Phone Modulation Spectrum Fail

[xinxin]

When testing the GSM phone , it shows fail at the 400K point for spectrum modulation of GSM band. Could you please tell me how to resolve it from sw or circuit?

 

[Debeli]

If you got the same effect on all working frequences, I would check modulator circuitry i.e. Gausian shaping filter (I believe that you can change coefficients in software). If the spectrum is different with working frequency you have problem in output network (PA, antenna,...)

Regards,
D.

 

[martinthorn]

GSM 400kHz modulation spectrum ?

Could be :

high intermodulation products into the PA input (check input with all '1'
data) : Check modulator performance

PWB/PCB layout bug : interference on PA power control lines, rf
feedback from output of PA to modulator/PA input/VCO. Check
performance before PA.

RF Shields could be leaky/broken : unwanted feedback from PA output

VCO/LO Problem : too much 400khz phase noise or too high PLL
(reference) spurious. Feed circuit with signal generator instead of
VCO to check.

DSP Bug : IQ data not predistorted/filtered correctly

measurement bug..

M.

 

[hr kim]

You add the big decoupling capacitor at the supply voltage of the Tranceicer IC.

 

[eecly]

Hi,

Pls check the 1. Phase noise of VCO. High phase noise may cause modulation spectrum fail
2. Loop BW of PLL. Try to reduce the loop BW a little bit.
3. Have u checked the receiving IQ data? Is that normal. If the received data is adnormal (means too much one or zero) may cause modulation spectrum fail.


eecly