Selecting coaxial cable and connector for PCB referring to required frequency

Question

I want to transmit my 24GHz signal from PCB through coaxial to antenna. I wonder how to chose best match for it, also the power is -5dBm, 50 ohm. It's based on ADF5901 2ch PA. I have found two good site but still I think I didn't found my way:

https://www.rosenberger.com/us_en/test-measurement/precision_connectors/

http://precisionconnector.com/

https://www.te.com

Most of them are for cable and not for connecting to PCB, specially I want it to be surface mount. The cable will be too small, the antenna is placed on other board and have few Centimeter distance from transmitter, I want it's cable be flexible and thin.

It is my first project on RF board I have zero experience, please help to chose the best cable and connector.

Answer 1

At 24GHz any flexible cable/connector system is going to have horrible losses, I would be thinking about handi-form if not hardline for this, probably with an isolator on each end to handle the inevitable mismatch. I would not be at all surprised to see 6dB of losses between the connectors, the isolators and the line loss!

Actually, I would just put the damn chip on the antenna board, less losses that way and overall a much simpler design.

Connectors for 24Ghz are NOT trivial, so go to some lengths to avoid them, far better to move the IF and reference frequency between boards and only have the quick stuff on the antenna board. A reference and IF at a few MHz to a few GHz are much easier to deal with then 24GHz across connectors.

To give you a flavour of just how tough this stuff is, my Gore test port extension cables cost well over $1,000 for a foot of cable with two 2.92mm precision connectors (And they are NOT specified to 24GHz!).

You do have access to a VNA good for 24GHz? And the cal kit to make it useful? Otherwise you have basically no chance of pulling this off.

Answer 2

There are plenty of connectors that can support 24GHz, I usually use Hirose, as their range is quite comprehensive, as well as they are usually available from most vendors.

See here

I would choose SMA as it is a widely available part and the cable assembly tools are easy to find.

When it comes to cables, look at the pre-made cables, as those are tested to work at those frequencies. If you still want to make your own, look at the datasheet for these cables to see what connectors/cables the use.

Please note that not all connectors or cables are made the same, so make sure you check the datasheet for the maximum frequency supported, and the impedance.

As others have mentioned, ensure that you choose a substrate that can take these frequencies, such as Rogers or other exotic materials.

Answer 3

EDIT: Subsequent to posting this answer, the OP has clarified his original question.

To get your match, you need your PCB tracks to have the same impedance as the coax and the coax connector. You'll need to calculate this based on your chosen track topology (ie. microstrip/stripline/cpwg) and your stack-up (specifically, the height and the permittivity (er) of the substrate).

There are calculators for these all over the place, such as on Microwaves101: https://www.microwaves101.com/calculators

Other users will probably give you a more detailed answer, but I do suggest you read up on some RF design guides/tutorials, such as this one on the Maxim website: https://www.maximintegrated.com/en/app-notes/index.mvp/id/5100

Answer 4

You need to select the best design for dielectric properties, controlled impedance, microvias, layout, connectors sources and have a VNA full calibration experience using semi-rigid copper cables to make this happen.