Accurate Testing of microwave circuits means eliminating measurement errors and ensuring repeatable results. This is only possible when the network analyzers can be calibrated with known and true calibration standards. These standards are not to be subjected to any kind of limitations, they must rather be designed to perfection.
The SMP is a 2.4mm connector, using solid dielectric interface. The female connector utilizes besides a slotted center conductor also a slotted outer conductor, which is unconventional. The female connector is designed to snap into the male connector, which is normally called the “socket”.
It is probably safe to say that millions of SMP connectors have been used, some even in critical parts of systems. The SMPs are popular because they are easy to install. Also, they are small, and therefore can be packaged densely, and they do not need to be torqued. Under most circumstances, the retention force of a fully detented pair of connectors is high enough to guarantee proper connection.
Unfortunately, all of these connectors (or cable assemblies or components terminated with these SMP connectors) could not be tested properly, or have only been tested by using adapters. Using these adapters, electrical tests results are questionable. The test data are always products of the component and the adapter. If an adapter employs compensation gaps, it could help to offset connector problems such as slots, recessed interfaces and untrue 50-Ohm areas. An adapter may also compensate for several dBs if it is matched to a specific connector design. An adapter can also make the test data look worse. The test data of a component may show that the component is electrically out of specification, when it would have been in specification if the adapter had not been used. This may happen especially when connector and adapter are manufactured by different suppliers.
Designing a Calibration Kit for SMP connectors and not using adapters makes it necessary to redesign the SMP connector for these test components and calibration standards to an extent that uncertainties are completely eliminated. But it always has to be maintained that any standard connector will mate properly with the connectors within the calibration kit.
The regular SMP connector uses PTFE as dielectric. This dielectric is a very soft material and it is even known for cold flow. It definitely eliminates the traceability of lines, adapters, and connectors and therefore cannot be used for absolute impedance standards. This would result into calibration errors and nil repeatability.
Slotting a line means changing the impedance of that line. The result will be an untrue 50 Ohm area. Corrections can be calculated, compensation by diameter adjustment, or gaps, or both. But fact is that slots, corrections and compensation gaps in short line lengths cannot be separated from each other and therefore also not from any measuring result. Therefore the use of slots and gaps, etc. in test components will bear uncertainties and cannot be recommended.
The SMP female connector employs in addition also a slotted outer conductor. Again, engineering can calculate compensation as well and try to take care of that mismatch area. But again: the result needs to be tested, it needs to be verified as true and valid. And it needs to be faced again: slots and compensation gaps in short line lengths cannot be separated from each other and any measuring result. Reflections can add or subtract in an uncontrolled manner. Slotted outer conductors are not usable for calibration standards.
Regular connectors are threaded. Therefore, when coupled, they remain securely in the proper mated position. Thus, components can be characterized precisely, and repeatable tests can be performed, showing identical results. PUSH-ON type connectors are different and much more difficult to test. The male and female connectors simply slide together. Although it is expected that they will end up in a predicted position, and the spring mechanism will lock them up in that position, it cannot be guaranteed. There are mechanical tolerances on all dimensions that can affect performance. If the interfaces of the connectors cannot meet properly, because of those tolerances, or if the interfaces end up in a different position each time (for example, if the insertion and withdrawal forces are inconsistent), the electrical performance will vary during testing.
Spectrum Elektrotechnik GmbH has developed a whole SMP Calibration System that allows testing SMP connectors with guaranteed repeatability:
The test connectors and adapters have been developed as ideal 50 Ohm lines. The center conductor of the female connector is designed as a seamless tube, loaded with a 0.004” (0.1 mm) slotted center contact, securing contact with the mating pin only at the very end, there where the tubing starts, ensuring a true 50 Ohm design. For the 40 GHz Calibration Kit the center contact is even spring loaded, striving for perfectionism. The outer conductor of the connectors of the calibration components is not slotted as well. This ensures a perfect 50 Ohm line. Instead of PTFE, a different dielectric has been selected. The dielectric constant is close to the one of PTFE, but the disadvantegeous properties of PTFE have been eliminated. The material is harder and it stays in place, ensuring true 50 Ohm lines, and traceable and repeatable test results.
To ensure proper and repeatable mating, the SMP Test Connectors employ additional threading. Therefore the Calibration Components can be threaded together during the calibration sequence, ensuring perfect and repeatable mating, avoiding uncertainties that cannot be prevented with pure snap on connectors. On the other hand, the test connectors will mate properly with any standard SMP connector.
Vector network analyzers require electrically stable cable assemblies for measurement in very critical areas, such as Phase, Amplitude and VSWR. Cables have to maintain calibration data, even when subjected to bending and twisting which can introduce distorting parameters. The cable assembly needs to be flexible without being flaccid. The Cables of Types 16, 18 and 22, as described in the Phase Stable Cable Assemblies’ section are ideal for this application. One end of the Assembly will usually be terminated with an NMD 2.4mm, 2.92mm, or 3.5mm, 7mm or N connector, as needed for connecting directly with the RF output of the network analyzer. These NMD Connectors are designed with a larger than standard coupling nut for greater stability. The other end of the ANA - Assembly can be terminated with the proper SMP Test Connector, avoiding any adapter, guaranteeing true and repeatable test results.
Regular SMP connectors are made to specifications that allow protruding dielectrics and center conductors. These connectors will damage mating precision connectors. Therefore Spectrum Elektrotechnik GmbH is offering also “Commercial” Adapters, accepting those connectors with protruding interfaces.
Where existing test cables are to be used, Spectrum Elektrotechnik GmbH offers between series instrument-grade adapters to those 2.4mm and 2.9mm test port or cable connectors. The other end of the adapter is supplied with the SMP Test Connector, equipped with the additional threading, allowing for proper calibration.
Spectrum Elektrotechnik GmbH has developed a complete line of SMP Components, as needed for proper and consistent LRL / TRL calibration. A Calibration Kit contains as a minimum for the Standard Calibration Kit: two Shorts (male and female), two Throughlines each (female/female, male/ female, male/male) in different lengths and a disc for the Calibration Characteristics. The Disc is supplied either for the Vector Network Analyzers of Hewlett Packard, or Wiltron, and is ready to be used. The Professional Calibration Kit contains also Interface Gauges, Calibration Blocks for the Interface Gauges, and a Torque Wrench.
The LRL / TRL Components are true 50 Ohm lines. The adapters are not using any supporting material between center contact and outer conductor. Outer conductors and the sets of center contacts are supplied separately. The set of center contacts contains all lengths as needed for adjusting the adjacent in specification connector interfaces that is usually recessed, as allowed, to perfect 50 Ohm lines. So length adjustments of 0.0004 inches (0.01mm) can be achieved, when all center contacts have been purchased. For details please refer to the LRL / TRL section in this Handbook. The Calibration Kits are supplied with discs that supply data on the properties of the Kits' components. Kits are available for the Network Analizers of Hewlett Packard and Wiltron. To characterize the Opens, Shorts, and the Loads of the Open-Short-Load-Through Kits, these LRL / TRL Components can perfectly be used.
For proper testing the regular SMP connectors, a variety of tools have been developed. For testing the SMP right angle connectors an adapter incorporating a bayonet clamp mechanism is offered. As outlined earlier, only snapping the components together may not guarantee the repeatability. The slightest interface gap will contribute to the meassurement results. Using the bayonet clamp mechanism will ensure secure fastening and repeatable testing. It guarantees proper and repeatable connection.
A withdrawal fork can serve for easy removal of SMP female connectors and “bullets”. For straight SMP females an Insertion/Holding/Withdrawal tool is supplied. It threads directly into the test connectors, holding the connector under test in place, ensures repeatability and helps as well for easy removal after testing, and it takes the stress off fragile assemblies.
To verify that the network analyzer system is working properly, the measuring of known devices, other than the calibration standards, is a straightforward method. Spectrum Elektrotechnik GmbH is offering verification kits that consist of precision airlines and mismatch airlines. Traceable measurement data are shipped with each kit.
The Calibration Kits can be divided into two groups:
The Industrial Type Calibration Kit is used for applications to 18.0 GHz. The Adapters marked “Commercial” are designed to accept all connectors, also those that are made to specifications that allow protruding center contacts.
The High Precision Calibration Kit is used for frequency ranges up to 40.0 GHz. With this Kit only precision connectors and adapters should be used, meeting Spectrum's SMP Test Connector Specification. Connectors manufactured to specifications that are allowing protrusion of the center contacts may ruin the precision components of the High Precision Calibration Kit.
As for other connector series, also for the SMPs the Standard, Expanded and Professional Kits are supplied.
Spectrum Elektrotechnik GmbH uses a unique system for easy identification of the individual calibration components. Therefore the Terminations, shorts and opens can easily be identified, at one glance:
Besides the regular Adapters supplied, some special adapters are furnished with the SMP Calibration Kit:
A Bayonet Clamp Mechanism is incorporated at some of the adapters for secure fastening and repeatable testing of the right angle SMP female connectors.
For straight SMP female connectors, Insertion/Holding/Withdrawal Tools are supplied for different connector lengths. This helps with these tiny connectors.
The tools screw directly into the test adapter and hold the connector under test in place. The withdrawal fork serves for easy removal of the SMP female connectors.
Four High Precision Gauges and two Calibration Blocks are supplied with the Professional Calibration Kit. Each of the Gauge are serialized and set for a certain task, marked properly on the back.
For proper test results and long life of the connectors the rules need to be obeyed. For each individual connector series a torque has been specified, a torque that ensures proper mating conditions, repeatable electrical performance and guarantees mechanically a long life of the connectors.
For the especially designed Calibration Components that are employing additional thread, the Torque Wrench should be used to ensure proper coupling and to avoid overtorquing.
The Torque Wrench is set to 0.7 Nm (6.2 inch-pounds).
Torque Wrenches are available also for most other connector series.
Spectrum Elektrotechnik GmbH offers a variety of test fixtures to allow professional testing in shortest time.
The fixture to the left is used to safely hold in place the Bayonet Mount 2.9mm to SMP Adapter
This is for testing small cable assemblies, that are terminated with an SMP right angle connector.