The frequency range (defined by the start and stop frequency) and the number of frequency points (defined by the length, in either linear or log spacing) are important factors to determine the sweep duration. Figure 1(a) shows a sweep in a log (or geometric) spacing with 7 points between 1 Hz and 1 MHz (boundaries included), with a remaining (duration) time of 8.8 s. Each data point requires enough dwell time to settle the transient on the device-under-test (DUT) caused by the frequency change, and then to average over enough filtered demodulated samples to improve measurement precision.

Let's now delve one step deeper, to find out how the settling and averaging are carried out in the LabOne Sweeper module. By clicking the Settings subtab, we can see a few application-specific modes as shown in Figure 1(b). The default application for lock-in users is parameter sweep, and for impedance users, it is impedance. The explanation of each application mode is listed in the field 'application description', but we may want to look one step closer at the details. For this, we need to click to enter the Advanced mode, where several setting parameters are shown in Figure 2. It should be noted that, in the scope of this blog post, all measurements and calculations were done in manual ranging. Enabling auto-ranging (available on MFIA Impedance Analyzer and MFLI Lock-in Amplifier with MF-IA option) will increase the duration of the sweep depending on how many range changes are required.

A comparison of two application modes

The most impactful parameter in Figure 2 is the bandwidth (BW). In the default Auto BW mode, the BW is dynamically adjusted during a sweep and hence balances the sweep rate and precision. It is also inversely related to the time constant (TC), with a filter-order dependent coefficient in front. The settling time and the averaging time can also be manually defined in the same subtab. Naturally, they both contribute to the sweep duration. But it is important to consider that the communication latency between a PC and an instrument also contributes. This communication latency is a constant at about 10 ms for the MFLI Lock-in Amplifier and MFIA Impedance Analyzer, but can differ on other instrument platforms. By referring to the detailed settings in Figure 2(a), for parameter sweep application without any averaging, we have:

Sweep duration at each frequency (parameter sweep) = settling time (16*TC) + averaging time (1/13 kSa/s) + communication time (10 ms)

In this simplified case, thanks to the fast data transfer rate on the MF platform (default at 13 kSa/s, highest at 107 kSa/s on MFIA, and 214 kSa/s on MFLI), averaging takes almost no time, much shorter than the fixed 10 ms communication latency. Users of Zurich Instruments lock-in amplifiers and impedance analyzers can certainly benefit from this fast data transfer rate, which is significantly higher than legacy interfaces. However, since TC is inversely proportional to BW (and roughly to the measurement frequency), it becomes significant at low frequencies. Figure 3(a) depicts such a result. A transition happens between 100 Hz and 10 kHz, and for all frequencies higher, the sweep duration will be only limited by the 10 ms communication time.

The parameter sweep application mode in Figure 1(a) optimizes the sweep rate. In contrast, the impedance application mode in Figure 1(b) sets a narrower BW limit of 100 Hz, which helps to reach a higher precision than the parameter sweep, yet at the cost of a longer sweep duration. A comparison of the two modes on the same 1 kOhm DUT is illustrated in Figure 4. The impedance mode lasts about twice the amount of the time (17 s), but the measured trace is flatter, with a smaller variation (higher precision) from the same true value (1 kOhm, 0 deg). 

The reason for such higher precision comes obviously from more averaging. However, its definition requires a clarification: the maximum among TC, min time, and the duration of samples. For instance, the averaging time at 1 Hz is about 7.15 s, corresponding to an average of about 100,000 samples! Readers with interest in these parameters are kindly directed to our LabOne programming manual. To quickly summarize based on the detailed settings in Figure 2(b), we have:

Sweep duration at each frequency (impedance)  = settling time (16*TC) + averaging time (TC-depending variable) + communication time (10 ms)

Figure 3(b) suggests that the averaging time (not TC) becomes the rate-limiting factor at high frequencies. The situation at low frequencies stays similar to Figure 3(a), also limited by a large TC.

Methods to speed up your sweep

At this point, we can summarize 3 main limiting factors for the sweep rate: the large TC at low frequencies, the communication latency at high frequencies, and the number of points needed. We will explain how to tackle them separately.

Method 1: one-period averaging of impedance

A low-pass filtering process is usually necessary to suppress the noise in the measurement, despite costing a long time at low frequencies. We could skip the process, for instance, when the DUT is known to be linear in I-V. Upon a sine wave test signal, we can safely assume the returning current signal also as a simple sine wave and force a sine fit. Using this approach, we can reach the theoretically fastest speed by measuring only a single period. 

Since the method is only valid at low frequencies, the sweep rate at high frequencies will not be affected. The default cut-off frequency for the one-period averaging mode is at 13 Hz, but can be increased up to 107 Hz on the MFIA or MFLI with MF-IA options. For details, you can have a look at this blog post.

Method 2: PID/AWG chirp

The bottleneck of sweep rate at high frequencies is the communication latency between PC and instrument. Naturally, the solution to this is to avoid frequent commands during the measurement, which can be done by preparing everything on the instrument beforehand. 

This blog post gives a good example. The MFLI Lock-in Amplifier (and MFIA Impedance Analyzer) can output a linear chirp using PID integration and capture the corresponding signal (voltage, current, and impedance) change in the time domain, with either the Scope module, the Plotter module, or DAQ module. However, as these modules display only time on the X-axis, we will need to post-process (outside LabOne) the multi-trace data by choosing frequency as the new axis. It should also be emphasized that PID integration can only generate a linear chirp, and without any settling time in between.  For reactive DUTs with potentially large RC time constants, this is likely not an ideal solution.

In such a case, the Arbitrary Waveform Generator (AWG) option, available on the UHFLI Lock-in Amplifier can be a good substitute. Considering it as inserting a slight delay between adjacent frequency points manually, which is still shorter than the communication latency. For details on using this method, you can have a look at here.

Method 3: fixed frequency measurement in the time domain

The easiest approach to save the sweep time is to not sweep at all, in other words, to measure at a fixed frequency (or a few fixed frequencies) in the time domain. This technique can be particularly useful if the DUT has a characteristic frequency that can be determined with a preliminary sweep. Afterward, we can just remain at this frequency and measure the signal evolution in the LabOne Plotter module. If more than one characteristic frequency is present, simultaneous multi-frequency measurements are also possible with the multi-demodulator (MD) or multi-frequency (MF) option, depending on the instrument platform you are using. Note that, a multi-frequency analysis is often a better way than a standard sweep in applications such as microfluidics where other physical processes involved are competing in speed with electrical detection. For more details, you can have a look at this application page.

Conclusion

We have presented 2 application modes of the LabOne Sweeper module. There are a few more for you to explore, which is often a good thing to do if the sweep rate and the precision need to be balanced and optimized. You might also notice that we didn't discuss the measurement accuracy throughout this blog post, as it depends only on the calibration and has almost nothing to do with the sweep rate. To this end, we would also like to offer you 3 tips to improve your future sweeps:

• Use existing Sweeper modes that are optimized for your applications;
• Fine-tune the Sweeper settings to adapt to real-world scenarios: for example, reducing the number of averages when it is not necessary, adding a minimal settling time for DUTs with large time constants, and etc.;
• Try other 'non-standard' sweep methods. They all come with tradeoffs, so we prepare a table below for your easy choice.

If you are interested, please get in touch to set up a discussion and demo.