RF2512 RF Micro Devices, RF2512 Datasheet - Page 7

• Manufacturer Part Number: RF2512
• Description: UHF TRANSMITTER
• Manufacturer: RF Micro Devices

tance is set by the loop filter output voltage through a
4k
To tune the VCO the designer only needs to calculate
the value of the inductors connected to pins 18 and 20
(RESNTR- and RESNTR+). The inductor value is
determined by the following 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-
tor, capacitor and parasitics. The factor of one-half is
due to the inductors being in each leg.
The setup of the VCO can be summarized as follows.
First, open the loop. Next, get the VCO to run on the
desired frequency by selecting the proper inductor and
capacitor values. The capacitor value will need to
include the varactor and circuit parasitics. After the
VCO is running at the desired frequency, then set the
VCO sensitivity.
The sensitivity is determined by connecting the control
voltage input point to ground and noting the frequency.
Then connect the same point to the supply and again
note the frequency. The difference between these two
frequencies divided by the supply voltage is the VCO
sensitivity expressed in Hz/V. Increasing the inductor
value while decreasing the capacitor value will
increase the sensitivity. Decreasing the inductor value
while increasing the capacitor value will lower the sen-
sitivity.
When increasing or decreasing component values,
make sure that the center frequency remains constant.
Finally, close the loop.
Rev B9 010509
resistor.
LOOP
FLT
L
=
æ
è
4 k
----------------
2
RESNTR+
1
L
f
ö
ø
2
--- -
C
1
RESNTR-
1
-- -
2
L
External to the part, the designer needs to implement a
loop filter to complete the PLL. The loop filter converts
the output of the charge pump into a voltage that is
used to control the VCO. Internally, the VCO is con-
nected to the charge pump output through a 4k resis-
tor. The loop filter is then connected in parallel to this
point at pin 23 (LOOP FLT). This limits the loop filter
topology to a second order filter usually consisting of a
shunt capacitor and a shunt series RC. A passive filter
is most common, as it is a low cost and low noise
design. An additional pole could be used for reducing
the reference spurs, however there is not a way to add
the series resistor. This should not be a reason for con-
cern however.
The schematic of the loop filter is
The transfer function is
where the time constants are defined as
The frequency at which unity gain occurs is given by
This is also the loop bandwidth.
If the phase margin (PM) and the loop bandwidth
(
constants. These are found using the equations
1
LBW
=
) are known, it is possible to calculate the time
------------------------------------------------- -
sec
2
Charge Pump
=
F s
PM
R
V
2
CC
=
LBW
C
–
2
R
tan
LBW
2
and
PM
------------------------------------------
s
=
Loop Filter
C2
1
------------------ -
R2
2
s
and
=
1
1
s
R
RF2512
2
C1
2
+
2
1
1
2
+
------------------ -
C
C
=
1
VCO
1
1
------------------------- -
+
C
2
C
LBW
2
2
1
11-17
1
11