Biomedical Engineering Reference
In-Depth Information
Core(100 µm)
Prepreg (500 µm)
Core(100 µm)
UWB and analog front end
Power plane
Ground plane
Narrow band and
micro-controller
Fig. 5.20
Layer stack up of the sensor node PCB
5.2.9 Sensor Node Integration and Design Considerations
The sensor node is implemented on a four layer PCB in order to preserve the
compactness of the sensor design while minimising the electromagnetic interfer-
ence that occur due to the simultaneous operation of the narrow band and UWB
RF sections. RO4350 by Rogers is chosen as the PCB core material due to its low
dielectric losses at the frequencies of interest and immunity against cross talk and
noise.
Interference mitigation is a major concern for the dual band sensor node design.
This design employs two techniques in order to minimise the possible interference
between the narrow band and UWB sections of the circuit. The first strategy is to
design a regionalised circuit structure using a four-layer PCB design as shown in
Fig.
5.20
. The use of power and ground planes in between the RF signal layers
improves the immunity against cross talk, and improves the noise performance of
the circuit by as much as 15 dB [
20
].
Use of separate ground layers for UWB and narrow band sections is employed
as the second technique to minimise interference between the two RF sections.
These ground layers are connected in a single spot using a ferrite bead that pre-
vents the exchange of high frequency noise between the two RF sections of the
sensor node. Power supplies to all the components are decoupled using bypass
capacitors. A ferrite bead is added in series with the power supply of the narrow
band receiver in order to prevent the high frequency noise that might leak to the
power plane from the UWB transmitter. Many grounded vias are used near all the
RF components, such as VCO and mixer, in order to prevent the occurrence of
current loops that degrade the signal quality. Stacked micro vias are used for
multilayer routing of RF and data signals in order to improve the immunity against
Electromagnetic Interference (EMI) [
21
].
All RF signals are routed using Co-Planar Wave Guides (CPWG) with
impedance matched to 50 X. The use of CPWG is preferred to route RF trans-
mission lines over the micro-strip lines due to the lower loss tangent and the
possibility of narrowing the RF traces in order to match the component pad
dimensions. The impedance of the RF tracks is obtained using (
5.11
)[
22
].
