Quick note: We usually post oscilloscope tips and tricks to this blog, but today we want to share with you about another test & measurement tool often used with or alongside scopes.
In my previous post, I outlined the different types of signal generators in the market today, and what you need to consider when selecting the right fit for your application. I also highlighted why the arbitrary waveform generator (AWG) is my recommendation for you to simulate real world stimulus.
Arbitrary waveform generators (AWGs) are the most versatile signal generators available. An AWG can generate any mathematically-characterized signal, including sine wave, pulse, modulated, multitone, polarized, and rotated signals. The AWG is commonly seen as the workhorse piece of test equipment and can perform the functions of any other generator type. A typical block diagram of an AWG is shown below. The signal flow through the functional blocks starts with a numeric description of a waveform stored in memory. Then the selected waveform samples are sent to a digital-to-analog converter (DAC), filtered, conditioned, amplified and output as an analog waveform.
A Closer Look at Each Arbitrary Waveform Generator (AWG) Functional Block
A digital representation of a waveform is loaded into AWG memory through a variety of software applications, such as MATLAB, LabView, Visual Studio Plus, IVI, and SCPI. The memory is clocked at the highest sampling rate supported by the AWG. The size of the memory will dictate the amount of signal playback time available. A rule of thumb to determine the playback time is: memory depth divided by sample rate equals playback time. The faster your sample rate, the quicker you will use up the available memory.
The sequencer circuitry can solve memory depth limitations by arranging (sequencing) the waveform into segments to create your desired waveform. Memory sequencing (or memory ping-pong) does this by only enabling memory during critical waveform portions and then shutting off. You can think about it like this: when recording a round of golf, imagine how much recording time you would save if you only recorded the players striking the ball and not all the walking and setup time. The sequencer does the same thing by only recording waveform transitions and not idle time. Synchronization is maintained by the trigger generator, which enables the waveform. Trigger events can be internal, external, or linked to another AWG.
3. Markers and Triggers
Marker outputs are useful for triggering external equipment. Trigger inputs are used to alter sequencer operation, resulting in the desired waveform entering the DAC. Hardware or software triggers can be used for applications requiring exact timing, like wideband chirp signals. They can also be used where multiple AWGs are synchronized together and need to be triggered simultaneously.
4. Clock Generator
The timing of the waveform is controlled by an internal or external clock source. The memory controller keeps track of waveform events in memory and then outputs them in the correct order to the DAC. The memory controller saves space by looping on repetitive elements so that the elements are listed only once in the waveform memory. Clocking circuitry controls both the DAC and the sequencer.
5. Digital-to-Analog Converter (DAC)
Waveform memory contents are sent to the DAC. Here the digital voltage values are converted into analog voltages. The number of bits within the DAC will impact the AWG’s vertical resolution. The higher the number of bits, the higher the vertical resolution and the more detailed the output waveform will be. DACs can use interpolation to reach an even higher update rate than what was supplied by the waveform memory.
6. Low Pass Filter
Because the DAC output is a series of voltage stair steps, it is harmonic-rich and requires filtering for a smooth sinusoidal analog waveform.
7. Output Amplifier
After the signal passes through the filter, it will enter an amplifier. The amplifier controls both gain and offset. This gives you the flexibility to adjust output gain and offset depending on your application. For example, you may need high dynamic ranges for radar and satellite solutions or high bandwidth for high-speed and coherent optical solutions.
Use this blog’s functional building blocks to help you understand just what is happening within your AWG and fully utilize the AWG’s capabilities. For a deeper understanding of arbitrary waveform generator fundamentals, I recommend that you download the comprehensive "Fundamentals of Arbitrary Waveform Generation" guide.
Takeaway and the Demands of Our Connected World
Our connected world demands increased speed and data complexity. To support this demand, today’s AWGs must:
- Reach higher frequencies while providing wider bandwidth
- Handle complex modulation techniques that cram more data into available bandwidths
- Work with ideal and real-world signals
- Generate signals that stress devices to their limits
- Provide reliable and repeatable results
More on this topic in my posts to come.