Single vs. Dual Directional Couplers for RF Measurements

The primary differentiator between single and dual directional couplers is the number of coupled auxiliary paths and their orientation. A single directional coupler, such as the Keysight 11691D, has one auxiliary coupled port. This configuration is optimized to sample power travelling in a single direction (typically the incident or forward wave). To sample the reflected wave with a single coupler, the device must be physically reversed in the transmission path.

Conversely, a dual directional coupler contains two independent auxiliary arms. Devices like the Keysight 11692D and Keysight 778D isolate and sample both the forward and reverse power waves simultaneously. This eliminates the need to reconnect or reverse the hardware, enabling real-time reflection and transmission measurements.

Directivity is a measure of a coupler's ability to differentiate between forward and reflected waves. Mathematically, it is the ratio of the power at a coupled port when the signal flows in the desired direction to the power when the signal flows in the opposite direction. High directivity is necessary for accurate reflection measurements because any leakage of the high-power forward signal into the reverse coupled port obscures the weaker reflected signal.

For example, the Keysight 11691D and 11692D provide a minimum directivity of 30 dB in the 2 to 8 GHz frequency range. This high performance allows precise separation of waves during broadband reflectometry. At lower frequencies, the Keysight 778D provides up to 36 dB minimum directivity (0.1 to 1.0 GHz), which is essential for preserving the accuracy of return loss and SWR measurements over a wide dynamic range.

When using dual-directional couplers for swept-frequency measurements, tracking between the two auxiliary arms is a key parameter. In the Keysight 778D, the auxiliary outputs typically track within 0.7 dB. This close matching ensures that forward reference power and reflected power are sampled with minimal relative amplitude error across the entire 100 MHz to 2 GHz span.

For high-frequency microwave setups operating up to 18 GHz, SWR mismatches at the ports can degrade measurement integrity. The Keysight 11691D and 11692D address this with maximum primary line SWR limits of 1.3 (2 to 12.4 GHz) and 1.4 (12.4 to 18 GHz). Low SWR minimizes internal reflections, ensuring that the sampled signals accurately represent the main transmission line environment.