nAN24-17 - Nordic Semiconductor

Using a 2450BM14A0002 Balun with
nRF24LE1 QFN32
nAN24-17
Application Note v1.0
All rights reserved.
Reproduction in whole or in part is prohibited without the prior written permission of the copyright holder.
2011-05-18
nAN24-17
Contents
1
2
3
3.1
3.2
3.3
4
Introduction .................................................................................................
Antenna matching network overview........................................................
Implementation............................................................................................
Schematic .............................................................................................
Layout ...................................................................................................
Measurement results on the 2450BM14A0002 balun...........................
Conclusion...................................................................................................
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Using a 2450BM14A0002 Balun with nRF24LE1 QFN32
1
Introduction
This application note describes how to use Johanson Technology’s 2450BM14A0002 balun (an SMD sixpin device with a small footprint of 1.6x0.8 mm (EIA 0603)) with the nRF24LE1 QFN32.
The 2450BM14A0002 balun greatly simplifies the RF front-end by considerably reducing component
count, system variability, implementation size area, and PCB sensitivity. Johanson Technology’s
2450BM14A0002 balun was originally designed for the nRF24L01(+) transceivers, and a technical note on
how to use it with nRF24L01(+) can be downloaded from Johanson Technology’s website (http://
www.johansontechnology.com/en/technical-notes.html)
Since nRF24LE1 uses nRF24L01+ as its radio core, it means the 2450BM14A0002 balun can also be
used on some of the nRF24LE1 variants.
nRF24LE1 has a different packet size to the nRF24L01(+) standalone chips, so the length of the bonding
wires is different. This means the impedance load on the antenna pins is different for nRF24LE1 when
compared to nRF24L01(+). This application note describes how to compensate for this difference.
Note: This application note does not apply to nRF24LE1 QFN24 or nRF24LE1 QFN48. The
2450BM14A0002 balun is not suitable for these devices because of the different packet sizes.
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Antenna matching network overview
The antenna matching network has four main tasks:
•
•
•
•
Balanced to unbalanced tranformation (balun)
Impedance transformation from a high impendance PA (Power Amplifier) load to 50 Ω
Low pass filtering to suppress harmonics
Power supply feed to the PA
All these tasks are handled by the antenna matching network’s three inductors and three capacitors, see
Figure 1.. The layout is critical since parasitic capacitance and inductance can be high.
L3
ANT2
ANT1
VDD_PA
J1
C5
4.7n
L1
6.8n
1.5p
C6
L2
1.0p
GND
6.8n
C3
C4
2.2nF
NA
GND
GND
GND
Figure 1. Standard antenna matching network
If space is limited, you can replace the antenna matching network with a single component; an integrated
balun. This performs the same tasks as the discrete balun, but with a single component only.
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Using a 2450BM14A0002 Balun with nRF24LE1 QFN32
3
Implementation
The balun replaces all RF components in the antenna matching network and performs the same tasks as
the antenna matching network (see Figure 1. on page 4) but with reduced space requirements.
3.1
Schematic
The schematic of the nRF24LE1 QFN32 and the 2450BM14A0002 balun are shown in Figure 2..
XC1
XC2
VCC_nRF
P1.5
P1.6
P0.0
C11
100nF
GND
22k 1%
U1
GND
GND
VCC_nRF
C10
33nF
1
C3
C4
2.2nF
N.A
GND
B1
2450BM14A0002
GND
6
DC
GND
NC Unbal
2
3
Bal2 Bal1
=Value
5
nRF24LE1
24
23
22
21
20
19
18
17
J1
GND
P1.3
P1.4
RESET
GND
C8
33nF
VDD
VSS
ANT2
ANT1
VDD_PA
RESET
P1.4
P1.3
VDD
P0.4
P0.5
P0.6
P0.7
P1.0
P1.1
P1.2
GND
C7
100nF
P0.1
VDD
DEC1
DEC2
P0.2
PROG
P0.3
VSS
9
10
11
12
13
14
15
16
C9
100nF
P0.2
PROG
P0.3
1
2
3
4
5
6
7
8
4
P0.1
P0.4
P0.5
P0.6
P0.7
P1.0
P1.1
P1.2
VCC_nRF
Ant
RF I/O
GND
P0.0
XC1
XC2
P1.6
P1.5
VDD
VSS
IREF
32
31
30
29
28
27
26
25
R1
GND
Figure 2. nRF24LE1 QFN32 schematic and the balun
The capacitor C3 (2.2nF) is needed to filter and decouple the power supply to the PA. All other matching
network components are replaced by the balun B1. You can connect the 50 Ω unbalanced output directly
to a 50 Ω antenna, or to the antenna match.
3.2
Layout
The nRF24LE1 QFN32 has shorter internal bonding wires than nRF24L01(+), so to compensate for this
difference, the balun is placed 2.34 mm away from nRF24LE1. This is a longer distance than used for the
nRF24L01(+) standalone chipsets and this extra length increases the inductance of the tracks from the
ANT1 and ANT2 pins to the balun, creating a more optimal load for nRF24LE1. See Figure 3. on page 6.
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Figure 3. Balun layout
Note: The distance is measured from the edge of the nRF24LE1 pad to the edge of the balun pad.
3.3
Measurement results on the 2450BM14A0002 balun
Figure 4. on page 7 shows the output power, measured on the 50 Ω unbalanced port on the balun. The
carrier is swept across the 2.4 GHz band (markers are placed at 2.40 GHz, 2.44 GHz and 2.48 GHz).
Figure 5. on page 7 to Figure 7. on page 8 shows the harmonic components (markers are placed at
maximum power). Figure 8. on page 9 shows the LO leakage in RX mode (a marker is placed at the
maximum power).
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Using a 2450BM14A0002 Balun with nRF24LE1 QFN32
Figure 4. Output power, carrier sweep
Figure 5. 2nd harmonic, carrier sweep
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Figure 6. 3rd harmonic, carrier sweep
Figure 7. 4th harmonic, carrier sweep
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Using a 2450BM14A0002 Balun with nRF24LE1 QFN32
Figure 8. RX LO, channel sweep
Frequency (GHz)
2.40
2.44
2.48
Measurement level (dBm)
0.351
-0.552
-1.022
Pass ETSI/FCC?
Yes
Yes
Yes
Table 1.Output power levels on the 2450BM14A0002 balun
Name
2nd harmonic
3rd harmonic
4th harmonic
RX LO
Frequency
(GHz)
4.802
7.299
9.876
2.761
Measurement level
(dBm)
-38.63
-54.95
-59.22
-55.24
Pass ETSI/FCC?
Yes
Yes
Yes
Yes
Table 2.Harmonics and RX LO levels on the 2450BM14A0002 balun
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Conclusion
As the measurement results show (see Table 2. on page 9), the output power is close to 0 dBm across the
band. The harmonics are well below the regulation limits for the ETSI and FCC regulatory standards and
so is the spurious in RX mode.
The discrete antenna matching network has similar measurement results:
Frequency (GHz)
2.400
2.480
Measurement level (dBm)
0.3
-0.6
Pass ETSI/FCC?
Yes
Yes
Table 3.Output power levels on the discrete antenna matching network
Name
2nd harmonic
3rd harmonic
4th harmonic
RX LO
Frequency
(GHz)
4.802
7.299
9.876
2.761
Measurement level
(dBm)
-44.8
-54.9
-51
-57.4
Pass ETSI/FCC?
Yes
Yes
Yes
Yes
Table 4.Harmonics and RX LO levels on the discrete antenna matching network
These measurement results prove that the discrete balun can be replaced with an integrated balun without
any performance loss.
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Using a 2450BM14A0002 Balun with nRF24LE1 QFN32
Liability disclaimer
Nordic Semiconductor ASA reserves the right to make changes without further notice to the product to
improve reliability, function or design. Nordic Semiconductor ASA does not assume any liability arising out
of the application or use of any product or circuits described herein.
Life support applications
Nordic Semiconductor’s products are not designed for use in life support appliances, devices, or systems
where malfunction of these products can reasonably be expected to result in personal injury. Nordic
Semiconductor ASA customers using or selling these products for use in such applications do so at their
own risk and agree to fully indemnify Nordic Semiconductor ASA for any damages resulting from such
improper use or sale.
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Revision History
Date
May 2011
Revision 1.0
Version
1.0
Page 11 of 11
Description