ST25R3916B matching network parallel tunable capacitor location

[wrig0421]

Hi,

Comparison of STEVAL-ST25R3916B UM3033 [pg 22] & ST25R Antenna Matching Tool...Specifically the matching networks show a difference in location of parallel tuning capacitor relative to antenna connection. 

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  • UM3033 antenna matching + filter [above].  Notice the location of the parallel tuning cap [C308] in relation to antenna connection ANT300. 
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  • The ST25R matching tool [above] shows antenna matching + filter.  Notice the parallel tuning cap is in spot closer to antenna connection than the UM3033 schematic. 
• The physical STEVAL D25R16BA board shows the tunable cap next to the antenna [see red box below].  Silkscreen confirms this also. 
  • 

     

Based on the above it seems that the STEVAL-ST25R3916B antenna matching schematic in UM3033 DOES NOT match the physical board. 

My question: 

1. Should tunable capacitor be placed as shown in the ST25R matching tool or should it be placed as shown in STEVAL-ST25R3916B UM3033?
   • I've created a custom PCB following the schematic in UM3033.  My results aren't great from matching perspective and trying to debug this.  I'm wondering if the location of the tunable capacitor is a concern. 

Thanks in advance.

[Travis Palmer]

Hi wrig0421,

electrically seen, the three setups (UM schematic, antenna matching tool schematic, layout of the STEVAL-ST25R3916B) are the same.

The EMI filter is filtering all frequencies above the 13.56MHz. Only the 13.56MHz voltage is resonating between antenna inductance and parallel capacitors. At this frequency range the electrical length is quite long and the placement of +/-2mm does not play a big role. 

It means, since the capacitors are all in parallel, the total capacitance value counts.

BR Travis