ML2712CH Просмотр технического описания (PDF) - Micro Linear Corporation
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ML2712CH Datasheet PDF : 22 Pages
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PRELIMINARY
ML2712
MODES OF OPERATION (CONTINUED)
RECEIVE MODE
The circuits active in RECEIVE Mode are shown in
Figure 6. All the transmitter circuits are normally disabled
in this mode.
Control
Interface
RSSI
Rx RF
Input
WB
Charge
Pump
WB
Phase
Comp
LPF
Control
x2
MUX
+
DAC
–
2LO VCO
PLL 2
Lock
Detect
PLL 1
1LO VCO
ML2712
RSSI A/D
Comparator
Output
2LO Output
To IF Transceiver
2LO Loop
Filter &
Tank Circuit
Lock Detect
Ref Frequency Input
1LO Loop
Filter &
Tank Circuit
1IF Input/
Output
Active
Disabled
Figure 6. Receive Mode Active Circuits
RECEIVE RF DOWN-CONVERTER
The Receive RF input amplifier converts the single
ended RF input to a differential signal which is then
fed to a mixer. The amplifier and mixer combination
down-converts the received RF signal to the receiver
1LO IF (1IF), which is optimized for 260MHz. The
output of the down-converter is differential with
340W of output impedance suitable for low loss
matching to an external SAW IF filter. The 1IF output
ports are bidirectional and are multiplexed with the
transmit reference IF input.
PLL & VCO CIRCUITS
Two independently programmable PLL circuits control the
1LO and 2LO VCO frequencies. These are programmed via
the Serial Control Bus (DATA, CLK, and ENABLE). Program
words are clocked into divider or control circuits when
ENABLE is asserted. The programming is operational
whether the ML2712 is in SLEEP, STANDBY, RECEIVE or
TRANSMIT Mode.
The reference signal, REF (Pin 10) typically from an
external crystal oscillator, is fed to a programmable
reference divider with programmable division ratios of 40,
32, 20 and 16. The reference divider output is fed to both
the 2LO and the 1LO phase/frequency detectors.
The polarity of the charge pump output current pulse is
programmable to give a positive or negative frequency/
voltage control. The value of the current pulses is
programmable via the Serial Control Bus. (See Table 7
and 10)
1LO VCO
The 1LO VCO operates at approximately 1.1GHz, and is
doubled to a final frequency of 2.2GHz. The 1GHZ VCO
signal is connected to the 1LO PLL circuits. The VCO
requires an external differential tank circuit design to
reduce the effects of frequency pulling due to signal
coupling. The tank circuit is tuned by the charge pump
output QP1(Pin 13) using a passive external loop filter.
The active circuitry for the 1LO VCO is a differential
cross-coupled pair providing a negative resistance
across the tank circuit to maintain oscillation. The
tank circuit must provide a DC path to RVCC2 (Pin
21). The layout of this circuit must be kept symmetric
to minimize interference or coupling from other
circuits in the radio.
2LO VCO
The 2LO VCO requires an external tank circuit and
loop filter. The 2LO VCO is phase locked to provide a
fixed frequency, nominally 236MHz. The differential
pair 2LO (Pin41) and 2LOB (Pin 40) providedrive to
the ML2713. The 2LO differential outputs are from
the collectors of a differential pair that require a pull-
up to a nominal 2.3V, normally provided by the
ML2713.
A calibration tone added to the 2LO output in
STANDBY Mode is intended for aligning filters and
discriminators in the IF circuits of the ML2713. The
calibration tone is an 8MHz square wave with a
16MHz or 32MHz reference input, or a 10MHz
square wave with af 20MHz or 40MHz reference
input.
The 2LO VCO is a cross-coupled pair, with one base
connected to RVCC3 (Pin 42) (2LO supply voltage). The
other base is connected to an external tank circuit through
T2LO (Pin 43). This design presents a negative impedance
across the tank circuit. Since a dominant oscillation, due
to bond wire inductance and parasitic capacitance on the
PCB, can lead to high frequency oscillation (of the order
of 1GHz), the circuit must be carefully laid out. The 2LO
tank circuit must provide a DC path from T2LO to RVCC3
(Pin 42).
10
PRELIMINARY DATASHEET January, 2000
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