Zurich Instruments Newsletter - Edition Q4/2013
- Customer Interview: Xavier Jehl, Electron Pumps at 50 mK
- Reduce Lab Setup Complexity: UHFLI Product News
- Reduce Lab Setup Complexity: UHFLI for Test & Measurement - Ultra-high Frequency Demodulation
- Premium Customer Care: New Software Releases Available for Download
- Premium Customer Care: New Blogs
- Tips & Tricks: Avoiding Pitfalls When Determining Signal to Noise Ratio
Xavier Jehl, Electron Pumps at 50 mK
Hello Xavier, you belong to the next generation of scientists in Grenoble, France. What is the scope of your research?
I am currently investigating extremely small silicon nanostructures, made in an industrial CMOS platform on 300 mm wafers. The bigger the wafers, the smaller the transistors we get! We now have feature sizes of the order of 10 nm. I am mostly involved with so called electron pumps, which are based on a regular transistor's design but with two gates. The application is mostly metrological: these pumps could be used to realize the future quantum ampere based on the electron charge, which would be fixed.
Which experiment are currently running and what are the objectives?
We are using two dilution refrigerators in which the sample is cooled down to around 50mK above absolute zero! One of them is for this electron pump, the other for studying single dopants, because the dimensions of industrial transistors are so small that transport through one, and recently two dopant atoms in series can be studied.
High-end measurement electronics is your daily bread. What is the reason for using 600 MHz lock-in amplifiers?
The dilution refrigerator has a radio-frequency (RF) resonant circuit combined with the electron pump. This is the first combination of this kind and where the Zurich lock-ins will help significantly, because it will allow us to demodulate directly the signal reflected back from the sample, which is typically in the frequency range from 200 to 600 MHz.
How many research groups do you know of in the world who are researching this or a similar topic?
On single dopants, there are maybe up to 10 groups now, involving electrical but also optical measurements. This is a field that is now growing very fast. Electron pumps are also in a "revival" period since metrology labs have shown that GaAs based devices may be suitable for the quantum ampere; our silicon option is very interesting for them as well. For pumps this is more restricted though, it's more like 5 groups at the moment, but some new ones are expected to join the race!
Can you name one of your publications that has been referenced the most often?
On single dopants it's probably this article "Single-Donor ionization energies in a Nanoscale CMOS channel" in Nature Nanotechnology 5, page 133-137 in 2010 (cited 91 times). We also have an article "A two-atom electron pump" in Nature Communications 4, 2013, article number 1581
Which piece of instrumentation would you like to have, but cannot currently find a viable commercial solution?
My main problem comes from the tens of thousands of samples I have, since they are produced in massive numbers. They require testing at 300K, then sorting to extract the very few we will be able to measure at low temperature. Both measurement apparatus and analysis software exist, but I'm missing something which links the two, a sort of giant database accessed by analysis software and fed by the measurement system, in order to have an overview of the millions of samples we have which are functional at room temperature!
Grenoble offers probably the best ski, paragliding and base jumping opportunities in the world. Which activities do you enjoy the most/least?
(I love this question!) It's true that Grenoble offers a fantastic playground for outdoor activities; I can say it without a personal bias since I'm originally from Paris, which is also a great place, but not for the same reasons! I mostly do ski touring, the sort of skiing activity that conserves energy: you go down but ALSO up by yourself... in the wilderness, far from ski lifts.
Reduce Lab Setup Complexity
UHFLI Product News
Just over one year after the launch of this unique product, the UHFLI lock-in amplifier has found its way into many research labs across 3 continents. At Zurich Instruments we believe that the success of the 600 MHz product is related to two situations. Firstly, we are encountering many research groups that are limited by the performance of older instruments and with the UHFLI lock-in amplifier are now able to push the boundaries of their research further. A second group of early adopters considers the UHFLI as the right investment for their current challenges and start to seize applications that they could not consider before.
Keeping in mind all the valued feedback from our community, we've worked hard to increase the number of features and therefore enlarge the number of applications supported. Today we're proud to present three new product options:
- UHF-BOX 600 MHz Boxcar Averager option (unique new feature)
- UHF-RUB Rubidium Atomic Clock option (unique new feature)
- UHF-10G 10 Gbit Optical Ethernet option (unique new feature)
- UHF-PID Quad PID/PLL Controller option (modification of feature)
Don't waste a pulse
The UHF-BOX Boxcar Averager option is the first of its kind - we are not aware of any previous commercial combination of a lock-in amplifier with a boxcar averager. This option upgrades the UHFLI into a powerful signal averager for all applications where the duty cycle of the signal is small. It's known that lock-in amplifiers are not particularly effective for measuring signals with a duty cycle less than 1:10, because the signal energy gets spread over a large number of harmonics of the reference frequency. Other signal acquisition methods are considered to be more effective. For example, boxcar averaging discriminates signal from noise by measuring only at well-defined time intervals, ignoring the signal of interest when there is no information, only noise.
The speed of our new boxcar is an increase of several orders of magnitude compared with the well known, old-school instruments available on the market. Zurich Instruments introduces the first boxcar averager capable of locking on to high repetition rate lasers, even up to a 600 MHz repetition rate, with an effective signal bandwidth up to 600 MHz, a specification that has never been seen before in a commercial instrument. For the scientist these specifications imply that he will be able to acquire a pulsed laser driven signal up to 1000 times faster than ever before. Furthermore, by combining a lock-in amplifier and boxcar in the same instrument we believe that researchers can easily switch between instrument operations without having to change a single cable in the setup.
The UHF-PID Quad PID/PLL Controller option replaces the previous UHF-PLL option. This new option offers 4 independent general purpose PID (proportional-integral-derivative) controllers that can be employed on a selection of internal input units, therefore providing closed loop control of several signals. In particular, when a PID controller is connected to the phase output of a lock-in demodulator this makes a PLL (phase-locked loop) for frequency regulation. This option allows the simultaneous operation of 2 PLLs and 2 PID controllers. Complex lab setups will benefit massively from such multiple locking and control options, all offered within one single instrument.
Powered by an atomic clock
The UHF-RUB Rubidium Atomic Clock sets new standards for clock stability for lock-in amplifiers. Up to now it has always been possible to externally add a rubidium powered device to the instrument. Doing this, users would require one external box, a potential source of ground loops. The first time combination of lock-in amplifier and state-of-the art rubidium-based atomic clock generation provides distinct advantages for setups with long term operation where frequency drift due to temperature or component aging and integrated errors are an identified problem, e.g. MEMS research and development. Another application group that profits from this option is high-speed atomic force microscopy (HS-AFM) scanning at ultra-high vacuum (UHV) and low temperature conditions.
The UHF-10G Optical Ethernet option serves to extend the data bandwidth between the UHFLI Instrument and the host computer from 1 Gbit/s to nominally 10 Gbit/s. Effectively this option provides a high-bandwidth data link capable of transferring more demodulated samples than the standard interface. The other main feature provided is the inherent electrical decoupling of the host computer from measurement setup by means of the optical link. This option requires a rather powerful desktop computer to reliably and continuously acquire and save the massive data stream; the bandwidth may be in excess of 100 MB/s, which demands a serious capability for copying and storage of data. The delivery of this option includes a hardware card to be inserted on the rear panel of the UHFLI and a PCI-Express card to be inserted in a desktop computer.
We are also announcing that we are adding 4 digital arithmetic processors (DAPs) to the UHFLI, capable of performing arithmetic computation on internal signals. More precisely it is possible to multiply and accumulate a wide selection of internal signals (e.g. demodulator output and auxiliary inputs) in order to generate a linear combination of them on the auxiliary outputs. This feature is available to all UHFLI users (no option required) and demonstrates the power of the Zurich Instruments' product architecture, capable of supporting features with ever increasing complexity for the benefits of its installed user base. We are convinced you are going to like this and many more new features we have in our roadmap!
Reduce Lab Setup Complexity
UHFLI for Test & Measurement - Ultra-high Frequency Demodulation
Engineers are discovering lock-in amplifiers. Historically, lock-in amplifiers were better known to physicists than engineers because the former require the flexibility of setting frequency and demodulation bandwidth whereas the latter need extensive signal processing and analysis capabilities. Furthermore, a large variety of commercial network and spectrum analyzers with a frequency range of up to tens of GHz are readily available to RF engineers working in communications and other industries. These analyzers operate on the same principle as lock-in amplifiers, but usually do not provide direct access to the demodulated samples and often need additional hardware for digital storage.
In reality, modern lock-in amplifiers can also meet the requirements of development labs when more flexibility is required. As frequency mixing is a concept familiar to engineers, a home-made solution can be built to demodulate a signal with a pre-defined modulation frequency at a fixed bandwidth. However, if the frequency is subject to change a discrete solution can quickly become unpractical. The choice of the demodulation bandwidth is even trickier due to the trade-off between signal-to-noise and measurement speed. Also, for many applications, the frequency and bandwidth requirements are not known from the beginning or change over time. This can quickly render a home-made solution obsolete.
For these reasons, we are seeing more and more that a flexible toolset (i.e. integrated spectrum analyzer, frequency sweeper, GHz oscilloscope and SW triggers) is becoming an essential requirement for determining the proper measurement conditions and detailed signal analysis.
Production testing of communication chips
An example application field for high-frequency lock-in amplifiers is the industrial testing of communication chips. Such devices emit a signal after being excited by an electromagnetic field which powers a circuit equipped with an antenna for transmitting a payload. In the simplest case, the communication contains an alive-or-not message. In more sophisticated cases, a bidirectional exchange between test station and chip is established.
While the UHFLI does not provide signal decoding (as some communication protocol testers do), the instrument provides a flexible solution adapted to the varying characteristics of the physical channel. Proprietary communication protocols transmitted above a few MHz are ideal examples of non-standard transmissions that require adaptive demodulation.
The UHFLI provides an out-of-the box, high-performance signal demodulation solution that provides a direct stream of demodulated samples. It features a comprehensive set of analysis tools and integrated data storage capabilities. Modulation schemes such as amplitude modulation (i.e. sideband demodulation), frequency modulation or more advanced techniques like double-sideband suppressed-carrier (DSB-SC) modulation are supported and can be set up with just a few mouse clicks. Finally, such an advanced lock-in amplifier controlled with a modern programming language such as MATLAB and Python can be configured as a complete test station with minimal additional hardware requirements.
More flexible than a network or a spectrum analyzer
Zurich Instruments offers lock-in amplifiers as a true alternative to network or spectrum analyzers for applications where time domain signal analysis is required. Network analyzers focus on the determination of S-parameters and used to be the de-facto choice for demodulation above several MHz. However this has now changed with the UHFLI, which covers the total frequency range from DC to 600 MHz and offers dedicated functionality for continuous time operation. In particular, the UHFLI provides demodulated samples at its outputs for high speed data storage or closed loop operation (e.g. PID control). Additionally, the capability of simultaneously measuring multiple harmonics or multiple arbitrary frequencies offers new ways for both frequency and time domain signal processing. Finally, the UHFLI also offers a cost-effective alternative for transmission and reflection measurements.
Premium Customer Care
New Software Releases Available for Download
We recommend that all Zurich Instruments users regularly update their instrument software in order to have access to new features and bug fixes. We work hard to release updated software packages for the HF2 Instruments (most recent release 12.08.1) and the UHF Instruments (most recent release 13.06). To download the latest software packages from the download center one needs user authentication that can be easily requested. If you have forgotten your password, it is not a problem to ask for your access credentials again.
Premium Customer Care
Lately we've been expanding the blog section on our website. We see the blog as an ideal way of capturing a few regular needs of our customers and, after enough time, the blogs will constitute an important knowledge base for the Zurich Instruments user community. The new posts cover a variety of tips and applications and we'll be adding more on a regular basis. Organized as personal blogs, all posts are also combined into a single Zurich Instruments blog.
Did you know that blogs are fully indexed and thus searchable not only from the blogs page but also from the Zurich Instruments homepage? Just try it out.
Did you notice that it is possible to leave comments about some of the blogs for non-registered as well as registered users? Your comments will be moderated by the blog owner.
Did you also realize that new blogs postings are announced on the Zurich Instruments Facebook page? We're not intending to send out general emails about new blogs but, in case you are into social media, liking Zurich Instruments on Facebook might be the appropriate way for you.
There are several ways how information is structured in the blogs. One can browse through the latest entries, check the categories such as "Measurements", "Applications", select the most interesting entries from cloud of tags, or simply searching. We hope that you'll find them a useful way of getting more from your Zurich Instruments products.
In two of our more whimsical blogs, we describe how to implement the most expensive FM radio receiver of the world or how to get into trouble by disturbing the electromagnetic spectrum in your neighborhood. Viel Glück!
Tips & Tricks
Avoiding Pitfalls When Determining Signal to Noise Ratio
Lock-in amplifiers are primarily used in measurement situations where signals are tiny and noise is abundant. Hence there is a strong motivation to understand and suppress existing noise sources as much as possible in order to improve the signal to noise ratio (SNR). In this article you will see how a Fourier Transform applied to Lock-in data can be used to measure and understand SNR.
Assuming the signal consists of a single frequency equal to the reference frequency and that the signal amplitude is stationary over a certain period of time, the corresponding histogram of lock-in output samples is usually well described by a Gaussian distribution. In this case the signal amplitude can be obtained by averaging over all recorded samples and noise is best quantified by calculating the standard deviation. A frequent error made here is to acquire data with an inadequate sampling rate over too small a time interval. Meaningful results can only be expected when the recorded and analyzed data include samples over more than about 100 times the low pass filter's time constant. Setting the sampling rate of the demodulated signal to about 7 to 10 times the filter's 3dB bandwidth typically leads to a good trade-off between statistcal accuracy and data redundancy.
However, there are many experimental situations where such a simple calculation of signal amplitude and noise leads to misinterpretation and even false results. For instance, whenever multiple frequency components are present within the demodulation bandwidth, or systematic effects like drifts lead to a non-Gaussian distribution in the histogram of the lock-in's output samples, a more thorough analysis is crucial. One possible approach is to analyze the signal in the frequency domain by subjecting the set of demodulator output samples to a complex Fast Fourier Transform (FFT) operation, i.e. using the zoomFFT functionality of a Zurich Instruments lock-in. The frequency span of the calculated spectrum is given by the demodulator sampling rate. The frequency resolution in Hertz is the inverse of the time recorded or alternatively the ratio of the sampling rate and the number of samples taken for the FFT. Two main features dominate the form of the spectrum, one being the input signal (often a sharp peak in the center of the spectrum) and the other one being the noise floor (usually shaped by the transfer function of the demodulator's low pass filter). By switching on filter compensation, the spectrum calculated is multiplied by the known inverse transfer function of the lock-in's low-pass filters before being displayed, which restores a flat noise floor. Now the cursors can be conveniently used to determine SNR for individual signals, even in the presence of a variety of different frequency components.
Moreover, one can normalize the spectrum with the actual FFT resolution and display the spectral densities, e.g. in units of V/√Hz, a quantity entirely independent of the zoomFFT resolution. Unplugging the signal when displaying spectral densities reveals the contribution of the instrument's input noise, which can be as low as 5 nV/√Hz dependent on the input signal range and input signal coupling.