Thank you all, I thought out some points:
1. Using single balance mixer will have the trouble of LO signal feedthrough, however, in Low IF architecture, the IF signal is normally less than 10MHz, which is 100 times less than the GPS frequency 1570MHz. This relaxes the design of the following filter design a lot and makes single balance mixer applicable.
2. Another problem is the voltage headroom mentioned by dsjomo above. Since LC load is not applicable in Low IF signal (otherwise it requires large value of L or C), only resistive load can be used. That will largely limit the linearity range like P1db. In this case another voltage drop for the tail current in Gillbert cell might not be acceptable, especially in low voltage IC design.
3. A double balance mixer requires an LNA with differential output, that will double the power consumption of the RF front-end (LNA+mixer), which is not acceptable in the trend of low voltage low power consumer market.
4. If using the single end (LNA output) to differential (Gillbert Cell input) scheme, the circuit may not be balance enough, image rejection ability will degrade.
All these reasons make single balance mixer very attractive in Low IF GPS receiver front end design.
Please input your comment, thanks.