Why in old GSM circuits both RX-VCO and TX-VCO were used?

In GSM circuits, I saw a few old platform with RX-VCO & TX-VCO.
Why can't it just use one VCO instead of two VCO ?
Thanks.

They can, and new GSM topologies do use a single VCO for both bands. You have to be concerned about lock time and the GSM class that you support through.

Dave

Thanks.
But I don't know why it rlates to GSM class.
Could you please advise ?
Thanks a lot.

GSM class refers to the number of slots that you can use for receive and transmit. If you can't get a PLL to lock fast enough, then you will need to use a seperate RX and TX so that you can toggle between receive and transmit.

Dave