Vector Signal Generator

The ability to generate any mathematically defined I/Q baseband waveform from digital sample memory. Unlike analog signal generators limited to continuous wave or basic modulation, a VSG plays back custom waveform files.

• Sample Rate: Determines the maximum modulation bandwidth (e.g., 2 GS/s for 1 GHz bandwidth).
• Memory Depth: Limits the duration of unique, non-repeating signal sequences.
• Waveform Sequencing: Allows complex test scenarios by chaining multiple waveform segments with precise timing triggers.

The precision with which the generator translates digital I/Q vectors into a physical radio frequency modulated signal. This is quantified by Error Vector Magnitude (EVM).

• Residual EVM: The VSG's own contribution to modulation distortion, typically specified as < 0.5% (-46 dB) for high-end instruments.
• I/Q Imbalance: Gain mismatch and quadrature skew between the I and Q paths that distort the constellation.
• Carrier Leakage: Unwanted local oscillator feedthrough that displaces the constellation origin.

The short-term frequency stability of the internal local oscillator (LO) , manifesting as random phase fluctuations. High phase noise masks the subtle nonlinear memory effects of a power amplifier.

• Specified at offsets: Measured in dBc/Hz at 1 kHz, 10 kHz, and 100 kHz from the carrier.
• Impact on EVM: Integrated phase noise directly degrades the EVM floor, limiting the observable linearization improvement from DPD.
• Ultra-low noise mode: Some VSGs offer a low phase noise option for characterizing high-performance, low-noise amplifiers.

Onboard Field-Programmable Gate Arrays (FPGAs) and Digital Signal Processors (DSPs) that manipulate the I/Q data stream in real-time without host computer intervention.

• Digital Predistortion Application: Real-time hardware applies the inverse PA model to the stimulus signal.
• Crest Factor Reduction (CFR): On-the-fly peak limiting to generate realistic, stressed waveforms.
• Additive Impairments: Real-time injection of controlled noise, fading profiles, or clipping distortion.

The ability to generate multiple phase-coherent signals for testing beamforming and MIMO systems. This is essential for massive MIMO DPD validation.

• Phase Coherence: A deterministic, repeatable phase relationship between all RF outputs.
• Channel Synchronization: Sample-level alignment of I/Q data across multiple channels.
• Calibrated Skew: Known, correctable timing differences between channels, often specified in picoseconds.

The broadband noise power density added by the VSG's output chain. A high noise floor obscures the out-of-band emissions and spectral regrowth that DPD is designed to suppress.

• Specified in dBm/Hz: A lower number (e.g., -155 dBm/Hz) is critical for measuring deep ACLR improvements.
• Noise Compensation: Advanced VSGs use digital techniques to pre-compensate for their own analog noise contribution.
• Dynamic Range: The ratio between the maximum output power and the noise floor, defining the measurable depth of spectral regrowth.

A mathematical representation of a modulated signal using in-phase (I) and quadrature (Q) components. This representation captures both amplitude and phase information at zero carrier frequency, which is the native format generated by a vector signal generator's internal DSP before upconversion to RF.

• Eliminates the carrier frequency from calculations
• Enables direct synthesis of arbitrary modulation schemes
• The I/Q data is the direct input to the VSG's arbitrary waveform generator

The ratio of the instantaneous peak power to the average power of a transmitted signal. A critical parameter that dictates the required back-off of a power amplifier to avoid compression. Vector signal generators must produce test signals with precise, repeatable PAPR to stress-test DPD algorithms.

• High PAPR (e.g., 10-12 dB) is typical for OFDM signals
• Forces the PA to operate at low average efficiency
• VSGs generate standardized PAPR waveforms for benchmarking linearization performance

A measure of the deviation of received constellation points from their ideal locations. EVM quantifies the in-band distortion introduced by transmitter impairments, including power amplifier nonlinearity. A vector signal generator provides the pristine reference signal against which EVM is measured.

• Expressed as a percentage of the ideal symbol magnitude
• 5G NR requires EVM as low as 1.5% for 256-QAM
• The VSG's inherent EVM floor must be significantly lower than the DUT's

A signal conditioning technique applied in the digital domain to reduce the PAPR of a transmission before it reaches the power amplifier. A vector signal generator can produce both raw and CFR-processed waveforms to compare the impact on spectral regrowth and amplifier efficiency.

• Uses techniques like peak windowing or clipping with filtering
• Trade-off: lower PAPR vs. increased in-band EVM
• VSGs enable direct comparison of different CFR algorithms on a single PA

A metric quantifying the ratio of transmitted power within an assigned channel to the power leaking into an adjacent radio frequency channel. ACLR is the primary regulatory compliance metric that digital predistortion aims to improve. The VSG must generate spectrally clean stimuli to isolate the PA's contribution to ACLR.

• Measured in dBc (decibels relative to the carrier)
• 3GPP specifies ACLR limits of -45 dBc or better for base stations
• VSG phase noise directly limits measurable ACLR floor

A complex-valued mathematical sequence with constant amplitude and zero autocorrelation. Used extensively as a training and characterization signal for power amplifier modeling because its flat frequency response and constant envelope isolate nonlinear behavior from signal dynamics.

• Ideal for extracting AM/AM and AM/PM distortion curves
• Used in LTE and 5G NR as a reference signal (SRS)
• A VSG can generate custom Zadoff-Chu sequences for model extraction