RF test points to measure low-level power supply noise - Page 1
Forum section: electronics / projects

author: matthuszagh
author_rank: Frequent Contributor
timestamp: August 18, 2023, 05:05:52 am
content: I'm designing a GNSS receiver PCB that requires low noise power supplies. As part of the design I was thinking of adding in small RF connectors on the power supply lines to be able to probe the supplies to measure noise. I was planning to use something quite small like U.Fl, or maybe MMCX for the greater mechanical durability.Does anyone else do this? Any thoughts, recommendations, pitfalls? I understand the test points can act like little antennas. I don't expect that would produce a large effect, but I could also place shielded opens on the connectors if that's a problem.Also I'm aware that placing a coaxial cable across a power supply will add parallel capacitance and change the PDN impedance. I think that's ok at the frequencies I'm working with and because there's already quite a bit of capacitance on the rail anyway. But, if it's an issue I can always add some series resistance at the connection to create a 10x attenuation effect.Another thing I was interested in was to potentially add a 2nd connector to perform PDN impedance measurements (using a shunt-thru method). On this page (https://resources.altium.com/p/impedance-testing-power-subsystem-design) it's recommended to place the 2nd connector at least one inch away from the first. I'm not sure I have the real-estate for that.Anyway, mostly just curious to hear what other people do.

author: tautech
author_rank: Super Contributor
timestamp: August 18, 2023, 07:33:47 am
content: TBH you are better to apply KISS principles and just allow for SMD test points to be added onto power rails if and when needed then remove them once tests are done.Because you can reuse them the outlay is small.Pick what fits your needs and check which local supplier has them:https://www.keyelco.com/category.cfm/Keyelco/Test-Points-Tips-Probes-Clips/id/518

author: Someone
author_rank: Super Contributor
timestamp: August 18, 2023, 07:58:04 am
content: Quote from: tautech on August 18, 2023, 07:33:47 amNot great coupling up into the GHz region with those, most people switch to some sort of solder in probe.Quote from: matthuszagh on August 18, 2023, 05:05:52 amGiven you're tight on space generally dont bother with footprints for test connections unless they are necessary for production testing. Hand wire a pigtail or tiny RF socket across a decoupling capacitor(s) as you've said perhaps with a series resistance (and a series capacitance if the measurement input cannot withstand/offet for the power rail voltage).

author: Smokey
author_rank: Super Contributor
timestamp: August 18, 2023, 08:11:52 am
content: I can't imagine the layout is so tight that you can't fit a U.Fl footprint.I'm not a real RF guy, but U.Fl sounds like a good option to me.

author: tautech
author_rank: Super Contributor
timestamp: August 18, 2023, 10:12:04 am
content: Quote from: Someone on August 18, 2023, 07:58:04 amYou imply a GHz region scope is required to see PSU noise ?Sceptical much.Screenshots please.

author: David Hess
author_rank: Super Contributor
timestamp: August 18, 2023, 10:26:17 pm
content: I have done what you describe using BNC connectors and it worked fine. I also used BNCs to bring out other signals for measurement. My preference would be to use something smaller and more distinctive like SMB connectors, but in practice any RF connector will work fine.

author: Someone
author_rank: Super Contributor
timestamp: August 20, 2023, 10:30:52 pm
content: Quote from: tautech on August 18, 2023, 10:12:04 amThat is what the OP asked for, GNSS (1.2/1.6GHz perhaps others) low noise RF.Quote from: matthuszagh on August 18, 2023, 05:05:52 amWhy would the OP ask for RF connectors if they were not trying to couple RF? Low noise receivers do need low noise power in-band, as the PSRR is usually abysmal. Same issues with GHz serdes. This is not an unusual measurement need.

author: nctnico
author_rank: Super Contributor
timestamp: August 20, 2023, 10:41:11 pm
content: Quote from: matthuszagh on August 18, 2023, 05:05:52 amDefine low noise. At several tens of kHz most regulators won't be able to regulate and all noise comes from the load doing switching.You have to split this 'problem' into two different problems: 1) measure noise coming from the regulators (line & load regulation) up to -say 100kHz-. For this a good tool is an LF spectrum analyser or signal analyser. A low bandwidth (<10MHz) high resolution oscilloscope will do as well. 2) Do a PDN analys using a network analyser. With this you can measure if the impedance of the power supply network is low enough along the bandwidth of your circuit.

author: David Hess
author_rank: Super Contributor
timestamp: August 21, 2023, 12:15:15 am
content: Quote from: Someone on August 20, 2023, 10:30:52 pmCoaxial RF connectors and cables are also convenient for making low level measurements down to DC. In may case I use them to get low level signals away from switching power supplies where the loop formed by the ground lead of a typical oscilloscope probe would pick up way too much noise from the coupled magnetic field. Plugging the probe tip directly into a coaxial socket also works but is often not convenient.

author: Someone
author_rank: Super Contributor
timestamp: August 21, 2023, 01:10:55 am
content: Quote from: David Hess on August 21, 2023, 12:15:15 amWhich was my first reply, coax pigtail... and questioning why a permanent connector was seen as valuable.

author: tautech
author_rank: Super Contributor
timestamp: August 21, 2023, 03:15:02 am
content: Quote from: David Hess on August 21, 2023, 12:15:15 amSmart PCB layout/design negates the need for scope probes with reference leads where just the reference spring can be used.