User Manual

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78 JN-DS-JN5139 v1.5 © Jennic 2008
B.3.7 VCO Tune Circuit Layout Considerations
The layout of these components is such that all three components are close together, and close to the VCO_TUNE
and VB_VCO pins on the wireless microcontroller IC. This improves the performance of the PLL by reducing parasitic
impedance and the likelihood of cross-talk.
B.3.8 Radio Front-End
The radio part of the wireless microcontroller device has an internal transmit-receive switch connected to the external
pins on the chip (RF- and RF+). The PHY controller of the radio configures the switch between transmit and receive.
In both configurations, the connection to the device is a differential 200-ohm configuration. As an example of how
this may be used, the 200-ohm differential antenna connection (RF- and RF+) can be fed to a miniature balun to
convert to a single-ended 50-ohm microstrip line which, in turn, can be connected to a small ceramic antenna.
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 Note: The PCB layout is very important for all of the external radio
connections and associated power supplies. In this respect, the tolerances
indicated in Figure 51: PCB Cross-Section are particularly important
B.3.9 Antennae
There are many different antenna configurations that could function for a 2.4-GHz transceiver. The free-space
wavelength at 2.4 GHz is approximately 12 cm, which means that a standard half-wave dipole would be
approximately 6 cm.
When advising on antenna design, it is dangerous to generalise. However, designers of any low-power radio device
must strive to ensure that as little power as possible is wasted in producing a radio signal transmission. This involves
careful consideration of the terms antenna efficiency, antenna directivity and antenna gain:
• Antenna Efficiency : This is a measure of how much energy fed into the antenna feed is actually retained in the
radio transmission. For example, a small antenna may exhibit an efficiency of approximately 50%, which
means that half the power fed into or out of the antenna is wasted. Clearly, it is important to keep efficiency as
high as possible. However, small antennae exhibit lower efficiency than large antennae.
• Antenna Directivity : An antenna radiation pattern indicates in which direction the power fed into an antenna
actually radiates. In situations where antennae can be aligned to “see” each other, this can be advantageous.
However, many situations do not allow this, since a path from one device to another may occur in any direction.
In general, larger antennae have a greater ability to radiate in a specific direction. In antenna terminology, this
is called the “directivity”. For instance, an antenna with a directivity of 3 dBi has the ability to radiate twice as
much power in one direction when compared with a theoretical omni-directional antenna. This is fine if both
antennae are aligned in this direction, but it is not good if they are misaligned.
• Antenna Gain : Often, the term “gain” is used when discussing antennae. This term should be treated with
some caution since it is the product of efficiency and directivity. A poor-efficiency antenna with a high directivity
can still exhibit a reasonable gain; however, power is still being wasted somewhere!
A list of antennae and suppliers can be found in the Application Note Antennae for use with JN51xx (JN-AN-1030) ,
available on the Jennic web site.
B.3.10 Ground Planes
The recommendation for a four-layer design allows the best use of the ground planes, with this in mind the following
restrictions should be placed on the layout:
• All RF signals are confined to the top layer.