Datasheet
45MHz to 650MHz, Integrated IF
VCOs with Differential Output
Detailed Description
The MAX2605–MAX2609 are low-noise VCOs designed
for fixed/single-frequency IF applications. The core
oscillator circuit is based on the well-known Colpitts
topology. The varactor and feedback capacitors are
integrated on-chip so that only an external inductor is
required to establish the frequency of oscillation and
produce a properly operating VCO. The tuning range,
biasing, startup, etc., are all managed within the IC. This
highly integrated design dramatically simplifies the
parts’ application.
The tuning range is wide enough so that, with the use
of ±2% tolerance inductors, no board-level adjustments
to the oscillation frequency are necessary. Once the
correct inductor value is chosen, the VCO is guaran-
teed always to tune to the desired operating frequency.
In addition, with the use of inductors of moderate Q (35
to 40), the VCO achieves excellent phase-noise perfor-
mance.
Applications Information
Desired Oscillation Frequency
The desired VCO operating frequency is set by the
value of the external inductance, L
F
. Figures 1–5 show
the inductance value L
F
required to achieve the desired
oscillation frequency. The inductor value can be taken
directly from these figures. Inductance must be select-
ed accurately to ensure proper operation over all con-
ditions.
Inductor Implementation
The inductance value required for the desired operat-
ing frequency may not necessarily coincide with a stan-
dard-value SMT inductor, which typically increases size
in ~1.2x steps. In such cases, the inductance must be
constructed from two inductors, L
F1
and L
F2
, in order to
achieve the desired inductance value. Choose L
F1
to
be a standard-value inductor with a value just less than
that required for L
F
. Choose L
F2
to be a standard-value
inductor with a value just less than (L
F
- L
F1
). L
F1
should adhere to the minimum Q requirements, but L
F2
may be implemented as a lower-cost, lower-Q, thin-film
SMT inductor. Its lower Q has only a small impact on
the overall Q of the total inductance because it is <20%
of the total inductance. However, the overall Q of L
F1
and L
F2
must be greater than the minimum inductor Q
(Table 1).
It is also permissible to use PC board traces to provide
a small amount of inductance, thereby adjusting the
total inductance value. On the MAX2608/MAX2609, the
inductance values for L
F2
are sometimes more exactly
implemented as a PC board trace (shorted to GND),
rather than an SMT inductor. When designing L
F
with
two inductors, use the simple model in Figure 7 to cal-
culate X
L
and L
EQ
.
The L
F
in Figures 1–5 represents an equivalent induc-
tance as seen by pin 1 (IND). The equivalent induc-
tance corresponds to the inductive reactance
connected to IND at the desired oscillation frequency
(f
NOMINAL
).
L
EQ
= X
L
/ (2π f
NOMINAL
) as seen in Figure 8
Design L
EQ
= L
F
at the desired f
NOMINAL
. The
MAX2605–MAX2609 are designed to tolerate approxi-
mately 0.5pF of external parasitic capacitance at IND.
This parasitic capacitance arises from the pad capaci-
tance at the device pin and pads for the inductor.
Additional shunt capacitance is not recommended
because it degrades the tuning range.
Bypass Capacitor on TUNE
The MAX2605–MAX2609’s oscillator design uses a vari-
ant of the Colpitts topology, where DC bias for the var-
actor is applied via a DC voltage on TUNE and a
ground connection through the external inductor L
F
.
TUNE must also have a high-frequency AC ground for
Table 1. External Inductor L
F
Range
Table 2. C
BYPASS
Values
403.9 ≤ L
F
≤ 15500 to 650MAX2609
40
35
35
35
MIN
INDUCTOR
Q
10 ≤ L
F
≤ 47300 to 500MAX2608
39 ≤ L
F
≤ 180
150 ≤ L
F
≤ 820
680 ≤ L
F
≤ 2200
INDUCTANCE
RANGE
(nH)
FREQUENCY
RANGE
(MHz)
45 to 70
70 to 150
150 to 300MAX2607
MAX2606
MAX2605
PART
≥ 39 pFMAX2609
≥ 100 pFMAX2608
≥ 330 pF
≥ 680 pF
≥ 820 pF
C
BYPASS
MAX2607
MAX2606
MAX2605
DEVICE
MAX2605–MAX2609
6
Maxim Integrated