Global
SDD = Differential-to-Differential S-parameter
SCC = Common-to-Common S-parameter
S21 = Input from Port 1,measure at Port 2，commonly used for transmission /insertion loss
S11 = Input from Port 1, reflect back at Port 1, commonly used for observe reflection/input return loss
1. YINT Summary mnemonic: S21 measures how much sighnal transmits through;S11 measures how much sighal reflects back.
| Written | Input port | Measurement Port | Enginering Defination |
| S21 | Port 1 | Port2 | Transmission Port 1 to Port 2 ;commonly used for insertion loss |
| S11 | Port 1 | Port 1 | Self-reflection at Port 1 |
| S12 | Port 1 | Port 1 | Reverse transmission form Port 2 to Port 1 |
| S22 | Port 2 | Port 2 |
Self-reflection, commonly used to evaluate output return loss |
2. Why does S21 usually represent transmission and insertion loss?
Take a cable, connector or PCB trace as the DUT. The VNA injects signals from Port 1 and measures the output at Port 2, and this power ratio is S21.
| S11 value | Intuitive meaning | Engineer's enterpretation |
| 0dB | Nearly no attenuation | Ideal condition or negligible loss;it is diffjicult for practical passive channel s to maintain 0 dB across the full frequency band. |
| -3dB | Approximately half of the power aremiane | Obvious attenuation; referring to insertio loss, it is usually written as IL=3dB |
| -10dB | Severe signal attenuation | Re-evaluation of the link budget may be required for high frequency channels |
| -30dB | Hardly any signal passes through | Signal transmission is extremely weak at this frequency point |
3.Why does S11 usually represent reflection/return loss.
After injecting a signal into Port 1, if the input impedance of the DUT does not match the system impedance, part of the energy will be reflected back from Port 1, and this reflection ratio is S11.
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Note: Pay attention to the positive and negtive signs of dB. S11 and S21 displayed on VNA are usually negative values, such as S21 =-3dB. S11=-20dB. However when speaking engineering, we use positive values, e.g.insertion loss 3dB.Therefore, for positive channels: Insertion Loss S-21(dB),return Loss≈-11(dB).
4.What's the meaning of SDD and SCC?
In high-speed differential systems, a pair of physical conductors can be decomposed into two modes: differential mode and common mode. Mixed-mode S-parameters describe channel characteristics by using these mode ports.
| Abbreviation | The full English name | What to Inspect |
| SDD | Differential-to-Differential S-parameter | Transmission and reflection behavirs of differential-mode signals |
| SCC | Common-to-Common S-parameter | Transmission and reflection characteristics of common-mode signals |
| SDC/SCD | Differential/Common mode-conversion S-parameter | Evaluate whether the differential signal has been converted into common mode,or whether the common mofr has been converted into differetial mode. |
The intuitive difference between differential mode and common mode.
Differential mode: The signal on the two lines are equal in amplitude and opposite in phase. For example, P=+V,N=-V.
Common mode: The signal on the two lines are equal in same direction. For example, P and Njitter upward at the same time.
High speed differential links mainly transmit differential mode signals. Excessive common mode energy is often related to EMI and raditation issures.
5.SDD21/SDD11/SCC21 specific explanation
| Parameter | English explanation | detailed explanation |
| Sdd21 | Differential-to-Differential Forward Transmission | Input differential-mode signal at Port 1, detect it at differential Port 2; It indicates how much differential signal is transmitted through the link. |
| Sdd11 | Differential-to-Differential Input Reflection | Input differential-mode signal at Port 1,reflected back to differential Port1;It reflects whether differential impedance matching is good. |
| Scc21 | Common-to-Common Forward Transmission | Input common-mode signal at Port 1 measured at common-mode port 2; It shows how much common-mode signal is transmitted. |
Sdd21: differential Pair 1 input→DUT→differential Pair 2 input
Sdd11: differential Pair1 input→DUT←differential Pair 1 reflection
Scc21:common mode Pair 1 input→DUT→common mode Pair 2 input
| Read the standard sentence structure of Sdd21: Sdd21 =output differential wave at mixed-mode port 2/input differential wave at mixed-mode port 1, 翻成工程语言就是：差分信号从输入端到输出端的传输能力，也就是差模插入损耗 |
6.Reading example: How to describe when you see the curve?
| test result | Recommendation statement | Implication |
| Sdd21 = -6 dB @ 10 GHz | The differential-mode insertion loss at 10GHz is approixmately 6dB | After being transmitted to the output end, the differential signal experiences significant attenuation. It is necessary to make a jugement based on the system budget to determine whether it is accept |
| Sdd11 = -15 dB @ 5 GHz | The differential mode return loss at 5 GHz is approximately 15dB. | The differential impedance matching at the input end is acceptable; the reflected power is approixmately 3.2%. |
| Scc21 = -25 dB @ 1 GHz | The common-mode transmission at 1GHz is approximately-25dB | The common-mode energy is stansmitted from the input to the output relatively weakly; this is usually beneficial for EMI, but it still needs to be evaluated in conjunction with the overall structure of the machine |
Experience in Judging Trends
The Sdd21: The closer the curve is to dB，the smaller the differential transmission loss is; the lower it goes, the greater the loss. To Sdd11: The more negative the curve is, the smallter the reflection is and the better the matching is; approaching 0dB indicates severe reflection. To Scc21: The lowwer the value is, the weaker the common-mode transmission is; in EMI scenarios, it is usually desired that the common-mode energy not easily spreads out.
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