ML2721 View Datasheet(PDF) - Micro Linear Corporation
Part Name
Description
MFG CO.
'ML2721' PDF : 27 Pages View PDF
OVERVIEW (continued)
PRELIMINARY
ML2721
TRANSMIT MODE
TXCAL AND TRANSMIT
In TRANSMIT mode, the VCO is directly modulated with
filtered FSK transmit data. The PLL is disabled or open,
with the charge pump output tri-stated. This stops the PLL
from contending with the applied modulation. The digital
transmit data input is generated by external baseband
circuits. This input signal is level shifted and filtered by the
transmit modulation filter prior to modulating the VCO
frequency. Due to the low leakage current of the charge
pump and tank circuit the ML2721 can be in TRANSMIT
mode for slot lengths of up to 10ms.
During RXCAL mode the transmit modulation data filter is
automatically tuned to remove the need for production
alignment. When the ML2721 is powered up (VDD first
applied) the tuning information is reset to mid-range. The
filter alignment mode should be used after power up or
XCEN enabled, and prior to transmission. This ensures the
modulation filters are aligned to prevent unwanted
spurious emissions.
Prior to transmitting the PLL must tune to the intended RF
center frequency of the transmission. This occurs in TXCAL
mode. The Transmit modulation is disabled and any input
on the DIN pin is ignored. The transmit output buffer is
enabled during TXCAL mode. To prevent spurious emissions
due to the PLL locking, any external RXON switch or PA
should be disabled during TXCAL mode.
Transmitter Circuit Operation
The ML2721 transmitter is a 2-FSK transmitter using a
directly modulated open loop VCO. See Figure 3. This
type of transmitter is simple, low power, and well suited
to a time-slotted system. The transmitter uses the stored
VCO tuning voltage on the PLL loop filter to set the VCO
frequency for the duration of the transmit slot. The
modulation is introduced through a second VCO tuning
port. This modulation port has a much lower tuning
sensitivity than the main tuning port in order to produce
the ±550kHz FSK deviation. Compensation circuits
stabilize the modulation deviation over the VCO tuning
range, and internal logic manages the correct transition
from TXCAL to TRANSMIT mode. The ML2721 design
supports transmit slot lengths up to 10ms, and the time
required to set up the transmitter for a new slot (TXCAL
mode) is 70µs.
The operating cycle of the transmitter starts with TXCAL
mode. See Figure 4. The data filter input is zeroed and the
PLL locks the VCO frequency to the desired RF channel
center frequency. A CW signal at the selected RF channel
frequency comes out of the Transmit RF output. When the
PLLEN control line is de-asserted the transmitter starts its
transition to TRANSMIT mode. The PLL charge pump is
disabled, leaving the PLL loop filter to hold the correct
tuning voltage for this channel. The data formatter injects
an NRZ bipolar data waveform into the Transmit data
filter. The Transmit data filter band-limits this waveform,
and feeds it to the modulation compensation circuits.
These scale the modulation voltage (depending on the
VCO tuning voltage) and drive the VCO tuning port with
MODULATION
COMPENSATION
TRANSMIT
DATA FILTER
LOOP FILTER
MODULATION
PORT
DIVIDER
PD
TUNING
PORT VCO
DIVIDE
12
BY 2
N
TRANSMIT
RF OUTPUT
Figure 3. Transmitter Block Diagram
DIN
VHIGH
VLOW
VMID
PLLEN
RXON
PLLEN
DATA FILTER
INPUT
DATA FILTER
OUTPUT
VCO TUNING
PORT VOLTAGE
OUTPUT
FREQUENCY
ON TRFO PIN
Figure 4. Transmitter Operating Cycle
12
PRELIMINARY DATASHEET January, 2000
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