Modular AWGs: How they work and how to use them
The AWG (arbitrary waveform generator), with its near universal selection of waveforms, has become a popular signal source for test systems. Modular AWGs let you add standard or custom waveforms to PCs as part of an automated test station. With an AWG, you can create waveforms by using equations, by capturing waveforms from digitizers or digital oscilloscopes, or you can create your own waveform with manufacturer supplied or third-party software. Waveform sequencing lets you switch among predefined waveforms.
Today's modular AWGs offer extended bandwidth, higher sampling rates, and longer waveform memory than previous models. In addition, they offer advanced operating modes and the ability to stream large amounts of waveform data from a PC's main memory. Before selecting a modular AWG, learn how they work and what they can do.
How AWGs work
AWGs are digital signal sources. They operate like digitizers or digital oscilloscopes in reverse. The AWG has a numeric description of the waveform stored in waveform memory. Selected samples from the memory are sent to a DAC (digital to analog converter) and then, after filtering and signal conditioning, samples are output as an analog waveform. Figure 1 contains a conceptual block diagram of an AWG.
Figure 1. An AWG consists of several functions blocks that take a digital representation of a waveform and produce a filtered analog signal.
Data representing the waveform first gets loaded into the waveform memory. Normally, the waveform memory has a larger data width and is clocked with a divided sampling clock compared to the DAC. An FPGA in between demultiplexes the samples and generates a waveform data stream at the DAC's sampling clock. When commanded by the memory controller, the contents of the waveform memory are sent to the DAC for conversion into an equivalent analog voltage. Some DACs allow additional interpolation, which results in a higher update rate at the output than supplied by the waveform memory.
The raw DAC output is rich in harmonics and requires filtering. An AWG's output stage filters and conditions the signal by adjusting gains and offsets to meet the user's waveform specification.The memory controller keeps track of the elements of each waveform component in the waveform memory, and any associated links, and outputs them in the correct order. To save memory space, the memory controller can loop on repetitive elements so that these elements need be listed only once in the waveform memory. A clock generator uses either an internal or an external clock to provide a common timebase. The trigger generator, which causes the waveform to be output or advanced based on a user specified event, provides synchronization. In addition to internal or external trigger events, the AWG can be linked to another modular AWG or digitizer.
The actual implementation of the functions above vary with specific models, but all AWGs have similar elements.
An AWG's specifications are quite different from standard signal generators because of the AWG's output waveform selection and its digital nature.
The key parameters, just as with a digitizer, are bandwidth and sampling rate. The bandwidth determines the highest sinewave frequency that the AWG can output with a loss less than 3 dB. Because many of the waveforms that an AWG can create are rich in harmonics, the bandwidth limit will determine the highest frequency waveform that can be generated. For example, a square wave generally must pass the fifth harmonic to be recognizable. For a given bandwidth, the highest frequency square wave is typically one fifth of the AWG's bandwidth.
An AWG's sampling rate is related to its bandwidth. According to sampling theory, the sampling rate has to be at least twice the bandwidth. The sampling rate also determines the AWG's horizontal resolution. This defines the smallest time increment that can be set within a waveform.
The size of the waveform memory determines the longest waveform that the AWG can produce without repeating (looping) any waveform components. The limit of signal duration, without looping, is memory length times the sample period. An AWG that has 2 Gsamples of waveform memory and a maximum sample rate of 1.25 Gsamples/s can produce a waveform that's 1.6 s long. Looping lets you repeat redundant waveform components, which can greatly increase the maximum waveform length.
Amplitude resolution specifies the minimum output signal level the AWG can generate, which is also the minimum amplitude step between adjacent samples. The amplitude resolution of the AWG is determined by the number of bits of resolution of the DAC and memory. In general, there is a tradeoff between DAC resolution and sampling rate. That is, the greater the number of bits in the DAC, the lower the maximum sampling rate. An AWG that has 14-bit resolution has a theoretical dynamic range of 16384:1 An AWG with 16-bit resolution has a theoretical dynamic range of 65536:1. Noise and other factors reduce dynamic range, just as they do with digitizers.
The maximum output amplitude that the AWG can generate is determined by the output amplifier stage. In general, there is also a trade-off between an AWG's sampling rate and output amplitude, with faster AWG’s having a lower maximum output amplitude. The minimum full-scale output range depends on the internal attenuators in the output stage. On any given full scale range, the theoretical minimum value is the full output divided by the amplitude resolution (an AWG with a 10 VP-P range and 16-bit resolution has a minimum output step of 10/65,536 = 152.5 µV). Internal noise and non-linearity limit the practical minimum signal output.
An AWG's number of channels typically ranges from one to four, but AWGs can often be synchronized to provide additional channels.
Output filtering improves the signal to noise ratio of the AWG output. Generally the types and cutoff frequencies of the filters can be specified.
All AWGs can create modulated waveforms by creating them analytically, in software, using manufacturer’s operating software like Spectrum’s SBench 6 or other third-party math software, and downloading them into the AWG’s waveform memory.
Another useful feature is having a trigger input to initiate the output or to advance the waveform through multiple segments. We'll cover that in more detail below.
AWGs can also produce an output trigger or marker output synchronous with the waveform output. These signals can then be used to trigger a digitizer, oscilloscope, or other instrument at appropriate times during the waveform.
AWGs may incorporate multiple operating modes that affect how they store and replay waveforms. The ability to repeat (loop) selected segments of the waveform and advance between segments based on triggers or gating signals adds flexibility and reduces the amount of memory required for complex waveforms. Here is a summary of common operating modes:
- Single shot: The programmed waveform is played once for each external or software trigger. After the first trigger, subsequent triggers are ignored.
- Repeated (continuous) output: The programmed waveform is played continuously for a pre-programmed number of times or until a stop command is executed. The trigger source can be either an external hardware trigger input or a software trigger. After the first trigger, additional trigger events will be ignored.
- Single Restart replay: This mode outputs the waveform data of the on-board memory once after each trigger event. The trigger source can be either hardware or software.
- FIFO: Some AWG's offer a FIFO (first in first out) operating mode designed for continuous data transfer between the host computer's memory or hard disk and the AWG. The AWG's on-board memory serves for buffering data, making continuous streaming extremely reliable. In this mode, the available waveform memory is limited by the host computer’s memory.
AWGs that support FIFO streaming can further extend waveforms by utilizing the host computer's memory. In FIFO mode, the AWG uses its on-board memory as a high-speed buffer between system memory and the DAC. This frees the AWG from the limits of its internal memory. Combining FIFO streaming with looping and linking functions lets you produce even longer waveforms.
- Multiple replay: The multiple-replay mode (Figure 2) provides fast output of waveforms on multiple trigger events without restarting the hardware. The on-board memory is divided into several equal size segments. Each segment can contain different waveform data, each of which is output with the occurrence of each trigger event. This mode allows very fast repetition rates.
- Gated Replay: The gated sampling mode outputs waveform data controlled by an external gate signal (Figure 3). Data is only replayed if the gate signal is at a preprogrammed level.
- Sequence mode: The sequence mode (Figure 4) splits the internal card memory into a number of data segments of different lengths. These data segments are chained in a user set order using an additional sequence memory. The sequence memory determines the order that segments are output as well as the number of loops for each segment. Trigger conditions can be defined to advance from segment to segment. Using sequence mode, an AWG can switch between replay waveforms by a simple software command or it can redefine waveform data for segments simultaneously while other segments are being replayed.