U.FL connectors to probe parts of the circuit -- need to terminate when unused?

[maelh]

Hi,

I am planning to make circuits with U.FL connectors added at some strategic places to probe the circuit (the places I want to test have 50 Ohm impedance).

Now I wonder how it will affect the circuit while no U.FL cable is connected (and therefore there will be no 50 Ohm termination). Will a short trace connecting to a signal trace and leading to an unterminated U.FL connector significantly affect the circuit/signal? For now I plan to stay below a few 100 MHz.

[RFDx]

Assuming the voltage/power on the main 50 Ohm line is sufficient, put a 450 Ohm resistor in series with the connector. By terminating the connector with a 50 Ohm load you get a 10:1 probe that doesn't disturb the rest of the circuit.

[magic]

On various Wi-Fi equipment I have seen weird things which look a little like U.FL connectors but aren't. They are placed in the middle of antenna transmission lines with no other components near them and this appears to be good enough. I suppose they are special test connectors designed for the exact purpose you have in mind, but I have no idea what they are called, whether you could buy them (with reasonable MOQ), and what sort of probes to use with them.

edit
Below 100MHz you can maybe get away with some degree of dodginess like a small unterminated connector hanging off the line...

[Weston]

If you connect anything 50 ohm terminated to the extra connector without removing the source / termination you no longer have a 50 ohm system.

If you just want to measure voltage waveforms, a series resistor will significantly reduce the loading.

If you want to measure the impedance or drive the load with an external source, consider RF connectors with a switch https://www.murata.com/en-us/products/connector/switchconnector

These connectors are pass through when a mating connector is not inserted, but when a mating connector is inserted one end is disconnected and the other end is connected to the mating connector.



These connectors are not super uncommon, but the nomenclature is pretty annoying. From what I can tell, these parts are mating compatible with U.FL but the female connectors are not footprint compatable https://www.digikey.com/en/products/detail/murata-electronics/MM8430-2610RA1/1775921

[maelh]

On various Wi-Fi equipment I have seen weird things which look a little like U.FL connectors but aren't. They are placed in the middle of antenna transmission lines with no other components near them and this appears to be good enough.

Do you mean the "RF connectors with a switch" mentioned by Weston, or something else? Is there a picture or do you know something more specific maybe?

[magic]

Here's a picture. Perhaps it's the "connector with a switch", this could explain how it works without introducing double loading on the circuit under test.

BTW, this particular connector is not compatible with U.FL. I tried and failed to plug a U.FL Wi-Fi antenna into it. Then I noticed that U.FL has a pin in the center and this one doesn't, so it's obviously a different thing.

[nali]

That connector looks like it's for a coaxial probe used in a "bed of nails" or flying probe test jig. Yes, it's the wrong gender for U.FL

Depending on your requirements you could maybe use a simple resistive splitter if you can accept the loss, although you'd need to keep the U.FL terminated when not in use.

[maelh]

Thanks for all the replies.

I suppose using an RF probe would also be an option.

Now I mostly wonder about U.FL connectors for applying signals to circuits for testing (such as a filter), and how it affects the signal quality when you later add a parallel connection to a purpose built-oscillator (and thereby effectively leave the U.FL connector unused and unterminated). I guess I'll make some experiments, and report back

[markus_jlrb]

Keep in mind, that using such a terminal with the switch inside
could cause problems at the following stage when the input load
is removed (oscillation, destruction) while connecting the probe.
Such terminals are ok for adding an external antenna or measuring
power level while disconnecting the antenna or other passive stage.
But in the case you cascade amplifiers or mmic gain blocks it's better
to use the 450 Ohm method as proposed for attaching probes in between.

Markus