ML2721
PRELIMINARY
CIRCUIT BLOCK DESCRIPTIONS
LOVCO AND TRANSMIT DRIVER
DATA FILTER AND DATA SLICER
The internal LO VCO operates at 1.8 to 1.86GHz, which
is two times the LO frequency of 900 to 930MHz. The
VCO output is divided by 2 to give accurately matched
quadrature signals at 915MHz. In Receive mode the LO is
offset from the wanted signal by +1.024MHz to produce
the low IF. This 1.024MHz shift is produced by
automatically adding an offset of +2 counts to the PLL
divider programming.
The FM demodulator is followed by a Gaussian lowpass
filter whose 768kHz cutoff frequency is matched to the
transmitted 1.536Mchip/s waveform. This Gaussian filter is
implemented with similar circuits to the IF filter, and is
shared with the Transmit modulation path. The filter output
can be AC coupled to the slicer because the spreading
code is almost DC balanced. The data slicer signal is
output to the baseband processor for timing recovery and
decoding.
In Transmit mode the LO is frequency modulated by the
transmitted data, using a modulation port of the VCO. The
VCO output is divided by 2 and a driver amplifier
typically develops 0dBm at 900 to 930MHz to directly
drive an ML2751 power amplifier.
POWER SUPPLY
The ML2721 uses multiple voltage regulators to protect
sensitive internal circuits from power supply noise.
Separate regulators supply the PLL dividers, RF circuits
and IF circuits. Each of these regulators takes its power
TRANSMIT DATA FILTERAND MODULATION DRIVER
from VCC , and supplies power internally to its respective
5
Logic level NRZ signals at DIN are scaled and filtered by
RVCC pin. External capacitors are required at each
n
th
a 5 -order lowpass filter. The lowpass filter is tuned to
RVCC pin to decouple the outputs of the internal
n
give a 1.35MHz 3dB point to pass the 1.536Mchips
transmit data. The filter data is then fed to the internal
modulation port of the LO tank circuit.
regulators. The VCO regulator takes its power from the
VCC pin which is normally connected to the RVCC pin.
2
6
An external decoupling capacitor is also used on the
internal bandgap voltage reference to improve the noise
performance of the regulators.
In the transmit closed loop mode the modulation port is
held at its midpoint so that the synthesizer locks to the
center channel frequency. In the transmit open loop mode
the VCO is modulated by Gaussian filtered data via the
VCO modulation port. The modulation driver contains
scaling circuitry to control the FM deviation over the
entire VCO tuning range. This circuit is inactive in the
receive mode.
These regulators are effective at supply voltages from
3.0V to 5.0V. As the VCC supply voltage drops below
3.0V chip performance gradually degrades, but the
ML2721 transceiver will maintain the link as long as the
supply voltage is greater than 2.7V.
5
RECEIVE MIXERS AND IF CHAIN
The Receive RF quadrature mixers down-convert the
signal to the 1.024MHz receiver IF. The input of the mixer
is single-ended and matched to 50W by a series inductor.
This gives a good terminating impedance for the
preceding RF filter. The quadrature outputs of the down
converter feed the IF filter. The quadrature mixer and IF
filter together achieve a typical image rejection of 35dB.
A quadrature combiner for the image reject mixer and a
th
12 -order Gaussian bandpass filter make up the active IF
filters. The active filters provide an accurate Gaussian
characteristic with a 1.408MHz, 3dB bandwidth which
improves both sensitivity and adjacent channel rejection.
The IF amplifiers provide the bulk of the receiver’s gain.
An RSSI signal is generated by using the outputs of the IF
amplifiers. The RSSI signal is conditioned and sent to the
baseband controller. A frequency-to-voltage converter
provides highly linear FM demodulation with good data
recovery from the low IF.
January, 2000
9
PRELIMINARY DATASHEET
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
