Resistance/impedance measurement

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mkrtich.nazaryan

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Hi folks,

I have faced a problem and need some help/suggestions.
I want to measure the pin impedance of RF IC (wifi chip) at point A (see below pic). The problem is a RF IC, Low-pass filter and ESD protections under the RF shielding and I have not access inside of shielding.
Below is simplified circuit.
Is there a way to measure the pin impedance of the RF chip? TDR? or anything else?
Thanks a lot for help.
 

In a finished circuit I think it is not possible to measure the IC pin impedance alone. As your IC in the schematic is capacitance-coupled to point A, you can cut the line or remove the capacitor to get directly to point A. Take care DC voltage is not present before you connect any impedance-measuring device to point A.
IC manufacturers usually give the IC pin parameters in data sheets, so designers can match IC pin impedance with connected circuits.
 
thanks for reply.
Yes, you right in the datasheet already mentioned input impedance. But in my case, the output power at antenna is low ~ 3-4 dBm than expected. I able to control (change) transmission power but I have some concern that ESD protection circuit or shunt cap inside of RF IC is damaged and made some impedance mismatching which will reflect transmission power from pin back to IC (the chip getting very hot when enabling that pin transmission).
Just wondering is a way to identify pin shorted? Or do some measurement and compare with good pin and see any differences (in my wifi IC there is identical pin for same frequency band but for 2nd antenna).
 

Another possible solution..
If you know the components' values and part numbers, you can build the same circuit on a seperate PCB and measure it standalone.
Then measure the input impedance of the circuit at the antenna input after that extract the s-parameters of the first copied circuit from these second measured s-parameters.
OK, it's not a very straightforward method but it's worth to try..
 

I think BigBoss is right but troubleshooting is always tough with RF modules with ICs. I would try to get a healthy receiver block to confirm the first suspect is damaged or not, by transmission testing.
I also wonder why the genius circuit designer located the EMC protection device AFTER the LNA?! EMC protection should protect just the sensitive LNA, otherwise it serves nothing.
 

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