AFR
Understanding Automatic Fixture Removal (AFR)
When an engineer designs a microscopic RF component (like a 2mm surface-mount filter or an integrated circuit), they cannot plug it directly into a Vector Network Analyzer (VNA). They must solder it onto a massive printed circuit board (PCB) test fixture, surrounded by long copper traces, vias, and SMA connectors. When they test the board, the VNA measures the massive insertion loss and phase delay of the entire PCB. To see how the microscopic filter actually performs, the engineer must mathematically subtract the entire test board. The modern software magic that achieves this is Automatic Fixture Removal (AFR).
Historically, removing a fixture required designing custom TRL (Thru-Reflect-Line) calibration standards directly onto the PCB, which consumed massive amounts of expensive board space and required an expert metrologist. AFR eliminates this burden. It is a highly advanced time-domain algorithm built directly into modern VNA software. It only requires the engineer to build one single extra trace on their PCB: a simple "2x Thru" line.
The 2x Thru Slicing Algorithm
The engineer connects the VNA to the 2x Thru trace (a copper line that is exactly twice as long as the trace leading to the actual component). The AFR software measures this Thru line, transforms the data into the Time Domain using a Fourier Transform, and mathematically slices the line exactly in half. It isolates the exact S-parameters of the left half of the board and the right half of the board. It then applies reverse matrix algebra to completely de-embed those halves from the measurement of the actual component.
The raw measurement of the entire board is:
[Tmeasured] = [Tfix_L] × [TDUT] × [Tfix_R]
The AFR software isolates the bare component instantly:
[TDUT] = [Tfix_L]-1 × [Tmeasured] × [Tfix_R]-1
Comparison
| De-Embedding Method | Custom PCB Standards Needed | Expertise Required | Speed |
|---|---|---|---|
| On-Board TRL Calibration | Many (Thru, Reflect, 3x Lines) | High (Requires custom kit definition) | Very Slow |
| Port Extension | None (Math only) | Low | Fast (But wildly inaccurate) |
| AFR (2x Thru) | One (A single 2x Thru trace) | Low (One-button software wizard) | Lightning Fast and Highly Accurate |
Frequently Asked Questions
What happens if the left and right sides of the fixture are not perfectly identical?
This is the primary weakness of the basic '2x Thru' AFR method. The math fundamentally assumes symmetry—that slicing the Thru line exactly in half represents the left and right sides accurately. If the left side has a massive via and the right side doesn't, the math fails. For asymmetric boards, modern AFR software allows you to use a '1-Port AFR' method, where you measure an Open or Short circuit at the component plane to characterize each side of the fixture independently.
How does AFR use the Time Domain?
By applying an Inverse Fast Fourier Transform (IFFT) to the wideband frequency data, the VNA converts the signal into a time-domain impulse. On the screen, the engineer can actually 'see' the RF pulse hit the SMA connector (spike 1), travel down the trace, and hit the component (spike 2). The AFR software uses 'Time-Domain Gating' to place mathematical windows around these specific spikes, cleanly isolating the impedance and loss of the fixture from the component.
Is AFR built into all VNAs?
No, AFR is typically a premium software option (often costing thousands of dollars) developed by companies like Keysight (PLTS) or Anritsu. Because the time-domain slicing algorithms and causality-enforcement math are incredibly complex and proprietary, it is highly monetized software crucial for high-speed digital and RF testing.