S-Parameters Tutorial – Part II: EMC Measurements and Testing

This part II of the s-parameter tutorial presents several examples of the s-parameter use in EMC measurements and testing (see [1] for the s-parameter theory). Since the vast majority of the s-parameter measurements in EMC utilize only s₁₁ and s₂₁ parameters we will discuss only these two measurements.

S₁₁ examples presented in this article include:

1. LISN impedance measurement,
2. capacitor impedance measurement,
3. log-periodic antenna VSWR measurement.

S₂₁ examples include:

1. preamp gain measurement,
2. attenuator loss measurement,
3. cable loss measurement.

Before the s parameter measurements are taken, the calibration process needs to take place to characterize the cables that connect the device under test (DUT) to the network analyzer.

Calibration Procedure

The calibration procedure utilizes a calibration kit, like the ones shown in Figure 1, which consists of a short, open, 50 Ω load attachment, and often a thru connector.

Figure 1: a) N-type calibration kit, b) SMA-type calibration kit

A few different types of calibrations can be performed, depending on the parameter of interest. If only the s₁₁ measurements are required then the calibration is performed at port 1 with a short, open and 50 Ω (load) terminations as shown in Figure 2.

Figure 2: Calibration for s₁₁ measurements

Figure 3 shows the network analyzer expected calibration plots for these three cases.

Figure 3: Calibration results a) short, b) open, c) 50 Ω load

For s₂₁ measurements of a preamp gain or an attenuator loss, the calibration procedure consists of two steps. In step 1, both connecting cables are first calibrated using the process described earlier for s₁₁ and s₂₂ calibration. Step 2 of the calibration involves the setup shown in Figure 4a; Figure 4b shows the expected calibration curve.

Figure 4: Step 2 of the s₂₁ calibration a) physical setup b) calibration plot

If the cable loss measurements are required then there are two options for the calibration procedure, depending on the measurement set up, as shown in Figure 5.

Figure 5: Cable loss measurement options

The calibration setup for both measurement options is shown in Figure 6.

Figure 6: Thru calibration options for cable loss measurements

The s₂₁ calibration associated with the option A is the same as the calibration described for the attenuator loss or pre-amp gain.

The s₂₁ calibration associated with the option B is different; it involves only one cable connected between port 1 and port 2 and does not require the prior s₁₁ or s₂₂ calibration. The expected calibration plot for option B is the same as the one shown in Figure 4b.

S₁₁ Measurements

S₁₁ measurement examples presented here include LISN impedance measurement, capacitor impedance measurement, and log-periodic antenna VSWR (reflection coefficient) measurement.

LISN Impedance Measurement

Figure 7 shows the measurement setup and the measurement result for determining the LISN impedance from the s₁₁ measurement, (see [2] for more details).

Figure 7: LISN impedance measurement

Capacitor Impedance Measurement

The experimental setup for capacitor impedance measurement and the measurement result are shown in Figure 8 (see [3] for more details).

Figure 8: Capacitor impedance measurement

Antenna VSWR Measurement

The experimental setup for a log-periodic antenna VSWR measurement and the measurement result are shown in Figure 9 (see [4] for more details).

Figure 9: Antenna s₁₁ VSWR measurement

S₂₁ Measurements

S₂₁ measurement examples presented here include preamp gain measurement, attenuator loss measurement, and cable loss measurement.

Preamp Gain Measurement

Figure 10 shows the measurement setup and the measurement result for measuring the 32 dB preamp gain (see [4] for more details).

Figure 10: Preamp gain measurement

Attenuator Loss Measurement

Figure 11 shows the measurement setup and result for measuring the loss of a 10 dB attenuator.

Figure 11: Attenuator loss measurement

Cable Loss Measurement

Figure 12 shows the measurement setup and result for measuring the cable loss.

Figure 12: Cable loss measurement

References

1. Adamczyk, B., S-Parameter Tutorial – Part I – Fundamental Background, In Compliance Magazine, August 2018.
2. Adamczyk, B., Teune J., Topology and Characterization of a DC Line Impedance Stabilization Network, In Compliance Magazine, July 2017.
3. Adamczyk, B., Teune J., Impact of a Trace Length on Capacitor Frequency Response, In Compliance Magazine, March 2018.
4. Bogdan Adamczyk, Foundations of Electromagnetic Compatibility with Practical Applications, Wiley, 2017.