S
Seed account
Connector choice looks trivial until the third board spin fails return-loss at the upper band edge or the cal kit no longer mates cleanly. A short comparison that matches the interface to the frequency and the mating life you need.
1. Frequency ceilings (typical precision grades)
- SMA: DC to 18 GHz common; some precision SMA to 26.5 GHz. Threaded, durable, cheap.
- 3.5 mm: DC to 26.5 GHz. Mate-compatible with SMA but air dielectric and tighter tolerance; do not torque an SMA male into a precision 3.5 mm female for cal work.
- 2.92 mm (K): DC to 40 GHz. Mate-compatible with 3.5 mm / SMA in a pinch, but best kept in its own family for metrology.
- 2.4 mm / 1.85 mm: 50 GHz / 67 GHz class. Not SMA-compatible.
- SMP / SMPM: push-on, blind-mate, roughly to 40 GHz (SMPM higher). Built for boards and modules that mate many times without a wrench.
2. Mechanical and repeatability notes
Threaded connectors (SMA / 3.5 / 2.92) need a torque wrench (typically 8 in-lb for brass SMA, check the vendor). Under-torque raises reflections; over-torque ruins the female fingers. Push-on SMP trades a bit of repeatability for speed and density; use the full- detent / limited-detent / smooth-bore option deliberately so modules do not unmate in vibration.
3. The adapter tax
Every adapter is a small mismatch and a radiation opportunity. A chain SMA-DUT → SMA-3.5 adapter → 3.5 cal plane can move S11 by several tenths of a dB at 20 GHz. Prefer:
- Same connector family from VNA port to DUT
- Or include the adapter inside the calibration (adapter removal / unknown thru)
- Never stack three adapters to "make it fit" for a plot you will publish
4. Board launch matters more than the logo on the connector
An SMA edge launch with a poor launch taper looks worse at 10 GHz than a clean 2.92 mm launch. Spend layout time on:
- Ground via fence around the launch
- Back-drill or thin board for the pin parasitics if needed
- Keep-out under the pin pad per vendor footprint
5. Practical picks
- Lab bench <18 GHz, students, general DUT: SMA.
- Precision work to 26.5 GHz: 3.5 mm kit and cables; keep SMA only on the DUT side with one known adapter.
- 40 GHz modules: 2.92 mm or SMPM depending on whether you need threaded metrology or blind mate.
- Dense multi-channel boards: SMP/SMPM or board-to-board bullets; budget for a test adapter coupon.
Buy the connector for the frequency you measure, not the frequency you hope to market. Then budget one good torque wrench and fewer adapters.
1. Frequency ceilings (typical precision grades)
- SMA: DC to 18 GHz common; some precision SMA to 26.5 GHz. Threaded, durable, cheap.
- 3.5 mm: DC to 26.5 GHz. Mate-compatible with SMA but air dielectric and tighter tolerance; do not torque an SMA male into a precision 3.5 mm female for cal work.
- 2.92 mm (K): DC to 40 GHz. Mate-compatible with 3.5 mm / SMA in a pinch, but best kept in its own family for metrology.
- 2.4 mm / 1.85 mm: 50 GHz / 67 GHz class. Not SMA-compatible.
- SMP / SMPM: push-on, blind-mate, roughly to 40 GHz (SMPM higher). Built for boards and modules that mate many times without a wrench.
2. Mechanical and repeatability notes
Threaded connectors (SMA / 3.5 / 2.92) need a torque wrench (typically 8 in-lb for brass SMA, check the vendor). Under-torque raises reflections; over-torque ruins the female fingers. Push-on SMP trades a bit of repeatability for speed and density; use the full- detent / limited-detent / smooth-bore option deliberately so modules do not unmate in vibration.
3. The adapter tax
Every adapter is a small mismatch and a radiation opportunity. A chain SMA-DUT → SMA-3.5 adapter → 3.5 cal plane can move S11 by several tenths of a dB at 20 GHz. Prefer:
- Same connector family from VNA port to DUT
- Or include the adapter inside the calibration (adapter removal / unknown thru)
- Never stack three adapters to "make it fit" for a plot you will publish
4. Board launch matters more than the logo on the connector
An SMA edge launch with a poor launch taper looks worse at 10 GHz than a clean 2.92 mm launch. Spend layout time on:
- Ground via fence around the launch
- Back-drill or thin board for the pin parasitics if needed
- Keep-out under the pin pad per vendor footprint
5. Practical picks
- Lab bench <18 GHz, students, general DUT: SMA.
- Precision work to 26.5 GHz: 3.5 mm kit and cables; keep SMA only on the DUT side with one known adapter.
- 40 GHz modules: 2.92 mm or SMPM depending on whether you need threaded metrology or blind mate.
- Dense multi-channel boards: SMP/SMPM or board-to-board bullets; budget for a test adapter coupon.
Buy the connector for the frequency you measure, not the frequency you hope to market. Then budget one good torque wrench and fewer adapters.