How to know antenna is properly matched with IC ?

[wanyancan]

Hi,

In application note 'nWP017 - Antenna tuning', it provides quite a lot information about how to tune antenna.

But, once it is properly tuned, how do I know the matching network is also proper for IC ?

I've got a VNA to tune the antenna matching network with calibration correction to the point where it is connected to IC.

But once the LC network is installed on the pcb between IC and antenna, how can I measure whether these two is still matched? 

If I have a spectrum analyser or oscilloscope, once I place probe tip on the ANT pin of IC,

it will surely introduce an excessive input capacitance and change the output RF signal. 

Obviously I can't add additional fixture for VNA to measure S11 any more. 

So my question is how to tune matching network with both IC and antenna in place? Is there any kind of "chip VNA" available? 

Best Regards,

Leo

[ketiljo]

You have to cut the track between the matching network at the 50 ohm point. Measure S11 into the antenna using the VNA as described on nWP017. Then measure S11 into the matching network with the chip in TX or RX mode. Both measurements should be as close to 50 ohm as possible. 

You have to physically break the connection between the chip and the antenna to do the measurements.