ML2721 datasheet(12/27 Pages) MICRO-LINEAR

12 / 27 page

ML2721

January, 2000

PRELIMINARY DATASHEET

PRELIMINARY

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.

Figure 4. Transmitter Operating Cycle

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

Figure 3. Transmitter Block Diagram

MODULATION

COMPENSATION

TRANSMIT

DATA FILTER

LOOP FILTER

DIN

VHIGH

VLOW

VMID

PLLEN

DIVIDER

P D

MODULATION

PORT

VCO

TUNING

PORT

TRANSMIT

RF OUTPUT

DIVIDE

BY 2

RXON

PLLEN

DATA FILTER

INPUT

DATA FILTER

OUTPUT

VCO TUNING

PORT VOLTAGE

OUTPUT

FREQUENCY

ON TRFO PIN

OVERVIEW (continued)

ML2721 датащи(PDF) 12 Page - Micro Linear Corporation

ML2721 датащи(HTML) 12 Page - Micro Linear Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

датащи скачать

ссылки URL