In this story, we focus on features in today’s signal generators that play an important role in testing the devices we use on a daily basis, and also learn how new technologies in signal generators are helping engineers optimise their design better
ABHISHEK A. MUTHA
Signal generators are electronic devices designed to generate repeating or non-repeating electrical signals. Arbitrary, RF, microwave, pitch and audio and video signal generators are some major ones. Customarily, signal generators were hardware systems, but since the advancement in multimedia and computing, flexible, software-based generators have also become available.
over the years, the software components have become the dominant design factor. Virtually, one can generate any frequency for test purpose as required; and this is the biggest flexibility for design engineers.
With many diverse kinds of signal-generating devices, for different purposes, functions and applications (and varying price tags of course), let us find out how these signal-generating devices have evolved over the years.
Evolution of signal generators
Signal generation tools have evolved over decades from the basic carrier wave (CW) signal generators to complex signal synthesis for the present and future wireless and EW system testing and validation.
The first generation of signal generators incorporated vacuum tubes and crystal oscillators which had very limited functionality. The next jump was to make it arbitrary where a big change occurred. The concept of arbitrary waveform generators gave way to software-defined signal generators, giving rise to modular architecture (such as PXI, VXI) based signal generators.
With the advent of digital systems everywhere, many of these systems needed to be tested differently with modified waveforms and signals. And this is where modulated signal generators with different types of modulation techniques helped. Signal and function generators are now much smaller in size with jam-packed features whereas they used to be desktop instruments (even today some of them are) a few years ago. So, in short, size has reduced with increase in capabilities.
Creating the modern signal. Today’s signal generators have gone a step further. They can generate virtually any kind of signal. One can simulate the signal in industry leading virtual engineering environment such as MATLAB, LabVIEW or other simulation software. One can also capture off-the-air signals using a combination of software and hardware tools such as vector signal analysers (VSAs) and also recreate these signals using these analysers.
Today’s use of signal generators includes the design, testing and maintenance of systems with complex modulation such as orthogonal frequency-division multiplexing (OFDM), multiple-input multiple-output (MIMO) and radar transmitters and receivers. Modern signal generator’s functionality and performance is enhanced to simulate various blocks of such communication and radar systems.
With the development of new technologies in millimetre-wave and terahertz range such as WiGig, millimetre-wave RADARs, etc, signal generators have also seen growth in terms of carrier frequency from a few gigahertz to terahertz, and bandwidth from a few megahertz to multi-gigahertz.
Stephen Hire, general manager—India, Aeroflex informs that the last year has seen new additions to the WLAN standards including 802.11ac which brings a requirement for very wide modulation bandwidths. He says, “To support this, our latest generators have 160MHz modulation bandwidth and operate up to 6GHz to support the full range of WLAN bands around the world.”
“In terms of functionality, the digital radio, cellular and WLAN industries have driven a lot of changes with current generators needing to support digital modulation techniques and have built-in waveforms for standards such as 3G and LTE. Cellular standards continue to evolve and, although LTE has been around for a while now, design engineers now need to start looking at LTE Advanced as well,” adds Hire.
Ability to test dynamic equipment. The functionality of signal generators was getting defined in software which was good enough for performing test on electronic circuits of the 20th century. “This evolution required a different set of functions to be generated at very different speeds for different kinds of models for different environments,” informs Satish Mohanram, technical marketing manager, National Instruments India.
For environments where equipment functionality is very dynamic, ensuring that the functionality of these equipment gets tested becomes a lot more complicated. One has to basically make the test system behave as if it is the real-world environment for that specific application.
Citing an example, Mohanram explains, “If you consider ECUs, you have to make the environment work as though it is the actual part, be it the transmission, the engine or the terrain on which it is going. This really brings in a lot of complexity into the type of signals that have to be generated; signals have to be generated really fast and change really quick. Software was the key but it was not providing the required speed and functionality wherever it was needed. This moved us into the next generation which is a software-defined signal generator having an on-board signal processing unit.”
Programmable signal generators. “These signal generators are where we stand currently and I think, with the kind of advancements that are happening in the field-programmable gate arrays (FPGAs), there is a lot of functionality that gets added to the signal generators in a way that they can generate or simulate different scenarios,” believes Mohanram.
In the past, FPGA was a way of incorporating your own intellectual property (IP) into any system as it is programmable. With this, you get great performance and at the same time have your functionality implemented. He adds, “FPGAs moved into the signal generators around a year or a year-and-a-half ago, which I believe is a significant move that has occurred whereby the functionality of signal generators has grown by leaps and bounds.”
New technologies that benefit the engineer. The functionality that gets into today’s designs is very much defined in software, which is a challenge faced by most of the design engineers today. As software evolves, being able to set different types of scenarios becomes very difficult. Earlier it was very simple because you could give a square or a sine wave at the input and see how the response was on the other side. But today the algorithms that are implemented over these devices also change in real time.
Citing an example, Mohanram says, “Consider an adaptive noise cancellation system. In such a system, when you give a certain input, the output will not be the same but it will vary based on the ambiance. For you to check that, the ambiance needs to be simulated first and then the actual input signal. The actual signal is simple but the ambiance is pretty dynamic in nature. One has to basically frame the mathematical model of ambiance and get that to run.”
These kind of things were never possible but, with the arrival of programmable signal generators, these have become possible because the model can be hard coded into the hardware. and it gets implemented in real time, unlike the earlier generation where if implemented in software would result in lags and inconsistencies. Hence, it provides design engineers the capability to comprehensively test the functionality, say, build better quality products for the market.
“Also, today’s signal generators have two outputs on a single instrument,” informs Madhukar Tripathi, regional manager, Anritsu India Pvt Ltd. He says, “This is very good for many applications where usage of two instruments and syncing them is necessary. Two-channel concept has eliminated the need for a second instrument, hence cost has receded, which leads to more accurate and faster measurements.”
Design engineers can now use these signal generators and make faster and accurate measurements. Local oscillator (LO) is now more stabilised, and this provides high performance and accurate measurement for many mission-critical measurements.
Talking about the architectures used in signal generators and how they help a design engineer, Manish Joshi, deputy CEO, Scientech Technologies Pvt Ltd informs, “In signal generators, for lower frequencies, DDS-based architecture is used as it is lower in cost and best in performance as compared to conventional discrete-based approach. For a higher frequency application like wireless communication, more than one channel is used with software connectivity for downloading different waveforms based on wireless communication standards.”
He further adds, “It will help a design engineer to get smooth and high-frequency resolution signal and to design him/her own customised signal.
Programmable versus basic signal generators
“Signal/function generator with software allows a user to easily create his own waveform and edit and download complex waveforms using the wave editor, whereas with the basic signal generators the user can access only factory-defined waveforms,” informs Joshi.
The basic signal generators are still being used for primitive tests that are being conducted on some of the electronic components that are designed, but when it comes to the high-speed verification/validation or the high-speed manufacturing and testing, the days of the stand-alone box-type signal generators are gone, believes Mohanram.
T. Anand, principal consultant, Knewron, says, “The buzzword now is programmable; hardcoded or fixed physical signal generation is thing of past. More so, it is need of today’s technologically evolved world.” On another note, Tripathi believes, “Industry looks for physical signal generators as they are popular. But both categories have own merits and final decision is made based on performance, application, cost and ease of operation.”
New initiatives to help design and test better
FPGA-based instruments. From signal generators’ standpoint, there are a lot of people developing and working on specific IPs that can be implemented on the FPGA and the signal generator, shares Mohanram. He says, “There is a good level of standardisation of these IPs, and hopefully we will be able to leverage all of these across platforms of signal generators. There are different vendors having different FPGAs on their signal generators; some of them are ‘open’ and some of them are not.”
Different vendors are coming up with open signal generators, and these IPs can be quickly leveraged by these open FPGA signal generators. There is some work happening in terms of standardising this architecture and coming up with IPs for the same.
On a similar note, Vishal Gupta informs, “FPGA-based signal generators have been there for quite a while. Some of the Agilent ARBs have in-built FPGAs where data generation comprises a sample memory which contains the sample data, a sequence memory which contains the required information for sequencing such as the sequence structure or loop counter values and a channel FPGA which combines both into a sequence-controlled sample stream.”
Software tools to control the instrument. Many signal and function generators now come with a facility to be controlled from custom programs such as those built in Visual Basic (VB) or otherwise, informs T. Anand. He says, “These facilities make these generators suitable candidates for getting included in automated test equipment (ATEs) such that feedback-loop-based tests could be easily designed.”
Talking about tools, Mohanram says, “There are tools coming into the marketplace which can integrate the design environment with these signal generators directly, because when you are talking about building this particular model which has to run on the signal generator, it generally happens in some kind of a design environment. So there is a good level of work happening to integrate these design environments along with software which actually hit the FPGA program on the signal generator.”
Sharing his thoughts on some other new initiatives, “Waveform generator companies are providing software/tools which will help designers to design and test their devices under test (DUT) in a much more simplified way. Portable handheld function generator is available for field applications,” explains Joshi.
Growth in usage
The evolving electronics industry is moving towards an approach whereby there are people who are developing platforms and people who are developing apps for the platforms. Also, programmable electronics is entering every part of life today, which is driving the need for being able to do other testing and really high-speed testing. “Those are two factors which are driving the growth of signal generators and I think that FPGA signal generators are an outcome of the need that we see in the market,” says Mohanram.
On a similar note, Vishal Gupta, senior application consultant (RF/MW, Surveillance), Agilent Technologies, informs that the driving factors are emerging commercial communication technologies, enhanced defence requirements and recreation of real-time field signals in the lab, and the restraining factors are hardware limitations, faster DAC and high component costs.
In the future
With so many different technologies that are being worked on and their rapid evolution, flexible signal generators need to be developed to ensure their long and useful working life. Developing common hardware platforms and software-defined radio techniques enable the same hardware to address many applications with customisable software that can be upgraded as technologies change. Modular platforms and software-defined radios also help companies protect their long-term investment.
The FPGA capability on the signal generator has provided the designer with a lot of new capabilities to do different types of testing. Mohanram says, “For instance, the concept of hardware-in-loop testing that people have been trying to incorporate to address the needs of today’s electronic equipment, is being achieved really well by these signal generators that have been enabled by FPGAs that are playing a significant role.”
Signal generators phased from controls and knobs on the front panel to software, but today people are able to develop custom test units which get connected to say a gyroscope. One can even go to the extent of simulating the whole gyroscope on the FPGA and generating the signal as though it comes from the gyroscope from the signal generator. IPs are being developed which can simulate all of these different functionalities and these are also being shared across different industries. “This I think is going to be the future where you will see a whole bunch of these IPs available for download and tweaked on these signal generators. Thereby making these signal generators behave as though they are a specific sensor or an instrument,” adds Mohanram.
Talking about touch screens in signal generators, Hire says, “Smartphones have led the world in showing how touch screens can simplify and improve the user experience. and we believe that touch screen control of test equipment also has the potential for delivering big productivity gains in R&D and production.”
The author is a senior technical correspondent at EFY, Bengaluru