Millimetre-wave (above 75 GHz) calibration
Above 75 GHz the wavelength drops below 4 mm, coaxial connectors run out (1.0 mm to 110 GHz) and measurements move into rectangular waveguide. Flange alignment, extender modules and the air itself become the main limits.
Instruments and typical accuracy
| Instrument | Typical range | Typical accuracy | Where it is used |
|---|---|---|---|
| Waveguide VNA with frequency extenders | 75 GHz to 1.1 THz | Depends on band and calibration, typically ±0.5 to ±1.5 dB on transmission | S-parameters of devices in waveguide |
| Waveguide power sensor (thermoelectric, diode) | 75 GHz to 170 GHz and above | 3 to 8 % | Power measurement in W and D band |
| Calorimeter power meter | 75 GHz to 3 THz | 2 to 5 % | Reference power at mm-wave and THz |
| Harmonic mixer and spectrum analyser | Up to 110 GHz and above | ±1 to ±3 dB level, frequency from the timebase | Signal analysis with external mixers |
| Frequency counter with harmonic mixer | Up to 110 GHz and above | Timebase accuracy | Frequency of mm-wave sources |
| Precision waveguide attenuator and variable attenuator | WR-12 to WR-3 | 0.05 to 0.2 dB | Level and linearity references |
| Waveguide noise source and radiometer | 75 GHz to 325 GHz | ±0.3 to ±1 dB | Receiver noise at 5G, radar and automotive bands |
| Antenna and gain horn | 75 GHz to 325 GHz | Gain ±0.2 to ±0.5 dB | Over-the-air radar and communications |
Above 110 GHz there is no coax: everything is waveguide, and calibration kits are specific to each band. The accuracy figures are typical ranges to help you compare instruments. Always use the specification of the instrument in front of you.
Waveguide bands above 75 GHz
| Waveguide | Broad wall a | Standard band (GHz) | TE10 cutoff (GHz) | Band letter (approx.) |
|---|---|---|---|---|
| WR-12 | 3.099 mm | 60 to 90 | 48.37 | E |
| WR-10 | 2.54 mm | 75 to 110 | 59.01 | W |
| WR-8 | 2.032 mm | 90 to 140 | 73.77 | F |
| WR-6 | 1.651 mm | 110 to 170 | 90.79 | D |
| WR-5 | 1.295 mm | 140 to 220 | 115.75 | G |
| WR-4 | 1.092 mm | 170 to 260 | 137.27 | — |
| WR-3 | 0.864 mm | 220 to 325 | 173.49 | — |
WR-10 overlaps with WR-12 so a measurement can stay in guide through the crossover. Above 325 GHz the guides become sub-millimetre sized and flange alignment dominates the uncertainty.
Reference standards and traceability
- Waveguide standards. IEEE 1785.1 gives waveguide bands and flange dimensions for 110 GHz and above. IEC and EIA list the lower WR sizes.
- Calibration. Waveguide calibration kits use TRL or offset-short methods whose definitions come from dimensional metrology of the guide and line lengths.
- Power. Waveguide power transfer standards are compared against a calorimeter or microcalorimeter, and their efficiency is given by band.
- Frequency. Referenced to the timebase through a harmonic mixer or multiplier chain, with phase locking to a known reference.
- Quality system. ISO/IEC 17025, scope listing the waveguide band, quantity and uncertainty.
How it is calibrated
Typical procedure
- Choose the waveguide band that covers the frequency, and inspect the flanges and alignment pins.
- Warm up the VNA and the extender heads. Let temperature settle, because drift in the heads is large compared with lower bands.
- Calibrate with the band-specific calibration kit (TRL or offset shorts), at the reference plane of the device.
- Fix and support the waveguide so that it is not under strain, and avoid moving the extenders after calibration.
- Measure the standard (an attenuator, a thru, a mismatch) as a check on the calibration.
- Measure the device and note the dynamic range limit at the top of the band.
- Record the temperature and report the uncertainty with the band and the flange type.
Where it goes wrong
- Flange misalignment. A few micrometres of offset at WR-3 is a large reflection.
- Poor repeatability. Re-connecting changes the result by more than the instrument uncertainty.
- Extender drift. Heads warm up and phase changes by degrees.
- Atmospheric absorption. Oxygen near 60 GHz and water near 183 GHz absorb a lot in free space.
- Low dynamic range at the top of the band. Noise dominates in transmission above 500 GHz.
What goes in the uncertainty budget
- Calibration kit definitions and standards.
- Flange repeatability and alignment.
- Extender drift, cable flex and temperature.
- Power sensor efficiency and mismatch.
- Noise floor and dynamic range, plus atmospheric effects in free space.
Combine the contributions as in the uncertainty budget calculator, then decide pass or fail with a guard band as explained on the calculators page.
Calculators
Everything runs in your browser. Nothing you type is sent anywhere.