MAS9081 데이터 시트보기 (PDF) - Micro Analog systems

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MAS9081

Stand-alone am Receiver ic

Micro Analog systems 

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

High

High

Open

-

High

Low

Low

High

Open

Closed

QO1

QO2

Low

Low

Closed

QO2

Description

Power down

Frequency 1

Frequency 2, RFI2 capacitor connected in

parallel with antenna

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

frequency is determined by antenna inductor LANT

(see Typical Application on page 5), antenna

capacitor CANT1 and parasitic capacitances related

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

to connect CANT2 in parallel with ferrite antenna and

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

The ferrite bar antenna having inductance LANT and

parasitic coil capacitance CCOIL is tuned to two

reception frequencies f1 and f2 by parallel capacitors

CANT1 and CANT2. The receiver input stage and

internal antenna capacitor switch have capacitances

(CRFI1, COFF2) which affect the resonance

frequencies. COFF2 is switch capacitance when

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.

Frequency f1 (higher frequency):

CTOT1=CCOIL+CANT1+CRFI1+COFF2=CCOIL+CANT1+6.5pF+37pF=CCOIL+CANT1+ 43.5pF, f1 = 2π

Frequency f2 (lower frequency):

CTOT2=CCOIL+CANT1+CANT2+CRFI1=CCOIL+CANT1+CANT2+ 6.5pF, f 2 = 2π

1

LANT ⋅ CTOT 2

1

LANT ⋅ CTOT1

3 (12)

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