why opll used in gsm

In some platform of GSM cell phone, we can find the TX sawfilter of GSM 900 band between transceiver and PA,and i have two question about that TX sawfilter:
1.why need to add that Tx sawfilter?
2.why some of GSM phone don't need the Tx sawfilter?

They need them to meet the receive band output power specification. (Transmitter output power at 20 MHz offset

Some phones don't have them because they don't need them to meet specifications.

Dave
www.keystoneradio.com

ALL the GSM phones have to meet the specifications for TX noise power in RX band.

The platforms that are using an Offset PLL approach don?t need a SAW filter between transceiver and PA, because the characteristic of the circuit is a low-pass filter rejecting in this way the TX noise in receive band.

Offset-PLL can be used only for GMSK modulation.

For 8PSK modulation (EDGE) cannot be used OPLL, and in this mode some transceivers need a SAW filter before the PA to meet the spec.

Hi vfone,

What about the noise floor too highin RX bands ?
I met this problem and have no clue to work.

I have checked PA is normal / control signal no noise interfer..
Could grounding of transceiver affect this ?
I use Locosto DRP

Pls helps

Can you measure the noise power in the receive band before the PA? That would help you isolate the PA or the locosto


Dave