This video demonstrates the characterization of a Mini-Circuits ZFDC-20-5+ directional coupler using a Cisco joint SV 4401A vector network analyzer (VNA) across a frequency range of 100 kHz to 2 GHz. The process involves setting up the VNA, performing a comprehensive calibration with appropriate adapters, and then conducting various measurements. Key parameters evaluated include the mainline loss, coupling ratio, directivity, and return loss for each port. The presenter details the methodology for measuring each parameter, compares the results against the manufacturer's datasheet specifications, and highlights the VNA's capabilities and limitations, particularly the manual calculation required for directivity.
Characterizing a Directional Coupler with a VNA
0:00 Introduction to Device and Goals: The video demonstrates using a Cisco joint SV 4401A Vector Network Analyzer (VNA) to characterize a Mini-Circuits ZFDC-20-5+ directional coupler, covering setup, calibration, and various measurements.
0:27 Understanding Directional Coupler Parameters: A directional coupler is a three-port RF device that samples signals in one direction. Key parameters to measure are: mainline loss (signal loss in the main path), coupling ratio (signal coupled to the coupled port), directivity (ratio of forward to reverse coupled response), and return loss (reflection loss of each port).
0:27 Datasheet Overview: The coupler operates from 100 kHz to 2 GHz, with a nominal coupling ratio of 19.5 dB (±0.5 dB), mainline loss varying by frequency, and typical directivity ranging from 30 dB (low frequency) to 22 dB (upper frequency).
3:52 VNA Frequency Setup: The VNA's frequency range is configured from 100 kHz to 2 GHz to match the coupler's specifications.
4:19 Calibration Setup with Adapters: To ensure accurate measurements, the calibration is performed as close as possible to the device under test (DUT) ports using N-to-BNC and BNC-to-SMA adapters, which are included in the calibration.
5:17 VNA Calibration Process: The VNA undergoes a full calibration, including open, short, and load for Port 1, followed by a "through" calibration connecting Port 1 to Port 2. The calibration data is then saved.
7:09 Return Loss (S11) Measurement: The return loss and SWR for the coupler's input and output ports are measured. The input return loss (S11) is found to be better than 23 dB, with SWR below 1.1, matching or exceeding specifications. The output port's return loss is also measured.
11:35 Mainline Loss (S21) Measurement: The mainline loss (transmission coefficient S21) is measured. Results show approximately 1.3 dB of loss at 2 GHz and 0.39 dB at low frequencies, closely aligning with the datasheet's typical values of 1.5 dB and 0.3-0.4 dB, respectively.
13:04 Coupling Ratio Measurement: The forward coupling ratio is measured, showing values around 19.4 dB at low frequencies and 18.9 dB at 2 GHz, confirming the 20 dB nominal coupling spec (19.5 dB ±0.5 dB).
14:18 Directivity Measurement & VNA Limitations: To measure directivity, the forward coupled response trace is saved, and then the reverse coupled response is measured. The VNA requires manual calculation of the difference between these two traces, as it lacks a direct trace or marker delta function.
17:05 Directivity Results: Manual calculation yields directivity values around 29.8 dB at 200 MHz, 22.7 dB at 1 GHz, and 9 dB at 2 GHz. These are reasonably close to the datasheet's typical values, considering the use of multiple adapters.
18:22 Conclusion: The video successfully demonstrates how to set up and use the VNA for comprehensive characterization of RF components like directional couplers.