Short Question: How can a Type-N Signal Analyzer operate to 26.5 GHz?
Long Question: Type-N connectors are not specified above 18 GHz. How does a Type-N Spectrum Analyzer operate to 26.5 GHz?
Answer: The RF circuitry behind the front panel of all 26.5 GHz Agilent signal analyzers is 3.5 mm geometry. For type-N analyzers the input connector is essentially an Agilent 1250-1745, type-N (f) to 3.5 mm (f) adapter so any performance degradation would be attributable to the type-N input adapter mounted to the front panel. This performance degradation would appear as “modes” in the frequency response of the spectrum analyzer. A “mode” is a narrow band resonance that would not be compensated by the spectrum analyzer calibration. These modes do not occur in precision type-N adapters for several reasons.
Advances in precision machining have allowed type-N connectors to operate mode-free to 26.5 GHz. For instance the Agilent 1250-1745 3.5 mm (f) to Type-N (f) adapter inner shield is solid. General-purpose type-N adapters (the Agilent 1250-1778 for example) have a slotted inner shield. The slots allow for mechanical tolerance variations, but they create a non-uniform ground plane on the outer shield. The slots can lead to fringing of the E-field which impacts connector performance.
Another area where Agilent has contributed to connector performance is in the center conductor support bead. Older type-N adapters have Delron or PTFE supporting the inner conductor. Looking into a general-purpose type-N adapter, one can see a white dielectric surrounding the center conductor. The 1250-17XX family of type-N adapters has a black support structure. This black support bead is an Agilent patented material that suppresses modes.
One simple test to look for modes can be done by mating an Agilent 1250-1744 type-N (m) to 3.5 mm (f) adapter with a 1250-1745 type-N (f) to 3.5 mm (f) adapter. The insertion loss of the assembly should be measured with a network analyzer over a very narrow bandwidth stepping from 18 GHz to 26.5 GHz. A mode would appear as a notch in the S21 response. Extensive testing on a PNA Network Analyzer with many connector combinations has shown the 1250-17XX family to be mode-free.
Here are some adapters of interest to you:
1250-1250 Type-N male, Type SMA female is available for purchase at Agilent. Grade: General Purpose Overall Length: 31.75 mm (1.25 in) Diameter: 21.15 mm (0.83 in)
1250-1744 Adapter, 3.5 mm (f) to Type-N (m), DC to 18 GHz is available for purchase at Agilent. Grade: Instrument Frequency Range: dc - 18 GHz Return Loss: 28 dB Electrical Length: 40.8 mm ( 1.61 in) Overall Length: 43.6 mm (1.72 in) Diameter: 20.8 mm (0.82 in)
Although I understand your point, we still have to deal with customers who only look at the specified performance of the adapter. For example, while the 1250-1744 may be usable to 26 GHz, it is only specified to 18 GHz. Since we integrate spectrum analyzers into systems, our customers frequently question the use of 18 GHz hardware on a 26 GHz measurement system. For now, I will reference your response, but it would be better if the adapter was specified as a 26 GHz adapter.