Micro wave designing for testability? - Page 1
Forum section: electronics / rf-microwave

author: Mr Simpleton
author_rank: Unknown
timestamp: January 24, 2019, 02:25:35 pm
content: Beeing out in very very deep waters, away from my safety zone, how to implement test points on a microwave PCB @ frequencies up to 10 GHz for a multi stage circuit?As the board is stuffed with 0402 components and LCC-chips, all at one go, it's like a "go/no-go" thing. Very hard to pin-point any problem such a a mismatch or low gain from a specific stage.Real life does not match simulated dataAny suggestion is welcome...

author: awallin
author_rank: Frequent Contributor
timestamp: January 24, 2019, 06:59:03 pm
content: nothing is going to be easy or cheap at 10 GHz??either of these two I guess:- make everything modular/connectorized, so you can test component-by-component in the signal chain with a 50ohm SA/scope- buy expensive or very expensive 10GHz active probe

author: daqq
author_rank: Super Contributor
timestamp: January 24, 2019, 08:29:35 pm
content: These test points/switches go up to 6GHz:https://www.amphenolrf.com/media/pdf/Market%20Solutions/Test%20Point.pdfhttps://www.rfmw.com/datasheets/rosenberger/15K101-40ME4.pdfOne of these goes up to 11GHz:https://datasheet.lcsc.com/szlcsc/Murata-Electronics-MM8030-2610RK0_C91632.pdf

author: Mr Simpleton
author_rank: Unknown
timestamp: January 24, 2019, 09:36:18 pm
content: Thanks for the help. I had seen those connectors before, but dismissed them as most seem speced up to 6 GHz, may have to look again.Buying probes, expensive or not, could pobably not help find impedance errors.Splitting up into small modules was my first suggestion, it was turned down. Still from an engineering side, seems the best vay though.CheersNever too late to give up.......

author: Howardlong
author_rank: Super Contributor
timestamp: January 24, 2019, 10:06:47 pm
content: Murata MM8030 series, rated to 11GHz. Specifically I use MM8030-2610 RKO on the board at various 50 ohm inter-module test points. The beauty of these is that they have an integrated isolation switch when you insert the reassuringly expensive test probe MXHQ87WA3000.Once characterised, replace the connector with an 0402 cap for production.

author: Mr Simpleton
author_rank: Unknown
timestamp: January 24, 2019, 10:50:26 pm
content: Thats great information! Will look at the Murata connector, and the possibility to replace with a 0402 cap is just icing on the cake! Excellent!Cheers

author: daqq
author_rank: Super Contributor
timestamp: January 25, 2019, 10:45:39 am
content: QuoteI haven't used them myself, but I saw them used in several RF devices. Two per 'block', one on the input and one on the output. Essentially, two were in series between every module, allowing you to completely isolate the block under test.

author: coppercone2
author_rank: Super Contributor
timestamp: January 30, 2019, 05:40:37 pm
content: If you get a hf rf probe, how do you connect it to circuit?Is there like a stripline geometry? Special test point footprint

author: coppercone2
author_rank: Super Contributor
timestamp: January 31, 2019, 09:19:36 pm
content: do you just put a via and a exposed bit of trace to probe next to it?Or do you need a blind via to touch the ground plane from the top?or something else? Dummy split?

author: Howardlong
author_rank: Super Contributor
timestamp: January 31, 2019, 09:52:14 pm
content: Take a look at the MM8030, all will become clear. It’s an SPDT switch, plugging in the coaxial probe activates the switch to the probe, isolating it from the rest of the circuit. As daqq suggests, you could add two in series allowing you to isolate bidirectionally. In practice, I just have one and unsolder it amd turn it around during characterisation.

author: ogden
author_rank: Super Contributor
timestamp: January 31, 2019, 10:22:10 pm
content: Quote from: Mr Simpleton on January 24, 2019, 09:36:18 pmSplitting into small modules is waste of money and time, also there is no such thing as non-intrusive MW probing. Usually RF engineers lift bypass capacitor (or other component) and solder test probe pigtail semirigid coax cables to (part of) circuit, like this:https://p1db.com/page/rf-pigtail-test-probesObviously you shall plan such "test points" in your design in form of exposed ground plane. This is design phase approach. If test-ability is needed for production and servicing, then already mentioned test connectors. It should be hellova (& expensive) device if dedicated tests points are needed.

author: DaJMasta
author_rank: Super Contributor
timestamp: January 31, 2019, 11:33:50 pm
content: Those connector style test points seem to be Rhode & Schwarz's preference in test equipment. You get the PCB mount connector, then an AC coupling cap or other small passive with an extra pad, one side goes to the rest of the device, one side goes to the connector (and sometimes a 50 ohm termination).Not suitable for dense boards, really, but you can also just have a bit of a microstrip coupler run along the signal of interest, terminate the ends, and put your connector on there. You'll always be losing a little bit of signal coupling into it while operating, but then your probing can be done with the device's full path complete and with a minimum of intrusiveness.

author: Howardlong
author_rank: Super Contributor
timestamp: February 05, 2019, 11:34:10 am
content: I agree that there's no such thing as non-intrusive probing.But what's the benefit of a pigtail over a designed-in connector with integrated isolation switch that is replaced with a coupling cap for production?A big benefit of the connector method is that you can build your own short/open/load for VNA calibration, something that would be difficult for soldered in pigtails.

author: kruszi
author_rank: Newbie
timestamp: February 05, 2019, 12:30:34 pm
content: I think that it really depends on the situation and what you are trying to achieve. Do you want to do end of the line tests with adjustments at hundreds of units? Or is it "only" during development with a few prototypes? How often will you need to probe at the specific point? How much room is available? Is it acceptable to desolder components before testing? Is it acceptable (consumes a lot of time) to solder a semi-rigid coax to the point?Think of tuning the antenna of a small and tightly built handheld device. There you have no room and you can't accept semi-rigid with a large SMA connector since you need to close the housing for proper tuning. For example, I decided to go for UFL connectors in a small remote control (PCB space extremely limited, rigid coax not possible - no possibility to fix mechanically) and drilling a small hole into the housing. But those UFL connectors have only like 10 plug-cycles so you can do OSL-calibration (=3 cycles) + calibration verification (-3 cycles) = 4 plug cycles remaining. After that -> replace UFL connector + cable. I only needed to do this once so it was acceptable to waste some RF-material. If I had to do this procedure in every unit I had to come up with a much better solution.The isolation switch is useful if you want to tune S11 / insertion losses (like at an antenna/filter) or S21 in two-port devices (filter, amplifier). You need to disconnect the stage you are tuning from the previous and next stage (possibly with perfect 50R termination) for tuning. Without the isolation switch in the test-port, you would need to cut traces (relatively wide gap which needs to be bridged by solder later on) or remove series components most often leaving you with stubs that might harm your measurement, tuning or network analysis.

author: Howardlong
author_rank: Super Contributor
timestamp: February 05, 2019, 04:33:29 pm
content: FWIW, the Murata MM8030-2610 SPDT receptacles are rated for 100 cycles, and cost about $0.30 each in qty 100.Page 12:https://www.murata.com/~/media/webrenewal/support/library/catalog/products/switch/switch/o33e.ashx?la=en-usThe MXHQ87WA3000 probe itself has a 2000 cycle durability.Page 20:https://datasheet.lcsc.com/szlcsc/Murata-Electronics-MM8030-2610RK0_C91632.pdfFor comparison, SMA connectors are typically rated with a 500 cycle durability. My own anecdotal evidence suggests that mechanical failure is around 6,000 cycles in production fixtures, but doubtless the electrical characteristics will have degraded long before this. Under the microscope, the failure mode is that the centre pin suffers abrasion from the receptacle, and eventually it becomes to small to make a consistent electrical connection. The change is almost cliff-edge, over the period of about ten cycles, the connector will go from consistently connecting to being consistently disconnected.