Question of spectrum due to switching on GSM mobile phone.

Question of spectrum due to switching on GSM mobile phone.

Would you kindly explain to me what's the reason for the difference between signal measurement with un-signal measurement?

On un-signal mode, my GSM mobile phone was in test mode that only transmitter is working , the spectrum due to switching of +/-400KHz has about 6~8dB margin at the whole DCS band.
On signal mode, I connect the mobile with GSM test set and check the spectrum due to switching of +/-400KHz , we found there is only about 0~2dB margin on high band. eg Channel 885.

GSM test set : CMU200.
I can confirm test set is in normal state and in calibration date.

At the old PCB version, there are the good margin 6~8dB whether in signal mode or in un-signal mode.

New PCB and old PCB are embedding one software image.

No DC-DC , No LCD , only GSM kernel circuit (BB & RF)
I have checked all control signal , no difference between signal mode with un-signal mode. (timming & signal quality)

No way I can do now , Who can give me some suggestion?

Thank you!

Re: Question of spectrum due to switching on GSM mobile phon

First thing that you have to check if is any difference between PA ramping signal in the signaling mode and non-signaling mode. Could be a bug in the radiocode that is calling for different ramps (look-up table) in different test modes. This is a very unusual situation, but could be a possibility.
Second, check if the output power for each PCL is the same for different test modes.
If the burst shape, timing and output power are the same for different test modes, shall be no difference for Transient Spectrum.

Re: Question of spectrum due to switching on GSM mobile phon

Hi:

Besides points raised by vfone, may be you can further check two things:
1. Any PA's VCC (like VBAT..) showed a spike during pre-TX burst ?
2. Any ripple on PVT modulation ripple ?

If yes, pls inform me the sympton, and I'll try to give you comments.

Regards,

Re: Question of spectrum due to switching on GSM mobile phon

Is this only happen under power level 0 or all power level.
If only power level 0 has this issue, please check the Tx power in link mode.
Maybe the PA is saturated in high channel.
Please reserve at least 0.5 dB margin for target power.
Ex: if target power is 30dBm, you should fine-tune PA matching to Max output power is 30.5 dBm for all channel, and mantain power flatness for all channel if possible.