ML2721 datasheet(13/27 Pages) MICRO-LINEAR

13 / 27 page

ML2721

January, 2000

PRELIMINARY DATASHEET

PRELIMINARY

the scaled, filtered modulation. The voltage on the

modulation port swings above and below its central value

to produce 2-FSK modulation on the VCO. The modulation

filtering is sufficient to meet the FCC occupied bandwidth

and out-of-band emissions requirements, and does not

introduce significant ISI (Inter Symbol Interference).

The Transmit modulation filter is a 5th order all-pole filter,

designed for minimum differential group delay and good

stop band attenuation at greater than 2MHz. The 3dB

Figure 5. Transmitter Eye Diagram

Table 4. Bandwidth as a Function of Divider Ratio

bandwidth of the filter is slaved to the reference

frequency. See Table 4.

OVERVIEW (continued)

may be disturbed, and the radio could transmit on an out-

of-band signal. To prevent this a comparator monitors the

supply voltage and trips a transmitter shutdown latch if

the supply voltage drops below 2.6V. This disables the

transmitter output buffer. The latch is reset by the next

rising edge of RXON so that the radio cannot transmit

)

(

)

(

NX FCOMP/2

DIVIDER

ADDER

P D

FCOMP

FREF

PLEN

BIT14...BIT3

INPUT SIGNAL AT

N X FCOMP/2

IF AT FCOMP

F/6

F/12

FCOMP

NX FCOMP

VCO

LOOP FILTER

DIVIDE

BY 2

Figure 6. Transmitter Low Voltage Lockout Sequence

Figure 7. Phase Locked Loop in Transmit Mode

RXON

TRANSMIT

R.F. OUTPUT

VCCA

SUPPLY

VOLTAGE

TRANSMIT

LOCKOUT

LATCH

PLLEN

2.65V

TRANSMIT

RF BUFFER

NX FCOMP/2

DIVIDER

ADDER

P D

FCOMP

REF

PLLEN

BIT14...BIT3

÷6 or

÷12

FCOMP

× FCOMP

VCO

LOOP FILTER

DIVIDE

BY 2

until the next complete transmitter cycle. See Figure 6. If

the ML2721 is DC powered on with RXON enabled the

transmitter buffer will be latched off. This helps prevent

unwanted emissions.

Programming the ML2721 integrated PLL.

For ease of use, the ML2721 PLL is programmed to the set

RF center frequency of operation of the radio. See Figure

7. The RF output center frequency is half the VCO

frequency during transmission. The VCO is automatically

offset by twice the IF, to give the correct receive LO

frequency, during reception. This guarantees the baseband

Figure 5 shows an eye diagram recovered from the

Transmit RF output of the transceiver with a data stream of

1.5Mb/s.

Transmitter Lockout (Low Voltage)

The ML2721 transmitter features a low voltage lockout

circuit to meet FCC spurious emissions requirements. If

the supply voltage drops below 2.5V the VCO frequency

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