Take Good Care of Your RF Calibration Kits

Calibration kits also contain connectors that attach the standards to your network analyzer or test cables (Fig. 1). Calibration terminators have either male or female connectors. Those connectors—like all RF/microwave connectors—require correct use and maintenance. Dirt or a poor contact can cause impedance mismatches resulting in source and load reflections that can degrade the integrity of signals, especially at higher frequencies.

Inspect Before Using
Inspect all calibration standards and mating connectors before every use; burrs and nicks can throw them out of calibration. You can minimize damage to the devices by handling them carefully. Place each device in the kit’s case—and not your desk drawer—between uses.

Remember that dirt, grease, and oil from your fingers can also alter a calibration standard’s impedance. Often, manufacturers receive calibration kits from users that contain apparently defective parts when, in fact, the parts just need cleaning. You can clean calibration standards and connectors yourself. Clean outer surfaces with isopropyl alcohol and a clean cloth.

You should clean the inside of the connectors, too. Often, a blast of clean, compressed air or nitrogen is all you need to remove dirt from a connector. If the connector still looks dirty, then carefully use a toothpick or other nonmetallic thin stick with a lint-free cloth soaked in alcohol. Make sure the stick is thin enough and narrow enough to get the cloth inside the connector so you can clean the connector’s threads and contact pins. Don’t use a metal stick or you could scratch the connector’s surface. Be sure to periodically check and clean your instrument’s connectors; they get dirty, too.

Once you have clean parts, you can attach them to your network analyzer and test cables. Take care in connecting a calibration standard—or any RF connector—to an instrument or cable. Turn the connector’s nut only. Don’t twist the cable or the connector’s body: You could damage the threads or contact pins.

When the mating surfaces make uniform, light contact, the preliminary connection is tight enough. Be careful not to overtighten the connection, you can check the calibration kit’s manual for specifications. For type N connectors, you typically need 6 to 10 in.-lbs. of torque if you tighten the connectors finger tight.¹ For better connections, use a tourque wrench, which can tighten the connectors to 12 in.-lbs.²

How They Work
You’ll need different connectors for 50-W and 75-W standards. Always mark the impedance of each type-N connector with a label so you don’t accidentally interconnect 50-W and 75-W connectors. The contact pins and fingers on 75-W type-N connectors are smaller than their 50-W cousins. Inserting a 50- W male connector into a 75-W female connector will damage the female fingers, rendering the connector useless.

Taking care of your calibration standards is important, but your goal is getting reliable measurements. Good standards won’t get you good measurements unless you calibrate your test setup. For that, you need to enter information about your standards into your network analyzer.

Calibration kit manufacturers machine their parts for specific electrical characteristics. Shorts and opens, for example, have specified characteristics such as offset delay (the time required for a signal to traverse the length of the standard relative to the calibration plane). Offset delay results from the actual mechanical offset between the short or open circuit and the mating plane of the standard’s connector. Each standard doesn’t have a unique set of calibration constants. Instead, each type of standard has typical characteristics that the network analyzer uses to compute correction factors.

To perform a calibration with a kit, you must specify which kit you’re using. Network analyzer manufacturers embed the calibration constants for their calibration kits into the instruments. You just select your kit from a menu. The constants also come on a floppy disk so you can enter them yourself (if the kit and network analyzer are from different manufacturers).

Once you select the calibration kit from a menu on the network analyzer, the instrument will prompt you to connect the calibration kit’s standards to its test ports. After completing the measurements with a calibration kit, the network analyzer will compute and store correction factors, which it then uses to correct its measurements. T&MW

FOOTNOTES
1. “Extending the Life of Type N Connectors,” Application Note, Wide Band Engineering, Phoenix, AZ, 1998. www.wbecoinc.com/napp.html.

2. “Coaxial Systems: Principles of microwave connector care.” Application Note 326, Agilent Technologies, Santa Clara, CA.

You can contact Martin Rowe at m.rowe@tmworld.com.