MAX2606EUT-T Maxim Integrated Products, MAX2606EUT-T Datasheet - Page 6

• Manufacturer Part Number: MAX2606EUT-T
• Description: RF Wireless Misc 70 to 150MHz VCO wit h Differential Outpu
• Manufacturer: Maxim Integrated Products

45MHz to 650MHz, Integrated IF
VCOs with Differential Output
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.
The desired VCO operating frequency is set by the
value of the external inductance, L
the inductance value L
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.
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
achieve the desired inductance value. Choose L
be a standard-value inductor with a value just less than
6
Table 1. External Inductor L
MAX2605
MAX2606
MAX2607
MAX2608
MAX2609
PART
_______________________________________________________________________________________
FREQUENCY
150 to 300
300 to 500
500 to 650
70 to 150
RANGE
45 to 70
Applications Information
(MHz)
Desired Oscillation Frequency
F
Detailed Description
Inductor Implementation
required to achieve the desired
INDUCTANCE
680 ≤ L
150 ≤ L
39 ≤ L
3.9 ≤ L
10 ≤ L
RANGE
(nH)
F1
F
F
F
F
F
≤ 2200
≤ 180
F
≤ 820
≤ 47
≤ 15
. Figures 1–5 show
and L
F
Range
F2
INDUCTOR
, in order to
MIN
35
35
35
40
40
Q
F1
to
that required for L
inductor with a value just less than (L
should adhere to the minimum Q requirements, but L
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
and L
(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
implemented as a PC board trace (shorted to GND),
rather than an SMT inductor. When designing L
two inductors, use the simple model in Figure 7 to cal-
culate X
The L
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
Design L
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.
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
TUNE must also have a high-frequency AC ground for
Table 2. C
NOMINAL
L
F2
F
EQ
in Figures 1–5 represents an equivalent induc-
L
must be greater than the minimum inductor Q
and L
= X
MAX2605
MAX2606
MAX2607
MAX2608
MAX2609
DEVICE
).
EQ
BYPASS
L
= L
/ (2π f
EQ
F
.
F
. Choose L
Bypass Capacitor on TUNE
NOMINAL
at the desired f
F2
Values
are sometimes more exactly
) as seen in Figure 8
F2
to be a standard-value
C
≥ 820 pF
≥ 680 pF
≥ 330 pF
≥ 100 pF
≥ 39 pF
BYPASS
NOMINAL
F
- L
F1
). L
F
. The
with
F1
F2
F1
F
.