Noise Figure vs. Phase Noise Test Sets
Filed under Test Sets
This guide examines the distinct roles of noise figure test sets and phase noise test sets in RF and microwave engineering. While both classes of instrumentation analyze noise in high-frequency systems, they serve fundamentally different purposes and evaluate different performance metrics. Noise figure test sets assess how much an active component, such as an amplifier or mixer, degrades the signal-to-noise ratio of a signal passing through it. In contrast, phase noise test sets measure the spectral purity, short-term stability, and phase or amplitude fluctuations of RF signal sources like synthesizers and oscillators. This comparison is intended for test engineers and technical buyers who need to determine the correct test configuration for evaluating active components versus characterizing signal generation sources. Understanding these differences ensures appropriate hardware selection for specific RF metrology requirements.
- 8971C RF Input Range
- 10 MHz to 26.5 GHz
- N5500A Carrier Range
- 50 kHz to 26.5 GHz (with microwave options)
- 11729C Carrier Range
- 10 MHz to 18 GHz
- N5500A Offset Frequency Range
- 0.01 Hz to 100 MHz
Active Component Characterization (Noise Figure)
Noise figure test sets analyze the signal-to-noise ratio (SNR) degradation introduced by a device under test (DUT) as a signal passes through it. The noise figure is a measure of the noise power added by an active component compared to the thermal noise at the input. To perform these measurements over broad frequency bands, auxiliary test sets are often paired with a noise figure meter to extend frequency coverage and filter unwanted signals.
For example, the Keysight Technologies (Agilent HP) 8971C extends the frequency range of the 8970B noise figure meter, enabling single-sideband measurements from 10 MHz to 26.5 GHz. It utilizes a stable YIG filter and an integrated low-noise preamplifier to reject image and odd harmonics by a typical value of 20 dB, while presenting a low single-sideband noise figure (<=18 dB from 10 MHz to 18 GHz, or <=11.5 dB from 18 GHz to 26.5 GHz when using Option 002).
Signal Source Evaluation (Phase Noise)
Phase noise test sets evaluate the short-term frequency stability and spectral purity of signal sources rather than active components. Instead of measuring a ratio of added noise through a device, these instruments analyze the phase and amplitude fluctuations of a carrier signal at specific offset frequencies from the carrier.
The Keysight Technologies (Agilent HP) N5500A serves as the core hardware for phase noise measurement systems, supporting carrier frequencies from 50 kHz to 1.6 GHz on the base model, and up to 26.5 GHz with microwave options. It measures absolute phase noise, residual phase noise, and AM noise across an offset frequency range of 0.01 Hz to 100 MHz, achieving a system phase noise floor of -170 dBc/Hz at offsets greater than 10 kHz.
Similarly, the Keysight Technologies (Agilent HP) 11729C down-converts microwave signals in the 10 MHz to 18 GHz carrier frequency range using internal microwave reference frequencies from 1.28 GHz to 17.92 GHz in 640 MHz steps. This allows the system to perform sensitive phase noise measurements using delay line/mixer or phase detector methods.
System Architectures and Interfaces
The physical and operational architectures of these test sets reflect their distinct application areas. Noise figure test sets like the Keysight Technologies (Agilent HP) 8971C often feature front-panel LED status indicators and rely on a host noise figure meter for display and processing via interfaces such as HP-IB (GPIB).
Phase noise test sets, such as the Keysight Technologies (Agilent HP) N5500A, also typically lack built-in displays, relying on status LEDs and requiring external PC control software, like the Keysight N5505A Phase Noise Measurement Utility. They also offer analog tuning outputs, such as a ±10 V internal tuning voltage output on a BNC female port, to establish phase lock with reference sources.
Example instruments
Frequently asked questions
- Can a phase noise test set measure AM noise?
- Yes. Certain phase noise test sets, such as the Keysight Technologies (Agilent HP) N5500A, support AM noise measurements in addition to absolute and residual phase noise.
- What is the purpose of the YIG filter in a noise figure test set?
- In instruments like the Keysight Technologies (Agilent HP) 8971C, a stable YIG filter is utilized to select the desired measurement frequency and provide image and odd harmonic rejection (typically 20 dB SSB rejection) to ensure accurate single-sideband noise measurements.