This Advent during the 24 days leading up to Christmas I thought it would be fun to have a quick introduction of scientific apparatus, in particular tools used in atomic physics research. This is a way to say thanks to their hard work for our research, and a look at how many things are really needed to make a laboratory work!
- Day 1: Acousto-Optic Modulator
- Day 2: Ion Pump
- Day 3: Photomultiplier Tube
- Day 4: Vapor Cell
- Day 5: Analog-Digital Converter
- Day 6: Magnetic Field Coils
- Day 7: Optical Fiber
- Day 8: Electro-Optical Modulator
- Day 9: CCD Camera
- Day 10: Ion Gauge
- Day 11: Residual Gas Analyzer
Arbitrary Waveform Generator
The arbitrary waveform generator (AWG) is more of a general instrument and not confined to physics labs, but physicists definitely take advantage of them. Most often they are specialized boxes that create any sort of voltage signal over time that the experiment requires (within limits, though).
In its simplest form it consists of the following parts. There’s a clock that drives stages that follow, as telling them “do the next step, now the next, now the next…” In the specifications usually the clock frequency is given, e.g 20 MHz, or 100 MHz, … The next stage is the waveform memory. This stores the desired output voltage signal as data. It is usually specified as number of data points, or bytes. That memory can be filled by the AWG itself if a default waveform is selected, or specified by sending data from a computer to the AWG. The clock signal will drive the output step through the data points in this memory. The data stored in the memory is converted to an analog signal by the Digital-to-Analog Converter (DAC), turning bit into actual voltage. Finally the analog output circuit will usually apply a low-pass filter to the signal to smooth over the steps in the output, that shows up at the clock frequency, and sends the output voltage to the connected devices. This is a very simplified description, of course…
While the AWG can indeed output any signal within its voltage limits and with frequencies below its clock frequency, it usually comes with a lot of pre-defined voltage waveforms too for simpler use.
One very basic one is the sine wave, this is a very natural signal. The function generator, a similar but simpler equipment can usually just create this or other simple waveforms, though through their specialization they can do that over a wider range of settings (e.g. much higher frequencies than AWGs can). For sine waves we would usually set the signal’s frequency, amplitude, offset, phase values.
Square wave is another simple form, just switching between a low and a high value periodically. Here we’d set the frequency, amplitude, offset, and duty cycle (the fraction of time spent at the high value, 25% duty cycle would mean 1/4 time at high value, 3/4 at the low value).
In the case of a triangle wave the frequency, amplitude, offset, and the symmetry is set (the slope of the increasing ramp compared to the decreasing ramp).
The sawtooth wave is a special case of the triangle wave when after gradually reaching the top of value of the signal, it jumps back straight to the low value and start increasing again. Here usually just set the frequency, amplitude, and offset.
Other more interesting signal types the chirp or sweep, where the frequency of the signal is changing over time . Here the base waveform (usually sine wave), the amplitude, start and stop frequency, and sweep time are the variables. Such signals can often be used in atomic physics to generate a time-dependent interaction in the experiment.
There are also burst, where a specific signal is output for just a specific (short) time, then there’s static voltage for a waiting period.
The more complex output modes that can be still among the default settings of an AWG are different modulation types: frequency modulated (FM), amplitude modulated (AM), phase modulated (PM) signals, and also telecom signal types, such as frequency-shift keying (FSK). Though these show up ,ore in the higher end, and wireless communication-focused devices.
An arbitrary waveform generator
AWGs usually have a screen so it is easy to see at a glance what is the current waveform setting. It’s even more necessary because there are quite a number of parameters to adjust (3-4 for each output type), and visual representation clarifies things much more simply.
For the arbitrary waveform functionality usually a computer is required. In an experiments maybe we measure some property of atoms or molecules we are working with, and based on those measurements a different signal is required to achieve our goal. Then the computer can take the measurement, calculate the the required waveform, and instruct the AWG to create the appropriate voltage signal. Another connected functionality is being able to be triggered: the AWG is instructed to wait for a “go-ahead” signal from a source (which can be another electronic equipment, or the computer), and then start to output the agreed waveform.
Of course the AWGs functionality can be replicated to a large extent by a computer and a digital-to-analog converter – because the AWG is just that too, except very much specialized. This is a common pattern in the lab. While the same functionality can be provided by multiple devices in general, the parameters of the problem matter devices cannot do all functionality equally well (e.g. computer + DAC combo might not be fast enough compared to the AWG’s clock, or conversely the AWG might not have large enough memory to hold a complex waveform pattern). Knowing when to use a specialized equipment, and which one, is a major part of an experimental physicist’s experience.
- Function & Arbitrary Waveform Generator Guidebook
- An example Arbitrary Waveform Generator user manual (pdf)
- Online tone generator (to experiment with waveforms in audio form)
If you liked this, come back tomorrow for another apparatus! And send this link to someone who you think would be interested! :)