Datasheet

Detailed Description
Oscillator
The MAX2754 VCO is implemented as an LC oscillator
topology, integrating all of the tank components on-
chip. This fully monolithic approach provides an
extremely easy-to-use VCO, equivalent to a VCO mod-
ule. The frequency is controlled by a voltage applied to
the TUNE pin. The VCO core uses a differential topolo-
gy to provide a stable frequency versus supply voltage
and improve the immunity to load variations. In addi-
tion, there is a buffer amplifier following the oscillator
core to provide added isolation from load and supply
variations and to boost the output power.
Linear Modulation
The linear modulation input offers a means to directly
FM modulate the VCO with a controlled amount of fre-
quency deviation for a given input voltage deviation.
The unique technique maintains a consistent modula-
tion gain (df/dV
MOD
) across the entire frequency tuning
range of the part, enabling accurate FM modulation
derived solely from the filtered NRZ “data” stream (the
modulation voltage input).
The modulation input is single-ended and centered
about +1.4V. The linear modulation full-scale range is
±1V around this point, for a +0.4V to +2.4V input volt-
age range. A very important point to note is that the
sign of the modulation gain is negative. A positive
change in V
MOD
results in a negative change in oscilla-
tion frequency. This convention for the modulation gain
is due to the practical implementation of the internal lin-
earizing circuitry. This gain inversion must be consid-
ered when designing the analog voltage interface that
drives the linear modulation input. The easiest way to
handle this is to invert the logic polarity of the modula-
tion data three-state output buffer (TX data output).
Where it is impossible to invert the data-stream logic
polarity, an external inverter and three-state buffer
would be required. These devices are offered in small
single-logic gates in SC-79 style packages from various
manufacturers (e.g., Fairchild—Tiny Logic, On
Semiconductor, or Rohm).
Figure 1 illustrates the frequency versus V
MOD
charac-
teristic of the modulation input. Note the negative slope
of the curve, df
MOD
/dV
MOD
< 0, where f
MOD
= f
OUT
-
f
NOM
.
Output Buffer
The oscillator signal from the core drives an output
buffer amplifier. The amplifier is internally matched to
50Ω including an on-chip DC-blocking capacitor. The
return loss can be improved to a minimum of 12dB over
1145MHz to 1250MHz by adding a 2.5nH series induc-
tor and a 3.0pF shunt capacitor. The output buffer has
a ground connection separate from the oscillator core
to minimize load-pulling effects. The amplifier boosts
the oscillator signal to a level suitable for driving most
RF mixers.
MAX2754
1.2GHz VCO with Linear
Modulation Input
_______________________________________________________________________________________ 5
Pin Description
PIN NAME FUNCTION
1 VREG Capacitor Connection to the On-Chip Linear Regulator Output. Connect a 330nF capacitor to ground.
2 TUNE
Oscillator Frequency Tuning-Voltage Input. High-impedance input with a voltage range of +0.4V (low
frequency) to +2.4V (high frequency).
3 GND1
Ground Connection for the Oscillator Core. Requires a low-inductance connection to the circuit-
board ground plane.
4 MOD Linear Modulation Input. High-impedance CMOS input with a voltage range of +0.4V to +2.4V.
5 SHDN
Shutdown Input. Drive logic low to place the device in shutdown mode. Drive logic high for normal
operation.
6 GND2
Ground Connection for Output-Buffered Amplifier, Linear Modulation Interface, and Biasing.
Requires a low-inductance connection to the circuit-board ground plane.
7 OUT
Buffered Oscillator Output. Incorporates an internal DC-blocking capacitor. OUT is internally
matched to 50Ω.
8V
CC
Supply Voltage Connection. Requires external RF bypass capacitor to ground for low noise and low
spurious content performance from the oscillator. Bypass with a 330pF capacitor to ground.