Conventional cell phone clock solutions compared to the STCD10x0 integrated solution AN2760

4/14

1

Conventional cell phone clock solutions compared to

the STCD10x0 integrated solution

1.1

Conventional clock solution - dedicated crystals for

dedicated modules

The easiest design of the clock tree in smart phones is to use dedicated crystals for different

application modules. This is illustrated in Figure 1.

Figure 1.

Conventional clock solution - dedicated crystals

Although the solution in Figure 1 is simple, an obvious drawback is the cost. Normally

WLAN and Bluetooth can accept the master clock in several different frequencies. For

example, 13, 16, 16.8, 19.2, 26, 3.6, 38.4 and 52 MHz are all accepted by WLAN, Bluetooth,

and FM radio. Different modules recommend different master clock frequencies and those

recommended values are not always the same. For example, 19.2 MHz is recommended for

Bluetooth by some chip vendors while 26 MHz is recommended for WLAN. Due to the

design risk and time-to-market, cell phone vendors are not willing to use the same crystal to

serve all application modules although this is possible.

A typical external clock source (typically 10 pF load capability) is always short of fan out

capability if several application modules are directly connected to its output.

1.2

Conventional clock solution - discrete buffer solution

Since the cost of a TCXO (temperature compensated crystal oscillator) is extremely high

and some of the common frequencies can be used for different applications, cell phone

designers have tried to use a discrete buffer solution (with emitter followers) to solve the

insufficient fan out issue. A two-stage discrete buffer is used as shown in Figure 2. The first

stage could solve the problem of the fan out of the clock source (normally TCXO) and also

provide a good isolation of input channel to output channels. The second stage distributes

the master clock to different application modules and also provides good isolation between

different output channels.

TCX

XO

XTAL

WLA

Media

WLAN

Bluetooth

Media

TCXO

XO

XTAL

ai14013