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Power Supply Requirements
The module does not have an internal
voltage regulator, therefore it requires a clean,
well-regulated power source. The power supply
noise should be less than 20mV. Power supply
noise can significantly affect the module's
performance, so providing a clean power supply
for the module should be a high priority during
design.
A 10Ω resistor in series with the supply followed by a 10µF tantalum
capacitor from V
to ground helps in cases where the quality of supply
cc
power is poor (Figure 88). This filter should be placed close to the module's
supply lines. These values may need to be adjusted depending on the
noise present on the supply line.
Antenna Considerations
The choice of antennas is a
critical and often overlooked
design consideration. The range,
performance and legality of an RF
link are critically dependent upon the
antenna. While adequate antenna
performance can often be obtained
by trial and error methods, antenna
design and matching is a complex
task. Professionally designed antennas such as those from Linx (Figure
89) help ensure maximum performance and FCC and other regulatory
compliance.
Linx transmitter modules typically have an output power that is higher
than the legal limits. This allows the designer to use an inefficient antenna
such as a loop trace or helical to meet size, cost or cosmetic requirements
and still achieve full legal output power for maximum range. If an efficient
antenna is used, then some attenuation of the output power will likely be
needed.
It is usually best to utilize a basic quarter-wave whip until your prototype
product is operating satisfactorily. Other antennas can then be evaluated
based on the cost, size and cosmetic requirements of the product.
Additional details are in Application Note AN-00500.
Vcc TO
MODULE
10Ω
Vcc IN
Figure 88: Supply Filter
Figure 89: Linx Antennas
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86
Interference Considerations
The RF spectrum is crowded and the potential for conflict with unwanted
sources of RF is very real. While all RF products are at risk from
interference, its effects can be minimized by better understanding its
characteristics.
10µF
Interference may come from internal or external sources. The first step
is to eliminate interference from noise sources on the board. This means
paying careful attention to layout, grounding, filtering and bypassing in
order to eliminate all radiated and conducted interference paths. For
many products, this is straightforward; however, products containing
components such as switching power supplies, motors, crystals and other
potential sources of noise must be approached with care. Comparing your
own design with a Linx evaluation board can help to determine if and at
what level design-specific interference is present.
External interference can manifest itself in a variety of ways. Low-level
interference produces noise and hashing on the output and reduces the
link's overall range.
High-level interference is caused by nearby products sharing the same
frequency or from near-band high-power devices. It can even come from
your own products if more than one transmitter is active in the same area.
It is important to remember that only one transmitter at a time can occupy
a frequency, regardless of the coding of the transmitted signal. This type of
interference is less common than those mentioned previously, but in severe
cases it can prevent all useful function of the affected device.
Although technically not interference, multipath is also a factor to be
understood. Multipath is a term used to refer to the signal cancellation
effects that occur when RF waves arrive at the receiver in different phase
relationships. This effect is a particularly significant factor in interior
environments where objects provide many different signal reflection paths.
Multipath cancellation results in lowered signal levels at the receiver and
shorter useful distances for the link.
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87
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