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## Remarks ## 1. **Don't trust, verify.** Off-spec connectors are common due to legacy standards, poor manufacturing or handling damages. To prevent damaging expensive instruments, it's essential to purchase a connector gage kit. Gauge all new, second-hand, lent-and-returned, dropped or shocked connectors. Damaged ports may fail silently, showing intermittent and connector-dependent signal losses, frequency-dependent reflections, and calibration drifts. One damaged connector may subsequently spread the damage to the whole lab. 2. **Damage monitoring.** Record connector dimensions when a new precision instrument or calkit arrives. Before each insertion, rotate the connector body to different angles. Make 5 measurements and take the average. Rotate the connector before gauging, never rotate the connector while it's still inserted. See [6]. 3. **Pin protrusion is forbidden in all standards.** Discard the connector immediately when the pin gage dial reads 0.001 inch, 0.01 mm, or higher. Due to measurement uncertainty, there's an ambiguous zone near 0 [6], but one graduation is too much! 4. **Dielectric protrusion is forbidden in modern connectors, and is never acceptable for precision-grade connectors even in legacy standards.** It's only acceptable for legacy non-precision connectors (IEC 60169-15, MIL-C-39012, Maury Standard, OSM 1979), but this has been banned by the IEC 61169-15. Flag the connector when the dielectric gage dial goes beyond 0, use them in legacy industrial systems only, never directly connect them to any test instruments. 5. **Legacy connectors**. IEC 60169-15 connectors are apparently still being produced en masse, even by respected vendors such as Rosenberger. Make sure to order MIL-STD-348A, IEC 61169-15, or 3.5 mm connectors for precision applications. 6. **Equivalence**. IEC 61169-15:2021 (Grade 0) and MIL-C-39012 (Standard Test, per MIL-C-39012B Amendment 1), MIL-STD-348A, Figure 405 are exactly equivalent. Both are precision-grade connectors. 7. **Mating customer/commercial-grade connectors with precision-grade connectors.** If confirmed to be protrusion-free, mixed-grade or SMA-to-3.5 mm connections can be safe, but with a potentially poor signal integrity due to pin recessions. IEC and MIL Standard Test (Grade 0) connectors are meant for performance characterization of IEC Grade 1 connectors, so they're perhaps too strict in that regard. The author suggests the *Maury Microwave Precision* dimensions as the go/no-go standard. Note that connecting 3.5 mm ports to SMA in fact improves signal integrity (if undamaged). 8. **Use sacrificial adapters.** A set of high-grade plug-receptacle adapters should be used as a "firewall" between test instruments ports and low-cost ports, acting as sacrificial adapters. These are hence known as connector savers. It's a backup option if a gage kit is unavailable, or an extra layer of protection in additional to a gage kit.