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1.2GHz VCO with Linear
Modulation Input
Pin Description
PIN
1
2
3
4
5
6
7
8
NAME
VREG
TUNE
GND1
MOD
SHDN
GND2
OUT
V CC
FUNCTION
Capacitor Connection to the On-Chip Linear Regulator Output. Connect a 330nF capacitor to ground.
Oscillator Frequency Tuning-Voltage Input. High-impedance input with a voltage range of +0.4V (low
frequency) to +2.4V (high frequency).
Ground Connection for the Oscillator Core. Requires a low-inductance connection to the circuit-
board ground plane.
Linear Modulation Input. High-impedance CMOS input with a voltage range of +0.4V to +2.4V.
Shutdown Input. Drive logic low to place the device in shutdown mode. Drive logic high for normal
operation.
Ground Connection for Output-Buffered Amplifier, Linear Modulation Interface, and Biasing.
Requires a low-inductance connection to the circuit-board ground plane.
Buffered Oscillator Output. Incorporates an internal DC-blocking capacitor. OUT is internally
matched to 50 ? .
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.
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
50 ? 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.
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