11-30
RF2513
Rev B8 010509
11
As an example, consider a loop bandwidth of 50 kHz, a
phase margin of 45°, a divide ratio of 64, a KVCO of
20MHz/V, and a KPD of 0.01592mA/2πrad. Time con-
stant
τ
1 is 1.31848 µs, time constant τ2 is 7.68468 µs,
C1 is 131.15pF, C2 is 633.26pF, and R2 is 12.14kΩ.
In order to perform these calculations, one will need to
know the value of two constants, KVCO and KPD.KPD is
calculated by dividing the charge pump current by 2
π.
For the RF2513, the charge pump current is 40
µA.
KVCO is best found empirically as it will change with
frequency and board parasitics. By briefly connecting
pin 23 (LOOP FLT) to VCC and then to ground, the fre-
quency tuning range of the VCO can be seen. Dividing
the difference between these two frequencies by the
difference in the voltage gives KVCO in MHz/V.
The control lines provide an interface for connecting
the device to a microcontroller or other signal generat-
ing mechanism. The designer can treat pin 16 (MOD
IN), pin 15 (DIV CTRL), pin 14 (MOD CTRL), pin 24
(OSC SEL), and pin 7 (LVL ADJ) as control pins whose
voltage level can be set.
General RF bypassing techniques must be observed
to get the best performance. Choose capacitors such
that they are series resonant near the frequency of
operation.
Board layout is always an area in which great care
must be taken. The board material and thickness are
used in calculating the RF line widths. The use of vias
for connection to the ground plane allows one to con-
nect to ground as close as possible to ground pins.
When laying out the traces around the VCO, it is desir-
able to keep the parasitics equal between the two legs.
This will allow equal valued inductors to be used.
Pre-compliance testing should be performed during
the design process. This can be done with a GTEM cell
or at a compliance testing laboratory. It is recom-
mended that pre-compliance testing be performed so
that there are no surprises during final compliance
testing. This will help keep the product development
and release on schedule.
Working with a laboratory offers the benefit of years of
compliance testing experience and familiarity with the
regulatory issues. Also, the laboratory can often pro-
vide feedback that will help the designer make the
product compliant.
On the other hand, having a GTEM cell or an open air
test site locally offers the designer the ability to rapidly
determine whether or not design changes impact the
product's compliance. Set-up of an open air test site
and the associated calibration is not trivial. An alterna-
tive is to use a GTEM test cell.
After the design has been completed and passes com-
pliance testing, application will need to be made with
the respective regulatory bodies for the geographic
region in which the product will be operated to obtain
final certifications.
Conclusions
The RF2513 is an FM/FSK UHF transmitter that fea-
tures a phase-locked output. This device is suitable for
use in a CFR Part 15.231 compliant product as well as
a local oscillator signal source. Two examples of show-
ing these applications have been provided. Further, the
RF2513 is packaged in a low cost SSOP-24 plastic
package and requires few external parts, thus making
it suitable for low cost designs.
C
1
τ
1
τ
2
-----
K
PD
K
VCO
⋅
ω
2
LB W
N
⋅
-----------------------------
1
ω
LBW
τ
2
⋅
()
2
+
1
ω
LBW
τ
1
⋅
()
2
+
----------------------------------------
⋅⋅
=
C
2
C
1
τ
2
τ
1
-----
1
–
èø
æö
⋅
=
R
2
τ
2
C
2
------
=
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