Test, problems, and solutions for the SMA to IPEX cable for ADALM-PLUTO second channel

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Another important piece written by Gianni IW1EPY about the long-standing problem of the IPEX-SMA cables used for the second channel of the ADALM-PLUTO. Here is his tutorial:

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A set of different SMA female to IPEX was purchased from China:

Question: Is the IPEX connector the problem?

What about the IPEX?

Same cable length, same SMA with adapter for RG 1.13 and an IPEX IPEX junction.
See also the channel 2 Pluto test of one of those cables.

This means that with a good SMA connection and a couple of IPEX, it can work up to 16 GHz.
First, no blame to the cable producer for usage up to 6 GHz is assured, but for us, that we take out the last still of lemon juice is not enough.
We use the second Rx as a reference for the VNA up to 12 GHz, so we would like a flat response in this range.
What is wrong?

These types of SMA cannot function properly; the correct Type 2 has many discontinuities and impedance changes that limit its use to around 6 GHz. On the left, Type 1 is its incorrect usage, as shown by my test with the proper coax cable. Inserting the entire coax cable, including the braid, into the SMA hole and soldering it is not correct. What happens? The tin solder connects the braid only at the rear entrance of the SMA, leaving a ground tube unconnected, which can be seen in the picture.

Solution

Find the SMA with the correct cable hole to enter with the insulated central coax cable and solder the braid to the outside of the SMA.

For RG 1.13, use a small-hole SMA but avoid using too much solder; connect all the braid tube to the inner tube of the SMA.

By now, the only solution to the problem is to purchase the IPEX cable and do the SMA connection.
Two good SMA-IPEX will allow the correct use of the second channel of ADALM-PLUTO up to 12 GHz.
Even without test systems is possible to use ADALM-PLUTO to make a comparison between cables to get the right one.

Due to the stiffness of the cable is a bit longer to accommodate a larger bend of the cable vs the RG 178, which allows a shorter path but a worse performance.

73 51 de IW1EPY