Understanding Baseband Waveform Data and Structure for Vector Signal Generators
2019-01-09 | 9 min read
You must have played music on your computer. A media player can play a different type of digital audio format like Waveform Audio File (WAV) and MPEG Audio III (MP3), and a sound card uses a digital-to-analog converter (DAC) which converts the digital data into an analog signal. The output signal is connected to an amplifier and speakers. First of all, you need to know if the media player supports the digital audio format that you want to play.
RF vector signal generators (VSG) use a dual arbitrary waveform generator (AWG) to generate baseband I (in-phase) and Q (quadrature) waveform signals and controls the playback sequence of waveform segments that have been written into the memory located in the internal baseband generator. Like an MP3 player that converts an audio file to an analog signal, the dual AWG enables you to play, rename, delete, store, and load waveform files in addition to building waveform sequences.
Creating baseband waveform files for VSG not only requires an understanding of the communication system, but also the waveform data requirements of the baseband generator. Before you create custom waveform files, you need to know the file formats that the VSG supports and the waveform data that is required for waveform playback. We will take a closer look at the basic characteristics of AWG first.
A dual AWG is flexible and can generate many complex modulation signals. You can simulate your design and create the waveform files from a PC, then use the dual AWG to convert the files into analog signals. However, you need to understand the key specifications of AWGs first. Figure 1 illustrates the basic block diagram for an AWG. Key specifications appear below.
- Sampling Rate: The maximum speed of conversion of the digital-to-analog converter (DAC). This parameter is equal to the speed at which samples are read from the waveform memory.
- Vertical Resolution: The number of the DAC bits in the AWG. For a given vertical resolution, N, the DAC will be capable of generating 2N different levels. For RF VSG, the number of bits of the DAC is from 12 to 16.
- Memory Size: The maximum number of samples that can be stored in the waveform memory.
- Output Characteristics:
- Amplitude and DC offset ranges
- Output impedance (typically 50 Ω)
- The availability of differential outputs
- The choice of output filters
Figure 1. A basic block diagram for an AWG
RF VSGs use dual AWGs to generate baseband I and Q signals. For example, the maximum sample rate of Keysight MXG N5182B’s baseband generator is 200 MSa/s, the number of bits of the DAC is 16 bits, and playback memory size is up to 1024 MSa. These specifications limit the signal maximum bandwidth, dynamic range, and waveform length correspondingly. The output connectors are BNC single-end or differential outputs with impedance 50-ohm, DC coupled.
Understanding Waveform Data
To generate baseband signals successfully, you need to understand what the AWG input data is and how to transfer a waveform file to the baseband generator. We will explore the input data that is required for the DAC first.
DAC Input Values
The DAC determines the range of input values for the I and Q data samples. A 16-bit (2 bytes) DAC means that the output range of the DAC can be divided into 0-65,535 (216) levels. A baseband generator divides the range with positive (32,767) and negative values (-32,768). Baseband generators accept binary or hex data formatted as shown in Figure 2.
Figure 2. 16-bit DAC input values correspond to output voltages
When you use the data in binary format, you need to be aware of byte order, little-endian or big-endian. The byte order describes how the system process stores integer values as binary data in memory. The Intel processor uses little-endian and the Sun processors use big-endian. The Apple processor is big-endian oriented, but it also supports little-endian. Keysight signal generators always assume that downloaded data is in big-endian order.
Figure 3. Byte order for multiple-byte waveform data – little endian and big endian
Waveform files are the most fundamental for baseband waveform generation. Waveform files can be generated by specific application software or general software tools, such as Matlab, C++, or Keysight’s software tools. There are two types of waveform files: segment and sequence.
A segment is a waveform file that you download to the signal generator’s memory and it consists of I/Q data, marker data, and a file header.
The file header contains key parameters of the waveform segment, including DAC sample rate, marker polarity, runtime scaling, and so forth.
Figure 4. Waveform header information on Keysight MXG signal generator
VSGs provide waveform markers to mark specific points on a waveform segment. You can turn on markers at specific sample points manually, or download a marker file with the waveform file together to the baseband generator. When the signal generator encounters an enabled marker, an auxiliary signal is routed to a real panel event output that corresponds to the marker number. The Keysight X-series signal generator provides four markers set at each waveform sample. A marker point consumes one byte. Figure 5 illustrates I/Q waveforms and markers 1-4. Using an oscilloscope to examine the Q channel and marker 1 signal, the waveforms appear at the bottom of Figure 5.
Figure 5. Shows the I and Q components of the waveform and the marker points
You need to set the marker polarity (positive or negative), and routing for each marker (1-4). You can also configure markers to enable or disable internal hardware, such as automatic leveling control (ALC) hold, and RF blanking.
An I/Q waveform consists of samples. A sample contains I/Q data (2*2 bytes) and marker data (1 byte), five bytes in total. When you create the waveform data, the I and Q data samples typically reside in separate arrays or files. You can also use a single file with I and Q data samples interleaved; the Q data follows the I data.
A waveform sequence is a file that contains pointers to one or more waveform segments, other waveform sequences (nested sequences), or both. This enables you to play multiple waveform segments or sequences. The signal generator allows you to set the number of times the segments and sequences repeat during playback.
Figure 6 shows the structure of waveform segments and sequences. In a segment, it includes a file header, I/Q waveform samples and marker files. Use a sequence to play back segments and/or other sequences.
Figure 6. Waveform files, waveform segments, and sequences
The dual arbitrary waveform player is a flexible way to generate complex baseband waveforms. However, you need to understand the hardware structure and the requirements for waveform data to generate your desired waveforms. To learn more about overcoming signal generation challenges, download the white paper “The Essential Signal Generator Guide”.