Global
Abbreviations:
SDD = Differential-to-Differential S-parameter
SCC = Common-to-Common S-parameter
S21 = Input from Port 1, measured at Port 2; commonly used for transmission/insertion loss
S11 = Input from Port 1, reflected back to Port 1; commonly used for reflection/return loss
1. Summary Mnemonic for S-parameters: S21 indicates "how much is transmitted through," S11 indicates "how much is reflected back."
| Notation | Input Port | Measurement Port | Engineering Meaning |
|---|---|---|---|
| S21 | Port 1 | Port 2 | Transmission from 1 to 2; commonly used for insertion loss / transmission gain |
| S11 | Port 1 | Port 1 | Reflection at Port 1 itself; commonly used for input return loss |
| S12 | Port 2 | Port 1 | Reverse transmission from 2 to 1 |
| S22 | Port 2 | Port 2 | Reflection at Port 2 itself; commonly used for output return loss |
2. Why does 21 typically represent transmission/insertion loss?
Taking a cable, connector, or PCB trace as the DUT, the VNA injects a signal from Port 1 and then measures the output at Port 2; this ratio is S21.
| S21 Value | Intuitive Meaning | Engineer Interpretation |
|---|---|---|
| 0 dB | Almost no attenuation | Ideal or very low loss; it is difficult for a practical passive channel to maintain 0 dB across the entire frequency band |
| -3 dB | Power is approximately halved | Significant attenuation; if referring to insertion loss, it is often written as IL = 3 dB |
| -10 dB | Signal is heavily attenuated | In high-frequency channels, equalization or link budget re-evaluation may be required |
| -30 dB | Almost no signal passes through | Transmission is very weak at this frequency point |
3. Why 11 typically represents reflection/return loss?
After injecting a signal from Port 1, if the input impedance of the DUT does not match the system impedance, a portion of the energy will be reflected back from Port 1. This reflection ratio is defined as S11.
| S11 Value | Reflected Power Percentage | Engineering Interpretation |
|---|---|---|
| 0 dB | Approximately 100% | Almost total reflection, very poor matching |
| -10 dB | Approximately 10% | Some reflection, often used as a basic threshold in engineering |
| -20 dB | Approximately 1% | Good matching; commonly referred to as return loss = 20 dB in colloquial terms |
| -30 dB | Approximately 0.1% | Very good matching |
Note the sign of dB. The S11 and S21 displayed on a VNA are usually negative values, e.g., S21 = -3 dB, S11 = -20 dB. However, in engineering jargon, when saying "insertion loss 3 dB" or "return loss 20 dB," the positive value is typically taken. Therefore, in a passive channel, Insertion Loss is approximately equal to -S21(dB), and Return Loss is approximately equal to -S11(dB).
4. What do SDD and SCC mean?
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 use these "mode ports" to describe the channel behavior.
| Abbreviation | Full English Name | Chinese Meaning | What to Look At |
|---|---|---|---|
| SDD | Differential-to-Differential S-parameter | Differential-to-Differential S-parameter | The transmission or reflection of differential signals in differential form |
| SCC | Common-to-Common S-parameter | Common-to-Common S-parameter | The transmission or reflection of common-mode signals in common-mode form |
| SDC / SCD | Differential/Common mode-conversion S-parameter | Mode conversion parameter between differential and common modes | Evaluate whether differential signals convert to common mode, or common mode converts to differential mode |
Differential mode: The signals on the two lines are equal in magnitude and opposite in direction, for example, P is +V and N is -V.
Common mode: The signals on the two lines are in the same direction, for example, P and N both jitter upward simultaneously.
High-speed differential links are intended to primarily transmit differential mode; when common-mode energy is excessive, it is often associated with EMI/radiation issues.
| Parameter | English Explanation | Chinese Meaning | Detailed Explanation |
|---|---|---|---|
| Sdd21 | Differential-to-Differential Forward Transmission | Differential-mode transmission / Differential-mode insertion loss | Input from differential-mode Port 1, measured at differential-mode Port 2; indicates how much differential signal is transmitted |
| Sdd11 | Differential-to-Differential Input Reflection | Differential-mode reflection / Differential-mode return loss | Input from differential-mode Port 1, reflected back at differential-mode Port 1; indicates how well the differential input is matched |
| Scc21 | Common-to-Common Forward Transmission | Common-mode transmission / Common-mode insertion loss | Input from common-mode Port 1, measured at common-mode Port 2; indicates how much common-mode signal is transmitted |
Sdd21: Differential Pair 1 Input -----> DUT -----> Differential Pair 2 Output Sdd11: Differential Pair 1 Input -----> DUT
Scc21: Common Mode Pair 1 Input -----> DUT -----> Common Mode Pair 2 Output
| Standard reading of Sdd21: Sdd21 = output differential wave at mixed-mode port 2 / input differential wave at mixed-mode port 1. In engineering terms, this translates to: the transmission capability of the differential signal from the input to the output, i.e., the differential-mode insertion loss. |
6. Reading Example: How to Describe the Curve When You See It?
| Test Result | Recommended Description | Meaning |
|---|---|---|
| Sdd21 = -6 dB @ 10 GHz | Differential-mode insertion loss is approximately 6 dB at 10 GHz | The differential signal is significantly attenuated when transmitted to the output; it must be evaluated against the system budget to determine acceptability |
| Sdd11 = -15 dB @ 5 GHz | Differential-mode return loss is approximately 15 dB at 5 GHz | The differential impedance matching at the input is acceptable; reflected power is approximately 3.2% |
| Scc21 = -25 dB @ 1 GHz | Common-mode transmission is approximately -25 dB at 1 GHz | Common-mode energy transmission from input to output is weak; this is generally favorable for EMI, but still needs to be assessed in conjunction with the overall system structure |
For Sdd21: The closer the curve is to 0 dB, the smaller the differential transmission loss; the lower it goes, the greater the loss.
For Sdd11: The more negative the curve, the smaller the reflection and the better the matching; close to 0 dB indicates severe reflection.
For Scc21: The lower the value, the weaker the common-mode transmission; in EMI scenarios, it is usually desired that common-mode energy does not easily propagate outward.
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