ML2721 Ver la hoja de datos (PDF) - Micro Linear Corporation
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ML2721 Datasheet PDF : 27 Pages
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OVERVIEW (continued)
PRELIMINARY
ML2721
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
Parameter
Divide by 6 Divide by 12
(RD0 = 0) (RD0 = 1)
Transmit data filter nominal
3dB bandwidth
REF/4.4
REF/8.8
Table 4. Bandwidth as a Function of Divider Ratio
bandwidth of the filter is slaved to the reference
frequency. See Table 4.
LOOP FILTER
PLEN
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
RXON
PLLEN
VCCA
SUPPLY
VOLTAGE
2.65V
TRANSMIT
LOCKOUT
LATCH
TRANSMIT
R.F. OUTPUT
Figure 6. Transmitter Low Voltage Lockout Sequence
VCO
NX FCOMP
DIVIDE
BY 2
NX FCOMP/2
INPUT SIGNAL AT
N X FCOMP/2
DIVIDER
12
ADDER
FCOMP
2
N
REGISTER 1
BIT14...BIT3
IF AT FCOMP
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
PD
unwanted emissions.
FCOMP
F/6
F/12
FREF
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
LOOP FILTER
PLLEN
Figure 5. Transmitter Eye Diagram
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
VCO
N × FCOMP
NX FCOMP/2
DIVIDE
BY 2
TRANSMIT
RF BUFFER
DIVIDER
FCOMP
12
ADDER
N
O REGISTER 1
BIT14...BIT3
PD
FCOMP
÷6 or
÷12
REF
Figure 7. Phase Locked Loop in Transmit Mode
January, 2000 PRELIMINARY DATASHEET
13
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