S21 Two-Port Shunt and Two-Port Series Methods
This is the second of two articles devoted to the topic of inductor impedance evaluation from the S parameter measurements using a network analyzer. The previous article [1] described the impedance measurements and calculations from the S11 parameters using the one-port shunt, two-port shunt, and two-port series methods. This article is devoted to the impedance measurements and calculations from the S21 parameters using the two-port shunt and two‑port series methods.
The overall conclusion of the previous article was that the inductor impedance evaluation from the S11 parameter measurements is not accurate. This article concludes that the two-port series method is the most accurate method for the inductor impedance evaluation from S21 parameters when using a network analyzer.
Two-Port Shunt Method
The two-port shunt configuration is shown in Figure 1.
For this configuration, the inductor’s impedance in terms of the S21 parameter was derived in [2] as
(1)
Two-Port Series Method
The two-port series configuration is shown in Figure 2.
For this configuration, the inductor’s impedance in terms of the S21 parameter was derived in [3] as
(2)
Impedance Measurement Setup and Results
The impedance measurement setup and the PCB boards are shown in Figure 3. The boards were populated with Murata RF inductors, LQG18HH47NJ00, LQC18HH15J00, LQG18HH27J00, of the values 47 nH, 150 nH, and 270 nH, respectively.
Figure 4 shows the impedance curves for a 47 nH inductor using a two-port shunt and two-port series methods. The shunt measurements were taken at 50 dB and self-resonant frequencies. The series measurements were taken at 60 dB and self‑resonant frequencies.
Figure 5 shows the inductor impedance curve obtained from the Murata Design Support Software “SimSurfing” [4].
The two-port shunt, two-port series measurements, and the Murata results are shown in Table 1.
| L = 47 nH | Two-port shunt | Murata |
| 1st 50 dB frequency | 257.44 MHz | 823 MHz |
| Resonant frequency | 279.49 MHz | 1.591 GHz |
| 2nd 50 dB frequency | 309.91 MHz | 2.985 GHz |
| L = 47 nH | Two-port series | Murata |
| 1st 60 dB frequency | 1.196 GHz | 1.29 GHz |
| Resonant frequency | 1.531 GHz | 1.591 GHz |
| 2nd 50 dB frequency | 2.087 GHz | 1.962 GHz |
Table 1: Impedances at 50 dB, 60 dB, and self-resonant frequencies (S21 methods)
It is apparent that the two-port series measurements are significantly closer to the Murata results than the two-port shunt measurements.
Figure 6 shows the impedance curves for a 150 nH inductor using a two-port shunt and two-port series methods. The shunt measurements were taken at 50 dB and self-resonant frequencies. The series measurements were taken at 60 dB and self-resonant frequencies.
Figure 7 shows the inductor impedance curve obtained from the Murata Design Support Software “SimSurfing.”
The two-port shunt, two-port series measurements, and the Murata results are shown in Table 2.
| L = 150 nH | Two-port shunt | Murata |
| 1st 50 dB frequency | 126.41 MHz | 320 MHz |
| Resonant frequency | 156.94 MHz | 810 MHz |
| 2nd 50 dB frequency | 194.88 MHz | 2.03 GHz |
| L = 150 nH | Two-port series | Murata |
| 1st 60 dB frequency | 557.78 MHz | 601 MHz |
| Resonant frequency | 825.01 MHz | 810 MHz |
| 2nd 50 dB frequency | 1.148 GHz | 1.29 GHz |
Table 2: Impedances at 50 dB, 60 dB, and self-resonant frequencies (S21 methods)
Again, the two-port series measurements at 50 dB and self-resonant frequencies are significantly closer to the Murata results than the two-port shunt measurements.
Figure 8 shows the impedance curves for a 270 nH inductor using a two-port shunt and two-port series methods. The shunt measurements were taken at 50 dB and self-resonant frequencies. The series measurements were taken at 60 dB and self-resonant frequencies.
Figure 9 shows the inductor impedance curve obtained from the Murata Design Support Software “SimSurfing.”
The two-port shunt, two-port series measurements, and the Murata results are shown in Table 3.
| L = 270 nH | Two-port shunt | Murata |
| 1st 50 dB frequency | 86.31 MHz | 184 MHz |
| Resonant frequency | 116.36 MHz | 638 MHz |
| 2nd 50 dB frequency | 156.97 MHz | 1.992 GHz |
| L = 270 nH | Two-port series | Murata |
| 1st 60 dB frequency | 361.67MHz | 395 MHz |
| Resonant frequency | 605.54 MHz | 638 MHz |
| 2nd 50 dB frequency | 933.99 MHz | 1.03 GHz |
Table 3: Impedances at 50 dB, 60 dB, and self-resonant frequencies (S21 methods)
Once again, the two-port series measurements at 50 dB and self-resonant frequencies are significantly closer to the Murata results than the two-port shunt measurements.
The overall conclusion is that the two-port series method is the most accurate method of the inductor’s impedance evaluation from the S21 parameter measurements.
References
- Bogdan Adamczyk, Patrick Cribbins, and Khalil Chame, “Inductor Impedance Evaluation from S Parameter Measurements – Part 1: S11 One-Port Shunt, Two-Port Shunt, and Two‑Port Series Methods,” In Compliance Magazine, April 2025.
- Bogdan Adamczyk, Patrick Cribbins, and Khalil Chame, “Capacitor Impedance Evaluation from S Parameter Measurements – Part 1: S11 One-Port Shunt, Two-Port Shunt, and Two-Port Series Methods,” In Compliance Magazine, February 2025.
- Bogdan Adamczyk, Patrick Cribbins, and Khalil Chame, “Capacitor Impedance Evaluation from S Parameter Measurements – Part 2: S21 Two‑Port Shunt and Two-Port Series Methods,” In Compliance Magazine, March 2025.
- Murata Design Support Software, “SimSurfing.”
