AC coupling needed in series with SAW filter

[borgestrand]

Hi, 

I'm combining a SAW filter with your nRF52840 and an RF MUX. The MUX requires AC coupling capacitors. Is that required in series with the SAFFB2G45MA0F0A as used on your nRF9160 dev kit? 

The MUX is Skyworks SKY13351-378LF.

Thanks,

Børge

[ketiljo]

That's hard to tell without knowing how the internal part of the filter is made. But I did a quick measurement on the nRF9160-DK and there is no DC short to ground on either side of the filter. So you shouldn't need DC block capacitor on the filter side of the switch. 

[borgestrand]

Dr K, 

Could you please check if there is a DC path through the filter from input to output? If there is, I guess I must AC couple since the filter sees just a series inductor to the radio's antenna pin. 

Cheers,

Børge

[ketiljo]

Good point! I checked this and there is not a DC path through the filter. 

You're correct about the antenna pin. This is DC grounded via the series inductor and  the internal balun.

[borgestrand]

Really? I suspected there was DC bias on the antenna pin. Good to know it's AC coupled on the inside.

Børge

[ketiljo]

The ANT pin is connected to the secondary winding of the balun transformer, where the other part ot the winding is grounded. No DC on the ANT pin anymore. On nRF52 and newer that is: