Combination Analyzers: Combining Network, Spectrum, and Impedance Measurements

The primary advantage of a combination analyzer is the consolidation of the RF signal source, receiver, and processing hardware. In standalone configurations, a separate vector network analyzer and spectrum analyzer require independent local oscillators and input circuitry. Integrated platforms optimize this by sharing components like direct A/D converters and synthesized local oscillators.

For example, the Keysight 4395A covers a frequency range of 10 Hz to 500 MHz with a 1 mHz resolution, while the Keysight 4396B operates from 100 kHz to 1.8 GHz with the same 1 mHz resolution. Consolidation significantly reduces the bench footprint, as both instruments utilize identical physical dimensions of 425 × 235 × 553 mm. However, sharing RF front-end components introduces design constraints. A dedicated spectrum analyzer often features specialized preselectors and low-noise preamplifiers that are difficult to implement in a shared path without compromising the vector network analyzer's port reflection characteristics.

When operating in vector network analyzer mode, combination instruments must maintain tight amplitude and phase accuracy. The 4395A offers a dynamic magnitude accuracy of ±0.05 dB and a dynamic phase accuracy of ±0.3°. It supports measurement sweeps across various modes, including linear frequency, log frequency, list frequency, power sweep, and CW time, displaying variables such as group delay, SWR, and polar charts across two independent channels.

In spectrum analyzer mode, key metrics shift to resolution bandwidth (RBW) and noise floor characteristics. The 4396B provides an RBW range from 1 Hz to 3 MHz in a 1-3-10 sequence, with a single sideband (SSB) phase noise profile of <-105 dBc/Hz at a 10 kHz offset (for carrier frequencies under 1 GHz). When operating as a network analyzer, the 4396B achieves a transmission dynamic range greater than 120 dB when using a 10 Hz IF bandwidth. These specifications demonstrate that high-performance RF metrics are achievable in a combined chassis, though dedicated standalone spectrum analyzers may still offer wider bandwidth options or lower noise floors depending on the application.

The third capability of these combination units is impedance analysis, which is highly beneficial for component characterization. Both the 4395A and 4396B support an impedance analysis option through configuration-dependent optional test kits. This allows engineers to extract parameters without transferring the device under test (DUT) to a separate impedance analyzer or LCR meter.

To manage these complex multi-mode measurements, test automation is vital. Both platforms feature built-in IBASIC as their programming environment, allowing automated calibration and measurement routines to run directly on the instrument without an external controller. Results can be stored internally using the built-in disk drive or RAM disk, and external communication is handled via a GPIB (HP-IB) interface.