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Spirit1 range extender & SAW filter, spurious emissions

Question asked by christian.hans on Sep 23, 2014
Latest reply on Apr 2, 2015 by vagelis.vagelis

according to AN4271 a SAW filter is necessary in order to reduce the spurious emissions above carrier in order to comply with ETSI regulations. This I understand. However, I would also believe that the SAW filter would reduce the spurious emissions below 862 MHz (which is the boarder for -54 dBm ETSI limit).

What I don't understand is how the circuit is capable of producing a spurious emission of only -60 dBm at 862 MHz for 27 dBm output (this value is taken from figure 8). The SAW filter used has about 3 dB attenuation @ 862 MHz. The Spirit1 transceiver has about -60 dBm at 862 MHz spurious emissions (AN4110 for 10 dBm output). The FEM has a S21 of 26 dB (small signal gain). I don't know what output level of the Spirit1 that is required to achieve 27 dBm output from FEM, but I guess a bit below 10 dBm.

So, -60 dBm (Spirit1 spurious output) - 3 dB (SAW) + 26 dB (FEM gain) = -37 dBm spurious output from FEM @ 862 MHz: Not -60 dBm which is measured.

If someone could please explain to me what is causing the emissions not to be amplified by the FEM, I would appreciate it. Or if there is something else that have skipped my mind in understanding.

Thank you.