Why De-Embedding Is Needed
When a VNA measures a PCB component through SMA launch connectors and PCB traces, the measured S-parameters include the fixture response. De-embedding removes the fixture mathematically to reveal the true device response at the component's solder pads.
ABCD Cascade De-Embedding Method
Setup: VNA (cal plane) → Left fixture → DUT → Right fixture → VNA
Step 1: Characterize the fixture:
Option A: Make a "thru" structure (connect left and right fixture pads with short trace)
Option B: EM-simulate the fixture (from PCB design files)
Option C: Measure fixture S-parameters separately
Step 2: Convert all S-matrices to ABCD:
[A_measured] = [A_left_fixture] × [A_DUT] × [A_right_fixture]
Step 3: Solve for DUT:
[A_DUT] = [A_left]⁻¹ × [A_measured] × [A_right]⁻¹
Step 4: Convert [A_DUT] back to S-parameters → S_DUT_true
Simple Symmetrical Fixture Removal
If fixture is symmetrical (left = right):
[A_fixture_half] = √([A_thru]) [matrix square root]
[A_DUT] = [A_fixture_half]⁻¹ × [A_measured] × [A_fixture_half]⁻¹
Alternatively: use half the measured thru delay as fixture delay
Fixture delay τ = τ_thru / 2
Remove by adding −τ phase shift to S11 and S22 (rotate on Smith chart)
Practical PCB Fixture De-Embedding Steps
- Design fixture with: DUT pads (standard), thru structure (connect DUT pads directly)
- Measure thru structure → save as fixture_thru.s2p
- Measure DUT in fixture → save as dut_in_fixture.s2p
- De-embed: [A_DUT] = [A_fixture]⁻¹ × [A_measured] × [A_fixture]⁻¹
- Load both fixture_thru.s2p and dut_in_fixture.s2p in RF View for visualization
- Verify de-embedded result makes physical sense (S11 should move toward expected impedance)
RF View: While RF View doesn't perform full de-embedding math automatically, load both raw and de-embedded .s2p files for comparison. Circuit Simulator can model fixture as T-line elements for manual de-embedding verification. Free on Android.