Datasheet

MAX2620
10MHz to 1050MHz Integrated
RF Oscillator with Buffered Outputs
8 _______________________________________________________________________________________
Keeping the resonant tank circuit’s real component less
than one-half the magnitude of the negative real com-
ponent ensures that oscillations will start. After start-up,
the oscillator’s negative resistance decreases, primarily
due to gain compression, and reaches equilibrium with
the real component (the circuit losses) in the resonant
tank circuit. Making the resonant tank circuit reactance
tunable (e.g., through use of a varactor diode) allows
for tuneability of the oscillation frequency, as long as
the oscillator exhibits negative resistance over the
desired tuning range. See Figures 3 and 4.
The negative resistance of the MAX2620 TANK pin can
be optimized at the desired oscillator frequency by
proper selection of feedback capacitors C3 and C4.
For example, the one-port characteristics of the device
are given as a plot of 1/S11 in the Typical Operating
Characteristics. 1/S11 is used because it maps inside
the unit circle Smith chart when the device exhibits
negative resistance (reflection gain).
V
TUNE
V
CC
V
CC
1
TANK
FDBK
SHDN
8
7
6
5
OUT
V
CC
2
GND
OUT
1
2
3
4
V
CC
V
CC
V
CC
1kΩ
D1
D1 = SMV1200-155 DUAL VARACTOR
C17
33pF
10Ω
51Ω
0.01µF
10µH
0.01µF
OUT TO
MIXER
C6
33pH
C5
150pF
C3
270pF
1000pF
1000pF
1000pF
27pF
1000pF
C4
270pF
SHDN
L1
2.2µH
OUT TO
SYNTHESIZER
MAX2620
Figure 3. 10MHz VCO LC Resonator