User Manual

Revision 1.0 Page 64 of 78
nRF24L01+ Product Specification
10 Peripheral RF Information
This chapter describes peripheral circuitry and PCB layout requirements that are important for achieving
optimum RF performance from the nRF24L01+.
10.1 Antenna output
The ANT1 and ANT2 output pins provide a balanced RF output to the antenna. The pins must have a DC
path to
VDD_PA, either through a RF choke or through the center point in a balanced dipole antenna. A
load of 15
Ω+j88Ω is recommended for maximum output power (0dBm). Lower load impedance (for
instance, 50
Ω) can be obtained by fitting a simple matching network between the load and ANT1 and
ANT2. A recommended matching network for 50Ω load impedance is described in chapter 11 on page 66.
10.2 Crystal oscillator
A crystal used with the nRF24L01+ must fulfil the specifications in Table 11. on page 19.
To achieve a crystal oscillator solution with low power consumption and fast start up time use a crystal with
a low load capacitance specification. A lower C
0
also gives lower current consumption and faster start up
time, but can increase the cost of the crystal. Typically C
0
=1.5pF at a crystal specified for C
0max
=7.0pF.
The crystal load capacitance, C
L, is given by:
, where C
1
’ = C
1
+ C
PCB1
+C
I1
and C
2
’ = C
2
+ C
PCB2
+ C
I2
C1 and C2 are SMD capacitors, see the application schematics in Figure 32. on page 66. CPCB1 and CPCB2
are the layout parasitic on the circuit board. CI1 and CI2 are the internal capacitance load of the XC1 and
XC2 pins respectively; the value is typically 1pF for both these pins.
10.3 nRF24L01+ crystal sharing with an MCU
Follow the rules described in sections 10.3.1 and 10.3.2 when using an MCU to drive the crystal reference
input
XC1 of the nRF24L01+ transceiver.
10.3.1 Crystal parameters
The MCU sets the requirement of load capacitance C
L
when it is driving the nRF24L01+ clock input. A fre-
quency accuracy of ±60ppm is required to get a functional radio link. The nRF24L01+ loads the crystal by
1pF in addition to the PCB routing.
10.3.2 Input crystal amplitude and current consumption
The input signal should not have amplitudes exceeding any rail voltage. Exceeding rail voltage excites the
ESD structure and consequently, the radio performance degrades below specification. You must use an
external DC block if you are testing the nRF24L01+ with a reference source that has no DC offset (which is
usual with a RF source).
''
''
21
21
CC
CC
L
C
+
⋅
=