Scalar vs. Vector Network Analyzer: Key Differences

This article compares Scalar Network Analyzers (SNA) and Vector Network Analyzers (VNA), highlighting their key differences. Traditional microwave measurements using slotted lines, while capable of measuring both amplitude and phase, are limited to single frequencies. Broadband frequency measurements over a wide range become time-consuming. Network analyzers address this by measuring both amplitude and phase across a broad frequency range in a reasonable timeframe. This article outlines the differences between SNA and VNA.

In essence, accurate reference signals are generated. Using this as a standard, the amplitude and phase of the signal emitted from the Device Under Test (DUT) are measured.

What is a Network Analyzer?

Figure: Components inside a Network Analyzer

As shown in the figure, a typical network analyzer comprises four main components:

• Signal Source: Provides the incident signal.
• Signal Separation Device: Separates incident, reflected, and transmitted signals.
• Receiver/Detector: Converts the microwave frequency to a lower Intermediate Frequency (IF) for easier processing.
• Signal Processor/Display: Processes the IF signal and displays the information.

Let’s delve into the network analyzer basics:

• Signal Source: It generates the incident signal that stimulates the DUT. The DUT responds by reflecting a portion of the signal and transmitting the rest. By sweeping the source frequency, we can determine the DUT’s frequency response. Two main types of sources are used: sweep oscillators and synthesized signal generators.

• Signal Separation: This module separates the incident, reflected, and transmitted signals. Once separated, their amplitude and phase can be measured to determine their differences. This is achieved using directional couplers, power splitters, bridges, or high-impedance probes.

• Receiver/Detector: This converts the Radio Frequency (RF) voltage to a lower IF or Direct Current (DC) signal for more accurate measurement. Three primary receiver techniques are employed:

  • Diode: A broadband detector that converts the RF signal to a proportional DC voltage. For amplitude-modulated signals, it strips the modulation. This is commonly used in scalar network analyzers.
  • Fundamental Mixing: A broadband tuned receiver technique that converts the RF signal to a low-frequency IF signal.
  • Harmonic Mixing: Similar to Fundamental Mixing. Both mixing techniques use Band-Pass Filters (BPF) at IF frequencies to reject spurious frequencies and extend the noise floor.

• Display: The final component displays the results as desired. Cartesian formats display magnitude, phase, or group delay as a function of frequency. Polar formats and impedance formats (e.g., Smith charts) are also common. Modern network analyzers can also display data in a tabular format.

Network Analyzer Types: Scalar and Vector Network Analyzers

The two main types of network analyzers are:

• Scalar Network Analyzer (SNA): Measures magnitude-related parameters.
• Vector Network Analyzer (VNA): Measures both phase and magnitude.

The following table summarizes the differences:

Network Analyzer Measurements

Network analyzers are used to perform various measurements, including:

• Transmission Measurements: Transmission coefficient, insertion loss, and gain.
• Reflection Measurements: Reflection coefficient, Voltage Standing Wave Ratio (VSWR), return loss, and impedance.
• Scattering Parameter (S-parameter) Measurements: S11, S12, S21, S22.

Refer to VNA tutorial for more information.

Manufacturers of Scalar and Vector Network Analyzers

• Scalar Network Analyzer 8757D from Agilent Technologies.
• E5072A vector network analyzer from Agilent Technologies.
• ANRITSU 37369C: Vector Network Analyzer, supports 40MHz to 40GHz.
• ANRITSU MS2026A: Handheld Vector Network Analyzer, supports up to 6GHz.