MAX7057ASE+T【PDF 10/20 页】IC TRANSMITTER ASK/FSK 16-SOIC

Due to the nature of the transmit PLL frequency divider,

a fixed offset of 16 must be subtracted from the trans-

mit PLL divider ratio for programming the MAX7057s

transmit frequency registers. To determine the value to

program the MAX7057s transmit frequency registers,

convert the decimal value of the following equation to

the nearest hexadecimal value:

Assume that the ASK transmit frequency = 315MHz

and f

XTAL

= 16MHz. In this example, the rounded deci-

mal value is 15,104, or 0x3B00 hexadecimal. The upper

2 bytes (0x3B) are loaded into the LOFREQ1 register,

and the low 2 bytes (0x00) are loaded into the

LOFREQ0 register. In ASK mode, the transmit frequen-

cy equals the lower frequency programmed into the

MAX7057s transmit frequency registers (see Tables 2,

3, and 912).

In FSK mode, the transmit frequencies equal the upper

(HIFREQ1 and HIFREQ0) and lower (LOFREQ1 and

LOFREQ0) frequencies programmed into the MAX7057s

transmit frequency registers. Calculate the upper and

lower frequency in the same way as shown above. FSK

deviations as low as ?kHz and as high as ?00kHz are

programmable (see Tables 2, 3, and 812).

The exact min and max values for the transmit frequen-

cy registers (HIFREQ1/0, LOFREQ1/0) are 2.9596

(0x2F42) and 12.0220 (0xC05A), yielding a synthesizer

ratio of 18.9596 and 28.0220, respectively. These limits

MUST be followed to prevent the delta-sigma modula-

tor from overflowing.

Whenever all of the fractional bits in the HIFREQ1/0 and

LOFREQ1/0 registers are zero (fhi[11:0] and flo[11:0]),

only an integer divider is used, and the delta-sigma

modulator is not in operation. This allows lower current

operation. The 600kHz PLL bandwidth should be used

in this mode to reduce phase noise.

Any change to the transmit frequency registers must be

followed by writing a 1 to the self-reset frequency load

register (see Tables 2, 3, and 12).

Crystal (XTAL) Oscillator

The crystal (XTAL) oscillator in the MAX7057 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 MAX7057 is designed to operate with a typical

10pF load capacitance crystal. It is very important to

use a crystal with a load capacitance that is equal to

the capacitance of the MAX7057 crystal oscillator

plus PCB parasitics and optional external load

capacitors. If a crystal designed to oscillate with a dif-

ferent 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 is always pulled higher in fre-

quency. Adding capacitance to increase the load

capacitance on the crystal increases the start-up time

and can prevent oscillation altogether.

In actuality, the oscillator pulls every crystal. The crys-

tals natural frequency is below its specified frequency,

but when loaded with the specified load capacitance,

the crystal is pulled and oscillates at its specified fre-

quency. 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

pulling is given by:

where:

fp is the amount the crystal frequency is pulled in ppm

C

m

is the motional capacitance of the crystal

C

case

is the case capacitance

C

spec

is the specified load capacitance

C

load

is the actual load capacitance

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

load

=

C

spec

), the frequency pulling equals zero.

Communication Protocol

The MAX7057 registers are programmed through an SPI

interface. Figure 2 shows the timing diagram of the SPI.

The GPO must be properly configured to act as an SPI

data output (SDO) by setting the configuration 1 register

(see Tables 2, 3, 15, and 16).

The SPI operates on a byte format, according to Figure 2.

f

C

p

m

case

load

case

spec

=

+

?/DIV>

2

1

10

6

f

Decimal value to program

RF

XTAL

-16

transmit frequency registers

?/DIV>

=

4096

300MHz to 450MHz Frequency-Programmable

ASK/FSK Transmitter

10 ______________________________________________________________________________________

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