Can a signal analyzer be used as a standard spectrum analyzer?

Yes. Modern signal analyzers can operate in a standard swept-tuned mode to cover broad spans of frequency, offering identical functionality to a traditional spectrum analyzer alongside their digital vector analysis capabilities.

Why does phase noise matter when comparing these instruments?

Phase noise defines the short-term stability of the analyzer's internal local oscillator. A low phase noise, such as -114 dBc/Hz at 10 kHz offset on the Keysight N9020A, prevents the analyzer's own noise from masking weak signals that are close in frequency to a larger carrier signal.

Spectrum Analyzer vs Signal Analyzer

Filed under Analyzers Signal

Choosing between a spectrum analyzer and a signal analyzer depends on whether your application requires basic amplitude-versus-frequency measurements or complex vector signal analysis. Traditionally, a spectrum analyzer is a swept-tuned instrument optimized for measuring power across a broad frequency span. In contrast, a modern signal analyzer incorporates a digital intermediate frequency (IF) architecture, enabling digitized phase and amplitude capture over a set analysis bandwidth. This guide explains these architectural differences, details when a swept spectrum analyzer remains sufficient, and outlines the primary specifications—such as analysis bandwidth, phase noise, and displayed average noise level (DANL)—that dictate instrument selection.

Traditional Swept Frequency Range: 9 kHz to 26.5 GHz (Keysight E4407B)
High-Performance Phase Noise: -129 dBc/Hz at 10 kHz offset (Keysight N9030A)
Standard Analysis Bandwidth: 10 MHz (Keysight N9000A, N9010A, N9030A)
Low-End Frequency Limit: Down to 3 Hz (Keysight N9030A)

Architectural Differences: Swept-Tuned vs. Vector Analysis

Traditional RF spectrum analyzers, such as the legacy Keysight 8591E and Keysight E4407B, operate on a swept-tuned heterodyne principle. A voltage-controlled local oscillator sweeps across a selected span, downconverting incoming RF signals to a fixed intermediate frequency (IF) where the power is detected. This method is highly effective for steady-state or repeating signals over wide frequency spans, but it cannot capture phase information or capture fast, transient signals that occur outside the instantaneous sweep position.

Modern signal analyzers, including the Keysight X-Series family comprising the Keysight N9000A, Keysight N9010A, Keysight N9020A, and Keysight N9030A, implement a digital IF architecture. Instead of analog detection after downconversion, the analog-to-digital converter (ADC) digitizes the entire IF bandwidth. This allows the instrument to extract both magnitude and phase data simultaneously, converting the signal into in-phase and quadrature (I/Q) components for comprehensive vector signal analysis.

Key Capabilities of Signal Analyzers

By digitizing the signal at the IF stage, signal analyzers offer several capabilities beyond traditional swept-tuned analysis:

Vector Signal Analysis and Demodulation: Because they capture phase data, signal analyzers can demodulate complex digital communications signals (such as QAM, QPSK, or custom wireless formats) to measure modulation quality metrics like Error Vector Magnitude (EVM).
Analysis Bandwidth: This represents the maximum instantaneous bandwidth over which the instrument can digitize signals. The standard analysis bandwidth for the Keysight N9000A and Keysight N9030A is 10 MHz. However, this can be expanded depending on the hardware, with the Keysight N9000A supporting up to 25 MHz (via Option B25) and the Keysight N9020A supporting analysis bandwidths up to 160 MHz.
Real-Time Spectrum Analysis: With digital processing, signal analyzers can execute continuous Fast Fourier Transforms (FFTs) fast enough to capture intermittent, transient signals without missing data.

When is a Swept Spectrum Analyzer Sufficient?

Despite the advanced features of modern signal analyzers, traditional swept spectrum analyzers remain highly effective for general-purpose RF testing. If your applications do not require phase information, digital demodulation, or real-time transient capture, a swept analyzer is fully capable.

Typical applications that are well-served by a standard swept-tuned architecture include spurious emission searches, harmonic distortion measurements, scalar network analysis, and basic electromagnetic interference (EMI) pre-compliance testing. Instruments like the Keysight 8591E (covering up to 1.8 GHz) or the Keysight E4407B (covering up to 26.5 GHz) provide robust, reliable basic frequency and power profiling without the overhead of modern digital signal processors.

Key Specifications to Consider

When choosing between these instruments, several key specifications must be evaluated to ensure compatibility with your device under test:

Frequency Range: Match the instrument limits to your target signals and their harmonics. For low-frequency work, the Keysight N9030A extends down to 3 Hz, while for high-frequency applications it supports up to 50 GHz. The Keysight N9010A covers up to 3.6 GHz.
Displayed Average Noise Level (DANL): This defines the instrument's noise floor and sensitivity. A lower DANL, such as the Keysight N9010A's -146 dBm, is critical for resolving low-level signals.
Phase Noise: Crucial for close-in signal resolution and local oscillator characterization. The Keysight E4407B offers a phase noise of <= -90 dBc/Hz at a 10 kHz offset (1 GHz carrier), while the higher-performance Keysight N9030A achieves -129 dBc/Hz at the same offset.
Analysis Bandwidth: Ensure the instantaneous digitizing bandwidth accommodates the full modulation bandwidth of your signal under test.