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DA9081.006

1 August, 2005

FREQUENCY SELECTION

The frequency selection and power down control is

accomplished via two digital control pins PDN1 and

PDN2. The control logic is presented in table 1.

Table 1 Frequency selection and power down control

PDN1

PDN2

RFI2 Switch

Selected Crystal

Output

Description

High

High

Open

-

Power down

High

Low

Open

QO1

Frequency 1

Low

High

Closed

QO2

Frequency 2, RFI2 capacitor connected in

parallel with antenna

Low

Low

Closed

QO2

Frequency 2, RFI2 capacitor connected in

parallel with antenna

The internal antenna tuning capacitor switch (RFI2)

and crystal filter output switches (QO1, QO2) are

controlled according table 1. See switch in block

diagram on page 1.

If frequency 1 is selected the RFI2 switch is open

and only crystal output QO1 is active. Antenna

(see Typical Application on page 5), antenna

to antenna inputs RFI1, RFI2 and RFI3 (see

Antenna Tuning Considerations below). Frequency

1

is

the

higher

frequency

of

two

selected

frequencies.

If frequency 2 is selected then RFI2 switch is closed

tune it to frequency 2. Then only crystal output QO2

is active. Frequency 2 is lower frequency of the two

selected frequencies.

It is recommended to switch the device to power

down

for

50ms

before

switching

to

another

frequency. This guarantees fast startup in switching

to another frequency. The 50ms power down period

is used to discharge AGC capacitor and to initialize

fast startup conditions.

ANTENNA TUNING CONSIDERATIONS

internal antenna capacitor switch have capacitances

which

affect

the

resonance

switch is open. When the switch is closed this

capacitance is shorted by on resistance of the

switch and is effectively eliminated. Following

relationships

can

be

written

into

two

tuning

frequencies.

1

1

2

1

TOT

ANT

C

L

f

⋅

=

π

2

2

2

1

TOT

ANT

C

L

f

⋅

=

π