Extend probe point with Coax? - Page 1
Forum section: electronics / projects

author: Thebrakshow
author_rank: Newbie
timestamp: December 17, 2020, 08:25:50 pm
content: It is an ok practice to extend a probing point of a PCB with a bit of coax cable. So for instance, on the PCB side, the coax is cut and soldered on the test point (and ground). The other side the cable in question has a male BNC connected to a BNC F-F-F T-coupler with a 50 ohm termination which would then allow you can use the scope probe tip attachment that is a male BNC? This way the scope probe connects to the scope as normal (using its in 10X mode, 1M termination). Or, would it just be better to connect the coax directly to the scope and use the channel in 1X mode (50 ohm scope termination setting) ?I do know that using 1X and or coax as a probe is very situational, in this case I am asking specifically about probing the gate's and drain's of each mosfet that create the 3 phase inverters.The reason I ask is because at work I'm trying to get some scope captures of brushless motor driver mosfets and by the nature of the product I physically can't attach a scope probe while the device is in operation (100% mechanical constraint). My only option is to run wires to the probing locations.Thanks!Brak

author: Marco
author_rank: Super Contributor
timestamp: December 17, 2020, 10:21:25 pm
content: 1X mode is not the same as 50 Ohm internal termination mode.Can your source drive an extra 50 Ohm parallel load without breaking a sweat? Then sure, you can solder in coax and terminate at 50 Ohm at the scope. If not, but if it can handle say 2K you can use one of the various forms of Z0/Lo-Z probes.If it needs Megaohm range impedance at DC you'll need to be trickier.

author: David Hess
author_rank: Super Contributor
timestamp: December 17, 2020, 11:57:34 pm
content: Either method can work but if a 50 ohm termination is used on the coaxial cable, then the coaxial cable might as well go directly to the oscilloscope.I sometimes use a short length of coaxial cable without any termination connected directly to the x1 or x10 probe tip. Bandwidth is limited by the capacitance of the unterminated cable and x1 probe if used so 6 inches of coaxial cable is not a significant length.

author: srb1954
author_rank: Super Contributor
timestamp: December 18, 2020, 01:24:14 am
content: Quote from: Thebrakshow on December 17, 2020, 08:25:50 pmWhat sort of voltage levels are you looking at?You need to be aware of the maximum voltages that can be applied to a 50terminator, particularly those on scope inputs as these are typically only rated for max 5V RMS.

author: PeteH
author_rank: Unknown
timestamp: December 18, 2020, 01:32:58 am
content: I do this a lot....Can use a 50 launch resistor from the measurement point or AC coupling cap for AC measurements if you're concerned with voltage levels.Also can use BNC AC coupling adaptors.Directly coupled into scope.... Watch your voltage rating as others have said.PCB layout with this in mind makes it a lot easier...

author: jonpaul
author_rank: Super Contributor
timestamp: December 18, 2020, 11:31:07 am
content: Hello Brak:Needed info to have a good answer:controler bus voltages, currents or power?Mains 3 phase/240/120V?Motor HP?Type of scopes available?As MOSFET gates may be elevated eg in H bridge, a differential measurment may be needed.Placing a 50 Ohm scope on a MOSFET gate may blow the driver or the FET. Certainly the driver wave will be changed.Are you experienced about dangers in probing high power equipment?Bon ChanceJon