Modulation problems on GSM phone

[xinxin]

modulation in gsm in( hindi)

When test a phone on spectrum due to the modulation for DCS , i found that when PCL =15,10 , fail happens at 400Khz , 600Khz away from the carrier , but when PCL=0 , it's OK. could u help me ?thanks

 

[vfone]

problems of gsm

-Try to evaluate separate the PA, looking for modulation spectrum at different PCL’s doing a load-pull.
-Check the control-loop, and the layout of this circuit.
-Check if the level at the input of the PA is inside of the PA spec

 

[martinthorn]

pcl gsm dcs

Check the DCS modulation spectrum spec carefully, at low power levels the 'dBc' spec may no longer be necessary : only the 'dBm' specification is applicable.

If performance is still outside specification then one possibility could be power control loop gain at low power levels. If the gain is low, and the bandwidth of the loop small, then suppression of any 400kHz AM on the PA output will not happen.

 

[xinxin]

cmu200 gsm burst

thanks to martinthorn and vfone

Our RF solution use low IF , the control loop for PA is open loop . the power into the PA is inside the spec of PA .
We use CMU200 to test modulation spectrum , the result is OK at low PCL , but for Agilent8960 , fail occurs. when the bursts to be test set 100 , it's OK at 8960 .
martinthorn , could you tell me the spec (dBm) for modulation spectrum at Low PCL , the spec said it's -60 dBc when more than 400K.

 

[vfone]

Xinxin
The relative modulation spec (dBc) for DCS is –60dBc @ 400kHz/600kHz
The absolute modulation spec (dBm) for DCS is –36dBm @ <600kHz
I am attaching pictures from latest GSM 05.05

Beware that 8960 may not have set the right RBW/VBW for the measurement.
According to GSM spec the RBW/VBW of the measurement (up to 1.8MHz from the carrier) shall be 30kHz.

 

[martinthorn]

Xinxin,

400kHz spec :

At power levels below about +30dBm , the spec limit to use is -36dBm
(the CMU200 knows this and adjusts the limit automatically, I am not
sure the Agilent does this)

600kHz spec :

the spec limit is 60dBc

Measurement Setup (from spec) :

"zero frequency scan, filter bandwidth and video bandwidth of 30 kHz up to 1800 kHz from the carrier and 100 kHz at 1800 kHz and above from the carrier, with averaging done over 50 % to 90 % of the useful part of the transmitted bursts, excluding the midamble, and then averaged over at least 200 such burst measurements. Above 1800 kHz from the carrier only measurements centred on 200 kHz multiples are taken with averaging over 50 bursts."

Note the number of averages : 200. Therefore the result from the 8960 may mean you are ok but with low margin !

One other thing to consider :

Are you testing in call ? The testers tell the phone what modulation
bit pattern to use (PN9/15 etc.). Different bit patterns can give different
results : the Agilent may be set differently to the CMU200.