Originally posted Apr 7, 2015
Is something wrong with this picture?
Many of the things that intrigue me do not have the same effect on an average person. However, you are also not an average person—or you wouldn’t be reading this blog. Thus, I hope you’ll find the following image and explanation as interesting and useful as I did. Take a close look at this Keysight X-Series signal analyzer and the bits I’ve highlighted:
The frequency range of this MXA signal analyzer extends to 26.5 GHz but it is equipped with a Type N input connector. Because N connectors are normally rated to 11 or 18 GHz, do we have a problem?
One up-front confession: I looked at this combination of frequency range and input connector for years before it struck me as strange. I vaguely remembered that N connectors were meant for lower frequencies and finally took the time to look it up.
The explanation is only a little complicated, including some clever engineering to optimize tradeoffs, and it’s worth understanding. As always with microwaves and connections, it’s a matter of materials, precision and geometry.
First, the short summary: The N connectors used in Keysight’s 26 GHz instruments are specially designed and constructed, and their characteristics are accounted for in the instrument specifications. If you’re working above 18 GHz and using appropriate adapters such as those in the 11878 Adapter Kit, you can measure with confidence. Just connect the N-to-3.5mm adapter at the instrument front panel and use 3.5 mm or SMA hardware from there.
Why use the N connector on a 26 GHz instrument in the first place? Why not an instrument-grade 3.5 mm connector that will readily connect to common SMA connectors as well? The main reason is the strength and durability of the N connector when dealing with the bumps, twists and frequent reconnections that test equipment must endure—and still ensure excellent performance. Precision N connectors offer a combination of robustness and consistent performance that is unique in the RF/microwave world. They’re also easy to align and are generally tightened by hand.
However, there is that small matter of limited frequency range. Standard N connectors are rated to 11 GHz and precision ones to 18 GHz. Above 18 GHz, conductor size and geometry can allow amplitude and phase errors due to the moding phenomenon I described in a previous post.
Moding is a resonance phenomenon from the larger dimensions of the N connector, and the solution involves a change in the construction of the instrument’s precision N connector. This special connector has a combination of a slotless inner shield, a support bead of a special material, and higher precision construction. As a result, resonances can be eliminated or reduced to such a small magnitude that the N connector is the overall best choice for test equipment over this frequency range.
There you have it, the practical advantages of N connectors over the full 26.5 GHz frequency range, without a performance penalty.