S21 passband
Keep forward transmission above the lowest acceptable level throughout the operating band.
S21 ≥ −2 dB900–1100 MHz
Practical RF limits
How to set S-parameter pass/fail limits.
An RF acceptance rule needs four things: the S-parameter, a start frequency, a stop frequency, and the direction of comparison against a threshold. The sign of a dB value makes that final choice easy to reverse accidentally.
Test a limit with synthetic measurements
Open the free limit checkerBenchReport includes a passband example with one deliberately failed unit.
Three common S-parameter rules
900–1100 MHz
S21 stopband
Keep unwanted forward transmission below the maximum permitted leakage level.
S21 ≤ −40 dB1500–2000 MHz
S11 input match
When using negative S11 magnitude in dB, require reflection to stay below the stated level.
S11 ≤ −10 dB900–1100 MHz
Why negative dB comparisons cause mistakes
With negative values, −20 dB is numerically less than −10 dB. For S11 magnitude in dB, −20 dB represents a smaller reflected wave than −10 dB and is normally the better match. That is why an S11 magnitude requirement written as “below −10 dB” becomes an at most −10 dB comparison.
Insertion loss is sometimes specified as a positive loss value even though a viewer plots S21 magnitude as a negative value. A specification saying “insertion loss no more than 2 dB” commonly corresponds to plotted S21 being at least −2 dB.
Confirm the quantity and convention first.
“Return loss greater than 10 dB” and “S11 below −10 dB” can express the same physical intent using opposite signs. Never copy a threshold without checking what the source document defines.
Apply limits only where they are valid
A pass/fail result should use samples inside the specified frequency band. A poor value outside the operating band should not fail a passband rule, while a gap with no measurement samples should not be treated as an automatic pass. BenchReport reports “no data” when a valid rule band contains no samples.
| Rule type | Trace | Comparison | Typical interpretation |
|---|---|---|---|
| Passband transmission | S21 | At least | Do not fall below the minimum transmitted level |
| Stopband rejection | S21 | At most | Do not exceed the maximum leakage level |
| Input reflection | S11 | At most | Keep negative reflection magnitude below the limit |
| Output reflection | S22 | At most | Keep negative reflection magnitude below the limit |
Acceptance-rule checklist
- Confirm whether the requirement describes S-parameter magnitude, insertion loss, return loss, VSWR, or another quantity.
- Use the same dB sign convention as the displayed trace.
- Enter the applicable start and stop frequencies, including the intended endpoints.
- Check that measurement samples exist throughout the region needed by the approved process.
- Inspect the worst value and its frequency instead of relying only on the pass/fail badge.
- Retain the source specification and measurement context with the final report.
Run the same checks across a batch
The free browser edition evaluates one rule across up to three measurements. BenchReport Pro adds unlimited measurements and rules plus reusable acceptance templates, so the same matrix can be applied again without re-entering it.
S-parameter limit questions
Should an S21 passband rule use “at least”?
Usually, when the plot shows S21 magnitude as a negative dB value. A limit of at least −2 dB rejects samples that fall below −2 dB.
Why does S11 often use “at most”?
For negative S11 magnitude, more-negative values represent lower reflection. “S11 below −10 dB” therefore uses at most −10 dB.
Does BenchReport interpolate at a band boundary?
No. The current calculation uses available measurement samples within the selected band. Review sampling density against your engineering process.
Does passing BenchReport prove compliance?
No. It documents calculations from supplied data. Calibration, uncertainty, measurement validity and formal approval remain the user’s responsibility.