Electronics > RF, Microwave, Ham Radio

Testing the effects of releaving ground plane beneth a component

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joeqsmith:
This experiment is just for the fun of it.  For some background, see the following thread:
https://www.eevblog.com/forum/rf-microwave/can-a-siggensadirectional-coupler-be-used-as-a-poor-mans-scalar-analyzer/msg3605548/#msg3605548

A few takeaway points from this thread are:


--- Quote ---I'd like to test the level of impedance mismatch caused by component footprints on PCB microstrip layouts, up to 3 GHz.
--- End quote ---

--- Quote ---... I already did some simulations and the results even looked real ....
--- End quote ---

--- Quote ---... motivation is to get a reality check - can I trust my simulation setup?
--- End quote ---

--- Quote ---A 0603 AC coupling capacitor footprint between a 50-ohm microstrip, far-end terminated by a 50-ohm coax load, One has solid ground plane and another has cut out.
--- End quote ---

My intent:

--- Quote ---I had made up a test board using some ATC (prior to the AVX debacle) 100B RF jelly been parts.  Three caps were mounted to a test board configured as a shunt thru, series and shunt.  I could mount a fourth capacitor to the same type PCB, series configuration, using the same connectors.  We could then compare the return loss for the two PCBs.  It's not much of a test but may give you some confidence in the variance.   I could then remove  2mm sq from the ground beneath the cap and remeasure it.    I could provide you with details about the build that you could then attempt to simulate and see how the results compare. 

--- End quote ---

I would also like to use both the original NanoVNA and along with the V2Plus4 to collect this data.   The OP wasn't forthcoming with details about their particular simulation but if anyone has access to ADS, HyperLynx or other simulators and want's to donate some time, that would be very helpful.  My plan would be to provide enough detail to allow you to create a model of the hardware (assuming I am able to detect any changes).  Again, the goal is to see if the simulation give even a rough result of the actual hardware.     

To give this test any sort of meaningful result, if that were at all possible, the first thing I plan to do is make a new set up standards and characterize them against a known set rather than using the ideal model.     

Any feedback on this experiment is certainly welcome.   

joeqsmith:
[attach=1][attach=1][attach=1]Recently I had posted a demonstration of my software for these low cost VNAs.    Part of this was measuring a capacitors ESR.   To make these measurements, I had mounted a few parts from American Technical Ceramics to a coplanar waveguide that I plan to use as a control for this experiment.   The parts I used area a 100B331JP 200X, 330pF 200V 5%.   These particular parts were produced prior to AVX taking them over.     



A second board will be assembled using a component from the same lot.  The same connectors and series coplanar waveguide will be used.  We will start by measuring the differences between two boards.   Assuming there is little difference, we will then relieve the area beneath the capacitor and remeasure the board.   Seems simple enough. 
 

joeqsmith:
Shown are the new calibration standards along with a second test board with the ATC device mounted to it.

The range will be limited to 1.5GHz.  This is the upper limit of the original NanoVNA.  The test boards behave poorly as we move beyond this.   

I plan to use the Short and Open from the PCB standards but will use one of the sorted Mini-Circuits ANNE parts for the load to improve the return loss. 

A MidWest Microwave terminator will be used to terminate the unused port on our test boards.  This part was also sorted based on the return loss. 

joeqsmith:
I wanted to get a feel for what sort of an difference we would see when removing material below the component as a sort of swag.  So a third board was populated with the same capacitor.   

For starts I inserted the test board several times, hand tightening the components.   I did this because so many internet experts have told me that hand tightening is every bit as good as using a torque wrench.   I then used my Dremel to cut a 1mm slit below the component and made one last sweep. 

joeqsmith:
Several people wrote about their horror stories of damaging connectors with a torque wrench.   A common theme was how the connectors on the NanoVNA are poorly soldered and were easily damaged. 

I set out to do some destructive tests on some edge mount connectors.  Some, fairly expensive, others bottom of the barrel.   Some where not even soldered to the board.  This video should give you some idea of just how much force it really takes to rip a connector from a board.   

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