This video provides a comprehensive tutorial on characterizing a Mini-Circuits ZFDC-20-5 plus directional coupler using a Siglent SV4401A Vector Network Analyzer (VNA). The process begins with an explanation of a directional coupler's key parameters: mainline loss, coupling ratio, directivity, and return loss. The presenter then demonstrates the full VNA workflow, starting with setting the 100 kHz to 2 GHz frequency range and performing a two-port SOLT (Short, Open, Load, Through) calibration. A key detail is the inclusion of BNC-to-SMA adapters within the calibration plane to ensure measurement accuracy at the device's ports. Each of the four main parameters is then measured sequentially, with the results compared against the manufacturer's datasheet. The video highlights a practical method for measuring directivity by saving the forward coupling measurement as a reference trace and comparing it to the live reverse coupling measurement. A limitation in the VNA's software is noted, as it requires manual calculation of the difference between the two traces. The final results show the coupler performs closely to its specifications.
Characterizing a Directional Coupler with a VNA
0:00:02 Introduction: The video's goal is to fully characterize the parameters of a Mini-Circuits directional coupler (model ZFDC-20-5 plus) using a Siglent SV4401A VNA.
0:00:50 Key Parameters Explained: The four primary characteristics of a directional coupler are defined:
Mainline Loss: Signal attenuation between the input and output ports.
Coupling Ratio: The amount of signal sampled at the "coupled" port (nominally 20 dB for this device).
Directivity: The ratio of forward-coupled power to unwanted reverse-coupled power, a measure of the coupler's directionality.
Return Loss (SWR): How well each port is matched to the system impedance.
0:03:53 VNA Setup: The VNA's frequency range is set from a start of 100 kHz to a stop of 2 GHz to match the coupler's specifications.
0:04:19 Calibration Strategy: To measure the BNC-equipped device accurately, adapters are used. The calibration is performed at the end of these adapters, effectively removing their influence from the final measurement.
0:05:17 Full Two-Port Calibration: A complete SOLT (Short, Open, Load, Through) calibration is performed for ports 1 and 2, and the resulting calibration data is saved to the VNA.
0:07:10 Measuring Return Loss (SWR): Before testing the device, the quality of the 50-ohm terminations is verified. The VNA is then used to measure the return loss of the coupler's input and output ports, which are found to be within the datasheet's typical specification of 1.2:1 SWR.
0:11:36 Measuring Mainline Loss: With the coupled port terminated, the transmission (S21) from the input to the output is measured. The loss is approximately 1.3 dB at 2 GHz, which is better than the typical 1.5 dB listed in the datasheet.
0:13:25 Measuring Coupling Ratio: The transmission from the input port to the coupled port is measured. The result is very close to the nominal 20 dB specification, measuring between 18.9 dB and 19.4 dB across the frequency range.
0:14:18 Measuring Directivity: To determine directivity, the forward coupling measurement is saved as a static reference trace. The coupler is then physically reversed to measure the reverse coupling. Directivity is the difference between these two measurements.
0:15:35 VNA Software Limitation: A limitation is identified where the VNA software does not have a built-in function to calculate the mathematical difference (delta) between a live trace and a reference trace, requiring the user to do it manually.
0:17:05 Directivity Results: By manually comparing marker values on the forward and reverse coupling traces, the directivity is calculated. The results align well with the datasheet, measuring around 29.8 dB at 200 MHz and 9 dB at 2 GHz.