MRF24J40 Datasheet, PDF (49/66 Pages) Microchip Technology – IEEE 802.15.4™ 2.4 GHz RF Transceiver

APPENDIX A: LAYOUT AND PART
SELECTION
A.1 Layout Considerations and RF
Measurements
Below is an example of the circuit diagram of a balun.
A balun is the impedance transformer from unbalanced
input of the PCB antenna and the balanced input of the
RF transceiver (pins RFP and RFN).
FIGURE A-1: EXAMPLE CIRCUIT DIAGRAM
MRF24J40
Figure A-2 shows the measured impedance of the
balun where the center of the band is very close to 50Ω.
When using low tolerance components (i.e., ±5%)
along with an appropriate ground, the impedance will
remain close to the 50Ω measurement. Figure A-3
shows the measured impedance of the PCB antenna
using a logarithmic scale magnitude. Figure A-4 shows
the same impedance using a Smith Chart diagram and
Figure A-5 using a voltage standing wave ratio.
C40
100 nF
ANT
0.3 pF
C7
L1
4.7 nH
C38
0.5 pF
0.5 pF
C23
10 nH
L5
C43
0.5 pF
L4
10 nH
L3
5.6 nH
0.5 pF
C37
RFP
RFN
FIGURE A-2: MEASURED IMPEDANCE
© 2006 Microchip Technology Inc.
Advance Information
DS39776A-page 47