Analog vs. Vector Signal Generators
Filed under Generators
Selecting the appropriate signal source requires understanding the fundamental differences between analog and vector signal generators. Analog signal generators produce continuous wave (CW) tones and support traditional modulation schemes such as amplitude (AM), frequency (FM), phase (PM), and pulse modulation. Vector signal generators (VSGs) introduce complex I/Q (in-phase and quadrature) modulation, allowing them to generate digitally modulated signals essential for modern wireless standards. This guide compares these two architectures, highlighting their distinct capabilities, performance parameters, and typical engineering applications.
- Analog Modulation Types
- CW, AM, FM, Phase (ØM), Pulse
- Vector Modulation Types
- ASK, FSK, MSK, PSK, QAM, Custom I/Q
- Analog RF/Microwave Models
- Keysight N5181B, Keysight E8257D
- Vector RF/Microwave Models
- Keysight N5182B, Keysight E8267D
Architectural Differences and Modulation
The primary difference between analog and vector generators lies in how they modulate the RF carrier. Analog signal generators, such as the Keysight N5181B and the microwave-class Keysight E8257D, alter a single parameter of the RF carrier—either its amplitude, frequency, or phase—using analog control signals. These systems are highly optimized for producing pure continuous wave signals and basic pulse shapes.
Conversely, vector signal generators like the Keysight N5182B and the Keysight E8267D utilize an integrated I/Q modulator. By splitting the RF carrier into two components shifted 90 degrees apart (In-phase and Quadrature), the generator can simultaneously vary amplitude and phase. This complex modulation is required to transmit digital formats like Quadrature Amplitude Modulation (QAM), Phase Shift Keying (PSK), and Frequency Shift Keying (FSK).
Baseband Generation and Technical Metrics
Vector instruments feature internal baseband generators to process complex digital data. For instance, the Keysight E8267D incorporates an internal baseband generator with a 100 MSa/s sample rate and 14-bit vertical resolution. This digital processing requires strict control over hardware impairments to prevent signal degradation. Key parameters for vector generators include I/Q gain balance (less than 0.2 dB on the E8267D) and quadrature error (less than 0.2 degrees).
Analog generators do not require complex baseband processors, focusing instead on spectral purity, frequency range, and power output. The Keysight E8257D analog PSG can reach operational frequencies up to 70 GHz and output levels exceeding +30 dBm when configured with Option 521, a power level typically more difficult to achieve in wideband vector architectures.
Choosing the Right Instrument for Your Application
Analog signal generators are the standard choice for applications requiring high spectral purity, low phase noise, or high output power without complex modulation. Typical use cases include local oscillator (LO) substitution, receiver sensitivity testing, radar testing, and system calibration. For example, the Keysight N5181B provides a phase noise of -146 dBc/Hz at 1 GHz with a 20 kHz offset, making it highly effective for checking receiver selectivity.
Vector signal generators are mandatory when testing components, transmitters, or receivers designed for digital communication standards. Designers of cellular, Wi-Fi, and military communication hardware rely on instruments like the Keysight N5182B to simulate real-world multipath, interference, and complex digital modulation formats.
Example instruments
Frequently asked questions
- Can a vector signal generator perform standard analog modulation?
- Yes. Most vector signal generators, including the Keysight E8267D and Keysight N5182B, are designed with backward compatibility and versatile internal modulation engines that allow them to perform standard AM, FM, Phase, and Pulse modulation alongside complex I/Q modulation.
- Which architecture is better for low phase noise applications?
- High-end analog signal generators are traditionally optimized for the lowest possible phase noise and the highest signal purity, making them the preferred choice for LO substitution. However, high-performance vector generators can achieve comparable phase noise performance, such as the Keysight E8267D matching the Keysight E8257D at -115 dBc/Hz typical phase noise at a 10 kHz offset on a 10 GHz carrier.