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September, 2021


Functional Testing of RF Boards with MMCX Connectors


By Matthias Zapatka, Field Application Engineer, INGUN USA M


icro-miniature coaxial connections (MMCX) are a very common type of


RF interface. These connectors are used for many different applications, across a wide range of industries. These include use in smart meters, industrial IoT devices, marine radio equipment, and professional audio. MMCX conforms to the


European CECC 22000 stan- dard. The connector is a “snap in type,” but since the mating cycles are limited it has become a chal- lenge to test these devices with the mating connector and spring- loaded interconnects provide a significant advantage.


Applications for MMCX Smart meters are devices


that record energy consumption. Unlike traditional power meters, these are equipped with two-way wireless communications that send and receive data from a cen- tral system. For antenna connections,


MMCX and U.FL are used. Typically, there are also minia- ture switch connectors on such boards to allow functional test- ing. To verify that an amplifier has enough power output, probes can be used on the MMCX or U.FL connectors. This is a con- ducted test, where the probe comes in direct contact with the DUT — the antenna would not yet be connected at this stage — so this is not an over-the-air test. Smart meters are typically


made in larger volumes and thus it can be cumbersome to use a mating connector for test, even though that typically yields the


best RF performance. A test probe may have a slightly higher loss and require a test fixture, but tens of thousands of boards can be tested with a single probe and the fixture would be needed to conduct a functional test to power up the board, as well. In the professional audio


industry, there has been pres- sure to go wireless for stage


testing to ensure the amplifier meets the specifications and the power is high enough to allow for a good connection to all receivers.


Spring-Loaded Test Probes A spring-loaded test probe


for MMCX is basically the exten- sion of a coaxial transmission line but with mechanical fea- tures that allow an easy connec-


is because the probe can be used for a high number of mating cycles. However, it requires that the probe be installed in a test fixture as it does not have self- retention.


Designing a Fixture If there is a fixture require-


ment, choosing the right one depends on several factors. How many test points are on the board? How many tests other than the conducted RF test must be performed? Does it require shielding or is an open architec- ture fine? These questions not only


drive the actual fixture selection, but also the price. But for exam- ple, if there are risks of RFI or EMI it may be wise to invest in a shielded enclosure which reduces the risk of interference. Sometimes shielded inserts


Smart meters and other industrial IoT products depend on proper RF testing.


events, in auditoriums, and for intercom systems. While a wired setup may be beneficial in a situ- ation prone to interference, the wireless trend does not seem likely to be reversed. That raises some challenges


for testing, as the components used to conduct a FCT also need to be immune to RFI and EMI — good shielding is a must. MMCX connectors are some


of the most the common for wire- less transmitters and trans- ceivers for antenna connection. Like smart meters, a very com- mon test scenario is power level


tion to that connector for test. “Easy” means a feature that


makes the test probe easily detachable after the test is done. The probe head must be shaped in a way that it does not deviate from the nominal impedance — for MMCX that impedance is 50W. The mechanical shape of a test device can interfere with the electrical behavior, especially in the GHz range. The MMCX mating connec-


tor has a ring around the head for snapping into the female por- tion — but the probe does not have this feature. This


and standard inserts can also be interchanged with the same fix- ture architecture. There are three basic types of standalone fixtures: manual ones with an overclamp (drive unit) that is closed by hand; vacuum fixtures, which are mainly used for a high number of probes); and pneumat- ic fixtures. The most important aspect


of the fixture is to drive the probes onto the DUT in a linear fashion, as side loads can dam- age the probe and the DUT over time.


Here are two examples of a


common fixture solution: a man- ual fixture that can be used for a variety of test systems, and one that can be used with both con-


Continued on next page


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