MAX7058ATG データシートの表示（PDF） - Maxim Integrated

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MAX7058ATG

315MHz/390MHz Dual-Frequency ASK Transmitter

Maxim Integrated 

MAX7058

315MHz/390MHz Dual-Frequency ASK Transmitter

When the particular capacitance control input pin is

high, then the corresponding amount of capacitance is

added at PAOUT; this capacitance tuning works only at

315MHz. The 16 capacitor values are selected by set-

ting CAP1–CAP4; the capacitance resolution is 0.5pF.

The total capacitance varies from 0 to 7.5pF. For exam-

ple, if CAP1 and CAP3 are high and CAP4 and CAP2

are low when operating at 315MHz, then this circuit will

add 2.5pF at PAOUT.

Table 2. Variable Capacitor Values and

Control Input Pins

CAPACITOR

CONTROL PIN STATE

(CAP4–CAP1)

0000

ADDED SHUNT CAPACITANCE

IN pF

315MHz (÷21) 390MHz (÷26)

0

0001

0.5

0010

1.0

0011

1.5

0100

2.0

0101

2.5

0110

3.0

0111

1000

3.5

0

4.0

1001

4.5

1010

5.0

1011

5.5

1100

6.0

1101

6.5

1110

7.0

1111

7.5

Phase-Locked Loop

The MAX7058 utilizes a fully integrated, programmable

PLL for its frequency synthesizer. All PLL components

including the loop filter are included on-chip. The divide

ratio is set at one of two fixed values: 21 (FSEL is set to

high) or 26 (FSEL is set to low).

Crystal (XTAL) Oscillator

The crystal (XTAL) oscillator in the MAX7058 is designed

to present a capacitance of approximately 6pF between

XTAL1 and XTAL2. In most cases, this corresponds to

an 8pF load capacitance applied to the external crystal

when typical PCB parasitics are added. The MAX7058 is

designed to operate with a typical 10pF load capacitance

crystal. It is very important to use a crystal with a load

capacitance equal to the capacitance of the MAX7058

crystal oscillator plus PCB parasitics. If a crystal

designed to oscillate with a different load capacitance is

used, the crystal is pulled away from its stated operating

frequency, introducing an error in the reference frequency.

A crystal designed to operate at a higher load capacitance

than the value specified for the oscillator will always

be pulled higher in frequency. Adding capacitance to

increase the load capacitance on the crystal will increase

the startup time and may prevent oscillation altogether.

In actuality, the oscillator pulls every crystal. The crystal’s

natural frequency is really below its specified frequency,

but when loaded with the specified load capacitance, the

crystal is pulled and oscillates at its specified frequency.

This pulling is already accounted for in the specification of

the load capacitance.

Additional pulling can be calculated if the electrical

parameters of the crystal are known. The frequency pull-

ing is given by:

ü=p§ü

Cm

2





C

case

1

+

C load

−

C case

1

+ C spec



 ×

6

where:

fp is the amount the crystal frequency is pulled in ppm

Cm is the motional capacitance of the crystal

Ccase is the case capacitance

Cload is the actual load capacitance

Cspec is the specified load capacitance

When the crystal is loaded as specified (i.e., Cload =

Cspec), the frequency pulling equals zero.

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