Simulate Telecom Signals With Digital Waveform Generators
Key takeaways:
- Digital waveform generators can synthesize any complex analog or digital signal by first generating a suitable digital data sequence.
- They are the only devices that can simulate the complex modulation schemes of modern communication and optical systems.
- Their versatility makes them useful in just about every industry that involves electronic signals.
Signal generators perform a critical role in innovation and technology advancement. Without them, smartphone manufacturers wouldn't know if their latest models will work correctly once they're in the hands of customers. Space engineers wouldn't know if the satellites and space telescopes they're sending to space will transmit and receive data properly once in orbit.
Helping to turn such technological dreams into realities is a powerful type of signal generator called the arbitrary waveform generator (AWG). For example, in January 2024, researchers used a Keysight AWG to achieve possibly the fastestwireless data transmission speed ever in the sub-terahertz frequency band.
Among AWGs, digital waveform generators are probably the most powerful generators available today.
What is a digital waveform generator?
Fig 1. Keysight M8199B arbitrary waveform generator
A digital waveform generator is an arbitrary waveform generator (or arbitrary function generator) that uses digital techniques to synthesize complex waveforms.
It can generate both analog and digital waveforms, ranging from simple analog shapes like sine or sawtooth to complex telecommunication digital signals that use complicated modulation techniques to accommodate thousands of parallel calls.
Digital waveform generators operate in the discrete-time domain, using various techniques to synthesize digital values corresponding to a waveform. The values undergo digital-to-analog conversion (DAC) and filtering to generate the final analog output signal.
What waveforms can digital waveform generators produce?
Fig 2. PAM-4 modulated signals
Digital generators can produce everything from simple analog shapes to complex, modulated digital signals, including:
- standard waveforms like sine waves, square waves, triangle waves, and sawtooth signals
- pulse and clock signals commonly used in digital circuits
- arbitrary waveforms corresponding to real-world signals
- multi-tone signals that contain multiple frequency components
- basic modulated signals using amplitude modulation (AM), frequency modulation (FM), or phase modulation
- sideband modulation signals that include a main frequency surrounded by sidebands at specific offsets
- intermodulation signals that comprise the sum and difference frequencies of two primary tones
- high-frequency radio frequency (RF) and microwave signals with advancedmodulation schemes like quadrature amplitude modulation (QAM), quadrature phase shift keying (QPSK), or orthogonal frequency division multiplexing (OFDM) used by telecommunication and satellite systems
- optical signals with phase-amplitude modulation (PAM)
What features should you consider when selecting a digital waveform generator?
Fig 3. Amplitude and phase expressed as I-Q components for modulation
When selecting a digital waveform generator, consider these key specifications and features:
- Waveforms: What waveforms does your application require?
- Output frequency range: Ensure that the generator supports the required frequency range. Also, remember that high-frequency gigahertz (GHz) range RF applications often involve intermediate frequencies in the megahertz (MHz) ranges, which can affect their precision.
- Modulation: Check the types of supported modulations (e.g., AM, FM, QAM, QPSK, and others) and the modulation parameters you can control.
- Number of channels: Most applications require multiple channels and synchronization between outputs.
- Programmability and precision: How precisely can the generator be programmed to generate the exact waveforms you need?
- Distortion and noise: Look for features that allow adding distortions or error signals to the waveform for simulating real-world environments.
- Frequency sweeps and bursts: The generator should generate frequency sweeps and signal bursts for characterizing devices over a range of frequencies.
- Sampling rate: The sampling rate determines how accurately the generator can reproduce waveforms, especially for high-frequency signals. It's specified in mega samples per second (MSa/s or MS/s) or giga samples per second (GSa/s or GS/s).
- Memory depth: The internal memory depth of the generator says how many sampling points they can store. It's specified as millions of data points (Mpts).
- Resolution: This is the bit depth of the DAC. High bit depth allows high resolution and fine control over the signal amplitude.
- Phase noise: For applications sensitive to signal purity, evaluate the phase noise of the generator.
- Connectivity and interfaces: Does it have connectivity options for integrating with the rest of your test setup? Does it have programmable interfaces for automated testing?
How do digital waveform generators differ from analog signal generators?
Some key differences between digital waveform generators and analog signal or function generators include:
- Flexibility: Digital generators can easily generate or replay any desired waveform, even irregular ones, with any type of complex modulation. Their digital signal processing components and field-programmable gate arrays just have to be programmed suitably to generate any arbitrary digital data sequence. In contrast, analog generators require additional specialized circuits for every capability.
- Precision: Digital synthesis allows precise control over waveform parameters like frequency, amplitude, phase, and shape. The precision of analog generators can degrade due to component drift over time.
- Synthesis techniques: Analog generators directly produce continuous analog waveforms. For example, state variable oscillators are analog differential equation models, and Wien bridges are resistor-capacitor oscillators. In contrast, digital generators use methods like direct digital synthesis (DDS) or recursive oscillators to first synthesize digitized signals and then convert them to analog signals with DACs.
- Operating domains: Analog generators operate in the continuous-time domain, while digital generators operate in the discrete-time domain.
What are the advantages of digital waveform generators in precision use cases?
For precision applications like digital communication, radar, and data acquisition, digital generators offer several benefits:
- Precise timing: Digital synthesis techniques enable very precise control over the timing.
- Synchronization capabilities: Digital generators support real-time synchronization across channels or with other instruments to create phase-coherent signals.
- Programmable phase adjustments: These features allow precise phase alignment required by phase-sensitive measurements, phased array systems, and phase modulation systems.
- Complex modulation formats: Digital generators support many schemes like QAM, QPSK, and OFDM that require precise timing and phase control to accurately encode and transmit data.
- Jitter injection: For stress testing, digital generators can introduce controlled amounts of jitter to analyze timing variations.
What are the principles used in digital waveform generation?
Fig 4. Digital waveform generation
The key principles used in all digital synthesis techniques include:
- Discrete-time domain operation: Signals are synthesized as sequences of values at discrete points in time.
- Algorithmic signal data synthesis: They mathematically model the desired waveforms using algorithms.
- Digital to analog conversion: The digitally synthesized data is converted to analog signals by a DAC.
- Reconstruction filtering: After conversion, the analog signal often undergoes reconstruction filtering to smooth the signal, remove unwanted artifacts introduced during the conversion, and ensure the output closely matches the intended waveform.
What are the signal synthesis techniques used by digital waveform generation?
Some common digital synthesis techniques are explained below.
1. Direct digital synthesis (DDS)
Fig 5. Direct digital synthesis
DDS involves fixed sample rate phase accumulation and phase-amplitude mapping. A phase accumulator updates the phase angle incrementally at each clock cycle. The amplitude for that angle is either looked up in a waveform table or calculated by a function. This effectively maps phase to amplitude. Arbitrary waveforms can be generated by changing the phase-amplitude mapping function.
2. Recursive oscillators
These techniques generate waveforms by recursively applying a mathematical formula, where each output sample is computed based on previous output samples. This method is effective for creating oscillatory waveforms and can be tailored for various waveform shapes.
3. Arbitrary waveform generation
This method can involve generating waveforms through the Fourier series or replaying a waveform captured by an oscilloscope. For the Fourier series approach, waveforms are computed directly according to user-specified harmonic amplitude and phase parameters. The replay method enables the reproduction of complex real-world signals. When devices synthesize waveforms using functions, they are termed as arbitrary function generators.
4. Point per clock (PPC)
One or more points of a waveform are output for each clock cycle by optimizing the number of arithmetic operations and memory accesses needed per sample period. This ensures that each clock cycle is used efficiently to contribute to the generation of waveform points.
5. Trueform generation
Trueform is Keysight's proprietary technique that overcomes the drawbacks of both DDS and PPC methods. It ensures that every point of the waveform is played, regardless of the set frequency or sample rate. It accurately reproduces fine details with very low waveform jitter.
How are digital waveform generators used for signal integrity testing and characterization of devices?
Digital generators are used to simulate real-world conditions and stress-test RF, optical, and electronic circuits under various signal characteristics. Here's how they are used in such workflows:
- Generate a wide range of signals: Real-world signals with complex waveforms, modulation, and encoding are generated.
- Test data communication: Digital generators enable testing of data transmission and reception under different signal conditions.
- Test fast edge rates: Digital generators can increase edge rates to stress circuits and expose problems like reflection, crosstalk, and ground bounce.
- Inject jitter: Introducing jitter (timing variations) into test signals helps to evaluate a circuit's tolerance to timing inaccuracies, which is critical for high-speed digital communication.
- Examine frequency response: By sweeping frequencies across a circuit's operational range, engineers can characterize its frequency response and identify resonances, bandwidth limitations, and filtering problems.
- Test impedance matching: Generating signals at various frequencies and amplitudes enables analysis of impedance matching across the circuit, crucial for minimizing signal reflections and losses.
- Create eye diagrams: By generating high-speed data signals, digital generators can be used in conjunction with oscilloscopes to create eye diagrams of a digital signal's integrity.
How are digital waveform generators useful for automated testing?
Digital generators prove very effective in automated test environments thanks to their inherent characteristics:
- Programmability: Digital synthesis allows extensive programmability of complex waveforms, modulation patterns, and signal sequences.
- Versatility: Their ability to generate a wide range of signals makes them ideal for testing a broad spectrum of devices and systems across different industries.
- Precision and accuracy: Parameters like frequency, amplitude, and phase can be programmed very accurately, leading to more reliable and repeatable test results.
- Speed: Their ability for high-speed digital synthesis makes testing very efficient in high-volume manufacturing and R&D environments.
- Remote operations: Modern digital generators are equipped with connectivity options like the general-purpose interface bus, universal serial bus, and local area network extensions for instrumentation. They facilitate the integration of waveform generators into distributed testing systems and cloud-based testing architectures.
How are digital waveform generators used in telecommunications testing?
Fig 7. I-Q eye diagrams and Trellis diagrams for telecom modulation schemes
Digital waveform generators are extensively used for testing telecommunication systems like 5G/6G and Wi-Fi as follows:
- Simulate modulation schemes: Digital generator configuration supports complex modulations like QAM, QPSK, and more.
- Conduct receiver tests: By varying the amplitude and adding controlled noise to the generated signals, the sensitivity and performance of receivers under different signal conditions can be tested.
- Test error correction: Digital generators can introduce bit errors and signal distortions to test the effectiveness of error detection and correction mechanisms.
- Simulate adverse conditions: Generators can simulate fading, multipath interference, and Doppler shifts.
How are digital waveform generators used with defense and satellite systems?
Digital generators are used in defense systems and use cases:
- test radar systems by simulating radar signals
- test battlefield communication devices by generating communication signals with realistic noise and interference
- generate electronic warfare signals for testing defense equipment like electronic countermeasure devices and jammers
Satellites for telecommunications, military use, and remote sensing use RF signals in different frequency bands with various modulation schemes. Additionally, their dynamics result in phenomena like Doppler shifts and atmospheric interference. All these aspects can be programmed into digital generators to simulate space-based and ground-based transmission and reception systems.
How are digital waveform generators used with optical systems?
Digital generators can synthesize high-speed signals with modulation schemes like phase-amplitude modulation that are common in optical systems.
Keysight's versatile digital waveform generators
Keysight's arbitrary waveform generators, arbitrary function generators, function generator amplifiers, and generatormodules cater to a wide range of defense, telecommunications, optical, calibration, and consumer device use cases.
Contact us for support and expertise in selecting the best arbs for your project.