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Spirit1 range extender & SAW filter, spurious emissions

hctiming
Associate
Posted on September 23, 2014 at 04:43

Hi,

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.

4 REPLIES 4
Nickname1920_O
Associate II
Posted on September 23, 2014 at 09:59

Hi,

you're right as regards the role of the SAW filter.

Anyway, there are two types of spurious emission from SPIRIT1: the ones from the crystal and those from the SMPS.

The ones from the crystal depend on the frequency of the crystal (25MHz, 26MHz, 50MHz, 52MHz) and are filtered by the SAW filter.

Those from the SMPS can be of 6.5MHz or of 3.25MHz from the carrier. Choosing 3.25MHz we get 864.75MHz and not the 862MHz you mentioned. At this frequency the spurious emission mask is -36dBm, so it results complaint with the ETSI regulations..

Regards,

Gaetano

nicolas239955
Associate II
Posted on January 27, 2015 at 19:15

Dear Gaetano,

I have merely same question for the 169MHz band. Yet, I've seen that you released a new version of the reference design with range extender at 169MHz. In this new one, SAW filter is not present (by the way I suspect there is some kind of mistake in the documentation schematics / guide / BOM do not match).

Have you improved the spirit1 output filter to the point that makes the SAW filter useless? Or has it to do something with the fact that you also shifted TCXO frequency from 25MHz to 50MHz?

Thanks,

Nicolas

Nickname1920_O
Associate II
Posted on February 11, 2015 at 12:14

Hi Nicolas,

yes, the right answer is that we have shifted the TCXO frequency.

Regards,

Gaetano

deathvag
Associate II
Posted on April 02, 2015 at 11:35

Hello,

Is it possible, using the STEVAL-IKR002Vx Kit, to see third-party meters in the WM-BUS GUI?

Regards,

Vagelis