Absolute Mode
Understanding Absolute Mode
A vector network analyzer internally contains multiple receivers. In a standard 2-port VNA, there are typically four receivers: R1, A, R2, and B. R1 and R2 are reference receivers that sample the source signal via internal directional couplers. A and B are test receivers connected to the physical test ports. Normal S-parameter measurement computes ratios: S11 = A/R1, S21 = B/R1, S12 = A/R2, S22 = B/R2.
Absolute Mode bypasses these ratios entirely. The engineer selects a single receiver (e.g., B) and the VNA displays its raw detected power across the sweep frequency range. This transforms the VNA into a tuned receiver or scalar power meter with the VNA's full dynamic range (typically 100-130 dB) and frequency resolution.
S21 = 20·log10(|b2/a1|) [dB]
Absolute Mode (Receiver B):
PB = 10·log10(|VB|2 / (2·Z0)) + 30 [dBm]
Receiver calibration correction:
Pcorrected = Praw + ΔPcable + ΔPadapter + ΔPreceiver
Without receiver cal: ±1 dB accuracy. With cal: ±0.1 dB.
Absolute Mode Use Cases
| Application | Receiver | What It Reveals | Alternative Instrument |
|---|---|---|---|
| Source power verification | R1 | VNA source output level | Power meter |
| Amplifier Pout | B | Raw output power in dBm | Power meter + attenuator |
| Harmonic analysis | B | 2f0, 3f0 levels | Spectrum analyzer |
| VNA diagnostics | R1 or R2 | Internal source/coupler health | Service center |
Frequently Asked Questions
Can you calibrate Absolute Mode?
Yes, via receiver calibration. Connect a traceable power sensor to the test port and perform a power sweep. The VNA creates a correction table mapping sensor readings to internal receiver readings, removing cable loss, adapter loss, and receiver nonlinearity. Accuracy improves from approximately ±1 dB (uncalibrated) to ±0.1 dB (calibrated).
Why use Absolute Mode instead of a spectrum analyzer?
Convenience. If the DUT is already connected for S-parameter testing, switching to Absolute Mode avoids disconnecting cables. Modern VNAs provide comparable dynamic range (100-120 dB) and the VNA also delivers phase information that a standard spectrum analyzer cannot provide.
How does Absolute Mode diagnose a broken VNA?
By viewing each internal receiver independently. If S-parameters appear erratic, viewing Receiver R1 in Absolute Mode reveals whether the source is delivering its specified power. If R1 shows -30 dBm when configured for 0 dBm output, the source amplifier or internal coupler is damaged, isolating the fault to a specific receiver chain.