Re: SMB Adaptor (again)

Joseph Strickland

My experience when I worked at Collins Radio in Richardson was that there were two types of BNC connectors. In the 50 ohm case there was a white nylon or Delrin plastic protection sleeve around the female center contact of the connector. The 50 ohm male pin that interfaces with the female contact is physically a little larger than the male pin in the 75 ohm mating BNC connectors. The 75 ohm BNC connectors with the female contacts have exposed contacts not surrounded by the white plastic seen in the 50 ohm connectors. That makes them more subject to damage from mating connectors pushed on to them at a slight angle. That is why you often see 50 ohm BNC connectors used on 75 ohm cables and their mating connectors on test equipment. In true 75 ohm connectors both the female and male contacts are exposed in the center of the connector and somewhat thinner than their 50 ohm family equivalents. When 50 ohm connectors are used in 75 ohm applications the main difference seen is that there is a slight reduction in return loss when compared to true 75 ohm connectors. Typical BNC connectors cause about 1/2 dB of signal loss whether 50 ohm or 75 ohm.

When it came to coaxial cables, the best quality ones specified in Collins/Rockwell telecommunication equipment used double shielded 1% 30dB or better return loss specified cables made by Times Cable Co. I don't know if it was a standard practice of Times Cable for their 75 ohm coaxial cable of the 1% return loss type or not, but they usually came with a violet stripe along the length of the coaxial cable. The high return loss (30dB or higher) had to be checked against 50 ohm or 75 ohm high accuracy loads in order to verify the true quality of the coaxial cable. Times Cables always were consistently best in that measure. 50 versus 75 ohm impedance connectors become more important the higher the signal frequencies are that are being handled by the equipment being tested or verified.

Collins often used variable trimmer capacitors on both baseband and RF circuits that required best impedance match to the highest expected frequency to be handled. They were adjusted with unit under test equipment having its normal cables within the equipment already connected while using a swept input signal covering the full range of signal frequencies to be handled by the equipment. The trimmer capacitors were adjusted for the best return loss over the full range of the signals being handled. Typically Selective Volt Meters of high accuracy by HP, Tektronix or Wandel & Goltermann were used for those checks and adjustment verification. Test equipment had to be verified against industry calibration standards on a regular schedule. Most of the analog equipment had to have good return loss at baseband frequencies from near DC to at least 30mHz. All the systems and unit test technicians I worked with insisted upon the very best performance that could be obtained with state of the art products.


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