Analog Switch Applications
in a MultiMode Cell Phone
Application Brief 28
By S. Banerjee
Summary: Currently a few semiconductor vendors are providing
chipsets for multimode cellular phones. The cell phone hardware
solution is solved with a few ASIC chips. Generally it is a three-chip
solution. Frequently several signal switching functions are needed
to communicate amongst these chips and also with other circuit
elements and modules. Because of oversight or lack of space on
the silicon or partitioning, it is not possible to integrate the func-
tions in the ASIC or in the chipset. In this situation, external small
geometry switches are used.
Figure 1. GSM Phone Block Diagram
Figure 2. Typical PLL Set Up
To perform switching functions in cellular phones, a high quality
low-loss analog signal switching device can be used. These
switches can be used to select different functions of the phone or
activate/deactivate circuit elements.
A few vendors are providing chipsets for digital phones.
In a digital GSM-type phone, the receiver and transmitter are locked
at the required frequencies (900 MHz, 1800 MHz and 1900 MHz)
through a PLL. In general the GSM phone consists of three main
blocks (see Figure 1) that are normally performed in an ASIC.
The PLL is the heart of the system for providing timing information.
The block diagram (see Figure 2) is a typical PLL set up. In a
multimode phone, different low-pass filters are required. In GSM
and DCS (2XGSM) phones, frequencies are filtered through a low-
pass filter. The method of modulation used in GSM is called Gaussian
Minimum Shift Keying (GMSK). The PLL is digital. In GMSK there
is a time slot by which the PLL has to be completed.