SMP3 vs. SMPS: why two standards?

The SPM3 and SMPS RF coax connectors are similar but slightly different. They are not entirely interchangeable, so we have two standards.

If you research SMP3 and SMPS RF coax connectors, you might conclude that they are the same, completely interoperable, and interchangeable. They are indeed quite similar, with comparable performance specs and dimensions. Both are appropriate for many of the same applications, including telecommunications, military/aerospace, and test and measurement (T&M).

However, they are not precisely the same. While SMP3 and SMPS connectors are often compatible, assuming that one type will mate seamlessly with the other could be a mistake — potentially a big one.

We’ll cut to the chase with our best advice for those whose designs require this class of RF coax connector. Pick one — SMP3 or SMPS — and stick with it as best you can.

SMP3 and (not “or”) G3PO

Those researching SMPS or SMP3 coax connectors are almost certain to also encounter G3PO connectors. These connectors are aimed at the same applications and will have performance specs familiar to anyone researching SMP3 and SMPS connectors.

If there are already two different standards (SMP3 and SMPS), why does the market need a third type in the same category? The answer is that it doesn’t – nor is there. G3PO and SMP3 are the exact same thing.

Some sources of information about connectors don’t mention that apparently because they assume everyone already knows that SMP3 simply formalizes G3PO specifications as a standard. That SMP3 and G3PO are the same doesn’t seem to be common knowledge. Perhaps that is to be expected, as the new standards were ratified during the pandemic in the summer of 2020.

Standard vs. standard

This still begs the question: why are there two standards — SMPS and SMP3 (aka G3PO) — for similar but not exactly the same connectors? The answer is that there was a disagreement that connector suppliers were disinclined to resolve.

RF connector types were developed starting in the 1930s with UHF for larger radio equipment, advancing to type-N in the 1940s thanks to Paul Neill of Bell Labs for medium-sized equipment to join coaxial cables. Soon after, Carl Concelman of Amphenol, who also designed the C connector, partnered with Neil to create the BNC type. By the 1960s, the SMA (subminiature version A) and SMB (subminiature version B) had been developed, and so on. Through time, RF coax connectors have become progressively smaller and simultaneously capable of transmitting higher-frequency signals. These were all standard sizes and connectors in each category were fully compatible across all brands. This held true through the introduction of SMP (subminiature push-on) and SMPM (subminiature push-on micro) connectors.

SMP connectors can transmit frequencies up to 40 GHz, and SMPM connectors operate at up to 65 GHz. Both were developed by Corning Gilbert, which called the two lines of connectors GPO (the line corresponding to SMP) and GPPO (the line corresponding to SMPM). Then, in 2014, Corning Gilbert developed a new, even smaller generation of RF coax connectors capable of frequencies up to 100 GHz. The company designated this product line G3PO. Other connector suppliers subsequently entered the market with similar RF coax connectors. Specifically, competition between Corning and Amphenol led to both developing their version of this connector.

Other manufacturers, like Cinch, started making their own crosses for those new connectors. However, some were very casual with the naming and often used them interchangeably (SMP3 and SMPS). The connectors were effectively the same for most applications, but for harsh applications, especially with shock and vibration, we’ve found that they are different enough to cause problems, and for mission-critical applications, that is unacceptable.

Intermateability issues

Some defense contractors using this generation of connectors began mixing and matching products from Corning Gilbert and its competitors, and some began experiencing “intermateability” issues. In 2014, the Defense Logistics Agency (DLA) began investigating the issue. The information gathered from the suppliers included “several incompatible dimensioning tables.” Some Gilbert competitors did not manufacture to precisely the same specifications.

Starting in 2017, the DLA tried to compel connector suppliers to agree to a common standard for this new generation of connectors. Corning Gilbert steadfastly championed its original design, while Amphenol remained adamant about continuing with the variation it was manufacturing and selling. The DLA wouldn’t add the connectors without resolving the differences because each wanted their own to be listed as the official specification.

In 2020, the DLA gave up trying to resolve the issue. A DLA document published that year states, “It was decided to create a new series in the SMP family. There will be an SMPS and SMP3 design.”

The agency washed its hands of the “incompatible dimensioning” between SMP3/G3PO on the one hand and SMPS connectors on the other. Those interested in comparing and contrasting can reference MIL-STD-348A and MIL-STD-348B (Figure 1).

The DLA added both in 2024, changing revision B of MIL-STD-348. Until then, neither connector was included in the standard. So, while the market often interchanges them and views them as different names for the same product, the DLA has now officially listed them as two distinct connectors to avoid mixing and matching.

Dimensioning

As this was being written, some sources of information about connectors — sources that one might have thought would be authoritative and reliable — nonetheless stated that SMP3 and SMPS are compatible, seemingly implying that they can be mixed and matched with no consequences. In reality, the tolerance differences between the two could be detrimental to signal performance in harsh environments with a lot of shock and vibration.

Some of the most potentially consequential differences between SMP3 (G3PO) and SMPS involve the degree of insertion misalignment that can be tolerated during connection. Accepting some degree of misalignment is always useful, but especially important for SMP3 blind mate connectors. Blind mate connectors are indicated in applications where end-users have minimal space (and/or time) to make a perfectly aligned connection.

These connectors are formatted with bullets, a construction that helps compensate for radial and axial misalignment. In this format, both sides of the connector have receptacles for the mating mechanism, a discrete part in the shape of a cylinder or bullet (Figure 2).

One side of the connector has full detent so that bullets are held fast once inserted (detent is the amount of force necessary to connect or disconnect). It is entirely possible to remove bullets from a socket with full detent, but the point of full detent is to make such removal difficult deliberately. The other side of the connector will be engineered to have a lower degree of detent. Different connector lines can offer two levels: full detent and smooth bore (Figure 3). If the two sides are disconnected, deliberately or inadvertently, the bullets should consequently all stay with the side of the connector with full detent. This keeps bullets from being lost and preserves the orientation of the bullet(s) so that reconnection can be accomplished correctly and swiftly, which can be especially important when using dual-port connectors.

The main functions of the bullet (Figure 4) are:

• Aligns the center conductor during mating, which helps reduce signal loss and maintain impedance matching
• Allows for blind mating when connectors are joined without complete visibility. The bullet allows for some misalignment during mating.
• It also helps prevent damage caused by misalignment during connection.

Mixing and matching SMPS and SMP3 connectors and bullets from different suppliers conforming to different standards, all with different tolerances for misalignment and detent could keep connections and reconnections from working smoothly or, in extreme cases, at all.

Shock and vibration worsen connector problems. Mil/aero applications need to withstand shock and vibration, but plenty of commercial applications do as well. We strongly advise against mixing and matching, especially in products that will experience environmental shaking.

Choosing SMP3

Cinch deliberately chose to work with one standard rather than both, in no small part to minimize possible confusion. We opted for SMP3 because that standard represents a larger share of the market.

SMP3 follows the prior generation, SMPM and SMP, and like those two lines, it features a floating bullet. The new SMP3 line has a higher frequency (up to 67 GHz), and the connectors are 30 percent smaller than SMPM.

One final note 

DLA August 28, 2020 memo: At the September 24th, 2014, US46F Working Group meeting held at DSCC, DLA Land and Maritime became aware of intermateability issues among the different SMP and SMPM connector manufacturers. An engineering practice study was conducted to identify possible root causes of intermateability problems and their subsequent solutions. Several incompatible dimensioning tables were submitted. Due to the nonconformities evident among the various manufacturers DLA recommended at that time to continue the effort to enforce standardization across the entire MIL-PRF-39012 and MIL-STD-348 ecosystem. Efforts were made in FY2017 to determine feasible dimensional requirements for the SMPS connectors. Results: It was decided to create a new series in the SMP family. There will be an SMPS and SMP3 design.