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Posts: 24
Registered: ‎07-26-2017

390Mhz with SI4468

I am using the Si4468 for a 915Mhz and a 390Mhz device.  With 915Mhz I can close the link with a 110dB of attenuation between transmitter and receiver using 2GFSK at 500kbps.  I use WDS and change the frequency to 390Mhz and then change the PA matching network according to table 5 in AN648.  What I have noticed is that at 915Mhz my power output is around +12dBm, at 390Mhz my power output is +18dBm however at 390Mhz the link will not work with anything more than 90dB of attenuation.  I have checked the matching networks and even changed parts but I can not get better than 90dB.  I figured the problem is not the transmit side of the link as I have +18dBm output and must be on the receive side. 

 

Has anyone seen such a problem before or does anyone have a suggestion of what to try? 

 

Thanks

Posts: 93
Registered: ‎02-25-2014

Re: 390Mhz with SI4468

The RX path matching network should also be updated for the proper frequency for sure. 

On the other hand, a 30MHz crystal harmonic falls into the RF band (13*30MHz=390MHz) which may also cause RX sensitivity degradation, so it could be suggested to try to use another crystal frequency - or check the RF link in a slightly different RF band.

Posts: 24
Registered: ‎07-26-2017

Re: 390Mhz with SI4468

Would you expect 20dB of reduce sensitivity for the 13th harmonic? 

Posts: 93
Registered: ‎02-25-2014

Re: 390Mhz with SI4468

Sensitivity measurements at a crystal harmonic could have pretty large spreading in results. Also, together with a mis-matched LNA it can easily become significant.