Real-time and Arbitrary Waveform Generation Mode: Which One is Right for Your Test?
2019-02-21 | 8 min read
When you download baseband waveform files to an RF vector signal generator (VSG), a question comes to mind. Does the VSG have enough memory available for my waveform files? Sometimes, you configure signal parameters on a VSG, but cannot find a button to download waveforms you set to the VSG! What’s the problem with your measurement setups?
RF vector signal generators offer two flexible baseband architectures with complementary features for generating complex digital modulation signals, real-time and arbitrary waveform generation. In addition, signal generators also allow external I/Q signal inputs for customized modulation signals.
You may not know which type of baseband architecture you’re using and why. Let’s take a close look at these two flexible baseband architectures and how you use them in your applications.
Real-Time Waveform Generator Mode
Real-time waveform generator mode is common for receiver test.
The real-time I/Q baseband generator interprets data bits and produces analog voltages that represent the modulation you want. Figure 1 shows a real-time baseband processor block diagram. It includes essential components of a transmitter, from payload generator, formatter (channel coding), I/Q symbol builder, spreading/scrambling encoding to FIR (Finite Impulse Response) filter. The processor is configured for specific modulation schemes or standards. You just need to configure parameters for each sub-block, then the baseband processor generates digital I/Q waveforms. There are no waveform files required in this mode.
The real-time baseband processor allows generation of multiple fully-coded logical channels, such as data channel, control channel, broadcast channel, etc. It simulates fully-coded signals so that a receiver can decode the signals and recover original payload data.
Figure 1. Real-time I/Q baseband generator block diagram
Signal generators also allow you to perform a bit error test (BERT) as shown in Figure 2. Your device under test (DUT) decodes the signal and recovers the payload data. When measuring bit error rate (BER), the DUT outputs the clock and the data stream to the signal generator’s rear panel BERT inputs. To remove headers in the data stream, input the "clock gate" signal to the BERT of a signal generator so that only a specific period is valid for BER analysis as shown in Figure 3.
Figure 2. Test configuration for various receiver tests
Figure 3. Gated clock and data
Dual Arbitrary Waveform Generator Mode
Dual arbitrary waveform generator (ARB) mode controls the playback sequence of waveform segments that are written into the internal baseband generator’s memory similar to an MP3 player which converts an audio file to an analog signal. This mode enables you to play, rename, delete, store, and load waveform files in addition to building waveform sequences. It also offers markers, triggering, clipping, and scaling capabilities. Learn more about baseband waveform data and structure in the blog post “Understanding Baseband Waveform Data and Structure for Vector Signal Generators.”
Figure 4 shows the example of downloading an I/Q waveform file to the baseband signal generator’s waveform memory. In this scenario, the waveform data will move to the I/Q digital data multiplexer (MUX). Then, digital signal processing (DSP) can perform advanced real-time signal processing, such as internal channel correction, increasing waveform sampling rate, and adding real-time Additive White Gaussian Noise (AWGN) to a signal.
Figure 4. Dual arbitrary waveform generator block diagram
Dual ARB mode is commonly associated with a component test because the limited memory size in baseband generators does not allow for a long period of play in the waveform. BERT typically uses a pseudorandom binary sequence (PRBS) as payload data. For example, PRBS15 produces a sequence length of 215-1. The baseband waveform length for a cycle of PRBS15 could stretch from several minutes to hours depending on the type of modulation scheme you use. The component test uses a stimulus-response test methodology to examine the difference between the input and the output waveform signal. In most test cases, payload data does not impact the measurement results.
However, some receiver tests may use the dual ARB mode such as a wireless local area network (WLAN) receiver’s packet error rate (PER) tests. The signal is in a packet or frame format. You can generate a waveform segment which includes a packet or several packets, then repeat the segment. The receiver receives the signal and determines whether each packet is corrupt or not with cyclic error-correcting code.
External I/Q Inputs
Signal generators accept externally supplied analog or digital I and Q signals for modulating onto the carrier as shown in Figure 4. You need to be aware of the input limitation of bandwidth, impedance, and levels for analog I/Q signals because the modulator may go into underdrive or overdrive. For digital I/Q input signals, data and clock rate must be matched what you set on the signal generator.
Figure 5. External analog and digital I/Q inputs
When Should I Use Real-time or Dual ARB mode?
Although dual ARB mode is more flexible in creating modulation signals for both component and receiver testing, it also has limitations. The comparison table below offers a quick overview of the differences between real-time and dual ARB modes.
Table 1. The differences between real-time and ARB signal generation mode
The Synergy of Two Baseband Architectures
To effectively test your DUT, you need to use the right baseband mode to generate signals. RF VSGs offer two flexible baseband architectures with complementary features for generating complex digital modulation signals. The flexible architecture of the VSG offers easily defined digitally modulated signals. Selecting appropriate waveform generation modes and understanding signal creation processes are the keys to successful vector signal generation.
Want to learn more about the key specifications of signal generators and why they matter? Download the "Essential Signal Generator Guide" and "9 Best Practices for Optimizing Your Signal Generator".