Definition
Gain Flatness = max(S21) − min(S21) over operating bandwidth [dB pk-pk] Sometimes specified as ±X dB variation from nominal gain Good LNA: gain flatness ±0.5 dB over 1.5–2.7 GHz PA driver amp: gain flatness ±1 dB over 3.3–3.8 GHz Wideband instrumentation amp: gain flatness ±0.1 dB over 10 MHz–3 GHz
Causes of Non-Flat Gain
| Cause | Frequency Behavior | Solution |
|---|---|---|
| Transistor ft roll-off | Gain decreases with frequency | Feedback or equalizer network |
| Input/output matching BW | Gain peaks at match resonance | Widen matching network BW |
| Bypass capacitor SRF | Gain changes at cap SRF | Use C0G caps with appropriate SRF |
| PCB resonance | Sharp gain spike/notch at one freq | Add ground via fence, check stub lengths |
Measurement in RF View
Load amplifier .s2p → S21 dB view → select operating bandwidth
Method 1: Delta marker
Set Marker 1 at S21 maximum in band
Δ marker at S21 minimum in band
Read ΔdB = gain flatness (pk-pk)
Method 2: FN (Flat Noise) Marker
FN marker automatically finds the ±X dB bandwidth around average
Use threshold = ±0.5 dB → reads frequency range where gain is flat ±0.5 dB
Specification example:
If gain flatness ±0.5 dB spec must be met from 880–960 MHz:
Load .s2p → FN marker at ±0.5 dB → verify frequency range ≥880–960 MHz
RF View Flatness Analysis: Delta marker and FN (Flat Noise) marker both measure gain flatness. Delta marker gives peak-to-peak variation; FN marker finds the flat bandwidth. Both free in RF View on Android.