ML2712
PRELIMINARY
MODES OF OPERATION
STANDBY MODE
Control
Interface
RSSI
Tx VCO
Tuning
Voltage
WB
Charge
Pump
In STANDBY mode all the PLL and VCO circuits are
enabled while all the transmitter and receiver circuits
are disabled. (see Figigure 1) The use of STANDBY
mode is recommended when the PLLs are locking, after
the PLL frequencies have been reprogrammed, or after the
IC has been transferred out of SLEEP mode. The VCO and
PLL circuits are enabled and reach a locked state in 150
usec indicated by an active Lock Detect (LD) signal.The
frequency divide ratio settings defined by Control Word C
and D (see Table 4 ) define the frequencies of the 1LO
PLL and 2LO PLL.
RSSI A/D
Comparator
Output
+
–
Control
DAC
WB
Phase
Comp
2LO Output
To IF Transceiver
2LO VCO
2LO Loop
Filter &
Tank Circuit
PLL 2
LPF
Lock
Detect
Lock Detect
Ref Frequency Input
Rx RF
Input
PLL 1
1LO Loop
Filter &
Tank Circuit
x2
1LO VCO
Rx RF
Input
1IF Input
Output
TRANSMIT MODE
MUX
The ML2712 uses a directly modulated VCO running
at the transmitter frequency to generate the
transmited signal. The VCO is then free of unwanted
spurious signals and has the advantage of requiring
no bandpass filtering for the transmitter signal prior
to or after the Power Amplifier. The transmitter VCO
is phase locked to the center frequency with the
transmitter modulation applied. The modulated signal
is applied directly to the VCO inside the loop
bandwidth of a phase locked loop. To allow
ML2712
Active
Disabled
Figure 1. Standby Mode Active Circuits
Control
Interface
RSSI
Tx VCO
Tuning
Voltage
WB
Charge
Pump
modulation rates in excess of 1Mbps requires a very
wideband phase detector, capable of operating with
loop bandwidths in excess of 5MHz. In this circuit
the noise floor is set by the transmit VCO rather than
by upconvert mixers. The circuits active in TRANSMIT
Mode are shown in Figure 2.
RSSI A/D
Comparator
Output
+
Control
–
DAC
WB
Phase
Comp
2LO Output
To IF Transceiver
2LO VCO
2LO Loop
Filter &
Tank Circuit
PLL 2
LPF
DIRECTLY MODULATEDTRANSMITVCO
Lock
Detect
Lock Detect
Ref Frequency Input
The transmitter is designed to enable a significant amount
of power to be generated at the required frequency using
a VCO, and is a technique for generating low noise,
phase/frequency modulated transmitters without the need
for bandpass filtering.
Tx RF
Input
PLL 1
1LO Loop
Filter &
Tank Circuit
x2
1LO VCO
Rx RF
Input
1IF Input/
Output
MUX
ML2712
The ML2712 transmitter architecture is shown Figure 3. A
tuning voltage applied to the transmit VCO, operating at
the final transmission frequency, ensures the correct
center frequency before modulation is applied. This closed
loop system, uses a PLL with the Transmit VCO phase
Active
Disabled
Figure 2. Transmit Mode Active Circuits
locked to a modulated reference signal (S
). The
MOD
modulated reference signal is generated at 260MHz. The
signal from the Transmit VCO is down converted with the
1LO signal, then filtered through a bandpass filter. The
output of the filter is fed to the very high-speed phase/
frequency detector Wideband PLL. This compares the
down-converted transmit signal with the modulated
TRANSMIT SIGNAL
(S + F1LO)
MOD
ERROR
CORRECTION
LOOP
FILTER
S
+
MOD
ERROR SIGNAL
RF DOWN
CONVERTER
reference signal S
.
PLL
S
MOD
MOD
Any frequency or phase error between S
and the
MOD
LOW PASS
FILTER
FREQUENCY = F1LO
S
= CARRIER FREQUENCY
+ MODULATION
MOD
down-converted signal is corrected by changing the
tuning voltage of the transmit VCO. The down-conversion
with the F1LO translates the reference signal S
final transmitter frequency.
to the
MOD
Figure 3. Directly Modulated VCO Transmitter
8
January, 2000
PRELIMINARY DATASHEET
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
