custom USB to BNC cable and impedance termination for oscilloscope - Page 1
Forum section: electronics / beginners

author: mehdifat
author_rank: Regular Contributor
timestamp: April 30, 2025, 01:47:52 am
content: i have some questions about bnc termination i hope you can help with.i learnt from my research on forums and web that a cable must be terminated with it's own amount of impedance.i made a usb to BNC cable so that i could have good grip and higher bandwidth at 1x. i ordered bnc plug and 50 ohm terminator but obviously my usb to bnc cable is of unknown impedance and i don't have anything to measure impedance with. i attached the cable pics.1- is there any way i could get the ballpark impedance of my cable with what i have? i have multimeters that measure capacitance but no inductance meters and i have two oscilloscopes Owon 1022 and Fnirsi 1013d.2-even if i can't measure the impedance , will it still be better to terminate it with 50ohm than no termination at all, right? i sometimes see different shape of the wave like it is ringing and clearly higher noise amplitude with the usb to bnc cable with no termination compared to 10x probe. it seems that higher frequencies are reported with higher amplitude and they ring sometimes. ringing seems to be more dependent on the waveform. i measure ripple and noise of the PSUs.3- is adding 50ohm or any other amount of termination dangerous and can it damage the scope ? i mean it is a load right and my under-test-PSU voltages go from 9V to 20volts.4-at what frequency, we normally start to see ringing or wrong amplitude without bnc termination?thanks for your help

author: TERRA Operative
author_rank: Super Contributor
timestamp: April 30, 2025, 05:13:41 am
content: The way you have constructed your cable means it is not a controlled impedance, so trying to match it or terminate it will be pretty futile.I suggest taking a look at some USB test fixtures (Keysight, Tektronix, etc sell or sold them) used for characterising USB comms and do what they did.

author: macboy
author_rank: Super Contributor
timestamp: May 07, 2025, 01:41:23 pm
content: Start from the beginning: what are you trying to measure?

author: radiolistener
author_rank: Super Contributor
timestamp: May 07, 2025, 07:51:08 pm
content: Since power supplies typically have very low output impedance (usually less than 1 Ω), a 50 Ω resistor should be placed in series at the power supply end of the coaxial cable.On the oscilloscope side, where the input impedance is typically high (around 1 MΩ), a 50 Ω resistor should be connected in parallel.This setup creates a matched 50 Ω impedance environment, minimizing signal reflections at both ends.However, modifying your cable to add a series resistor at the power supply looks impossible, but matching the impedance at the oscilloscope side is generally more important, as it prevents reflections from that end. You can typically ignore impedance matching at the power supply side in this case, since no signals are propagating from the oscilloscope toward the source.That said, impedance matching is only necessary when measuring high-frequency signals. For DC voltage measurements, such precautions are not required, and a direct connection is usually sufficient.