Preliminary
11-41
RF2516
Rev A10 010613
11
Designing With the RF2516
The reference oscillator is built around the onboard
transistor at pins 1 and 2. The intended topology is that
of a Colpitts oscillator. The Colpitts oscillator is quite
common and requires few external components, mak-
ing it ideal for low-cost solutions. The topology of this
type of oscillator is as seen in the following figure.
This type of oscillator is a parallel resonant circuit for a
fundamental mode crystal. The transistor amplifier is
an emitter follower and the voltage gain is developed
by the tapped capacitor impedance transformer. The
series combination of C1 and C2 act in parallel with the
input capacitance of the transistor to capacitively load
the crystal.
The nominal capacitor values can be calculated with
the following equations6:
and
The load capacitance is usually 32pF. The variable freq
is the oscillator frequency in MHz. The frequency can
be adjusted by either changing C2 or by placing a vari-
able capacitor in series with the crystal. As an exam-
ple, assume a desired frequency of 14MHz and a load
capacitance of 32pF. C1=137.1pF and C2=41.7pF.
These capacitor values provide a starting point. The
drive level of the oscillator should be checked by look-
ing at the signal at pin 2 (OSC E). It has been found
that the level at this pin should generally be around
500mVPP or less. This will reduce the reference spur
levels and reduce noise from distortion. If this level is
higher than 500mVPP then decrease the value of C1.
The values of these capacitors are usually tweaked
during design to meet performance goals, such as
minimizing the start-up time.
Additionally, by placing a variable capacitor in series
with the crystal, one is able to adjust the frequency.
This will also alter the drive level, so it should be
checked again.
An important part of the overall design is the voltage
controlled oscillator.The VCOisconfigured asadif-
ferential amplifier. The VCO is tuned via internal varac-
tors. The varactors are tuned by the loop filter output
voltage through a 4k
Ω resistor.
As mentioned earlier, the inductors and the varactors
aretuningadifferential amplifier.Totunethe VCO the
designer only needs to calculate the value of the induc-
tors connected to pins 12 and 13 (RESNTR- and
RESNTR+). The inductor value is determined by the
equation:
In this equation, f is the desired operating frequency
and L is the value of the inductor required. The value C
is the amount of capacitance presented by the varac-
tors and parasitics. For calculation purposes 1.5pF
should be used. The factor of one-half is due to the
inductors being in each leg. As an example, assume
an operating frequency of 433MHz. The calculated
value of each inductor is 45nH. A 47 nH inductor would
be appropriate as the closest available value.
X1
C2
C1
V
CC
C
1
60 C
load
⋅
freq
MHz
------------------------
=
C
2
1
1
C
load
-------------
1
C
1
------
–
--------------------------
=
4k
Ω
LOOP FLT
L
L
RESNTR+
RESNTR-
L
1
2
π f
⋅⋅
----------------
èø
æö
2
1
C
----
1
2
---
⋅⋅
=
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