GSM frequency error peak at low Basestation downlink power

[Wilson_yu_chen]

Hi guys,

I met problem for my phone with peak frequency error at low TCH
downlink power as -90dBm while sensitivity BER is quite good. Other
higher TCH level, the PFE is within spec.

PFE will goes up high as 0.1ppm more, and independent of TX PCL
and supply voltage.

Can anybody tell what should be checked ?

Thanks!!

Wilson

 

[vfone]

Re: GSM frequency error peak at low Basestation downlink pow

AFC or TCXO

 

[Wilson_yu_chen]

Re: GSM frequency error peak at low Basestation downlink pow

Hi Vfone,

Crystal 26MHz..
And I found problem is gone after removal of TXRX regulator / RF LDO bypassing .
Could you pls address morehow comes AFC correlate to my problem ?

BTW, I met a serious problem for adjacent channel rejection on DCS band -400KHz. based on TI Locosto platform. TI has patch for fix the bug resulting
from their prefilter saturation in Receive path (DCS). Now we gound not even one
dB improvement after patch integration and wondering what dominant factor that
make this software patch nothing. Normally from other project there will be 3dB
improvement.

What could I do for verfication the root cause?
What I have done:

 

[RFDave]

Re: GSM frequency error peak at low Basestation downlink pow

Hi:

If you are at low input power on receive, you should be at high PA output power. When the PA comes on, you suck a lot of current, and your grounds can move around. That may be pulling your TCXO off frequency a bit. The AFC probably can't correct for that sort of fast frequency offset, it should be a pretty slow correction loop. Your comment about changing the LDO bypassing points at that.

Dave

 

[eecly]

1. It may be cause by frequency pulling. Have you checked the time domain of frequency error?
2. The base frequency 26 MHz may be shifted during PA switch on. It may be caused by a lot of reason. Voltage drop or temperature change may be one of the reason. Have you checked the frequecny error for differnt transmit power level?

 

[Wilson_yu_chen]

Re: GSM frequency error peak at low Basestation downlink pow

FE is independent with Pwer level (TX) in my case.
FE is getting high only on VBAT on PCB=3.8V
It works normal when VBAT=3.4 or 4.2V

Strange to me. If really is pulling, then 3.4V supply should put PA
voltage drop more severely.

Helps!

 

[RFDave]

Re: GSM frequency error peak at low Basestation downlink pow

Hi:

Sounds like some sort of power supply bypassing strangeness to me. If you use a bench power supply, does the same thing happen? How about removing the TCXO and using an external signal generator?

Dave