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The receiver features a RSSI output (Received Signal Strength Indicator). The

RSSI pin gives a linear voltage output corresponding to incoming signal level.

The RSSI output has many valuable uses including interference assessment,

signal strength indication, external data squelching and qualification, and

transmitter presence indication. Since RSSI values vary from part to part and

correspond to signal strength and not necessarily distance, the pin is not

recommended for range-finding applications.

When RF transmissions are not present, or when the received signal strength is too

low to ensure proper demodulation, the data output is squelched to a continuous

high state. This ensures compatibility with common serial UART’s. It is important to

note that the squelch level is set just over the transceiver’s internal noise threshold

and any external RF activity above that threshold will produce hashing on the

transceiver output. While the data output is qualified high in low-noise environments,

the designer should always plan for the potential of hashing. This is a key area of

consideration when considering the overall protocol structure.

The two most obvious ways a wireless link differs from a wire are timing and

susceptibility to interference. Brief reviews of each of these critical areas follows:

INTERFERENCE CONSIDERATIONS

It must be recognized that many bands are widely used, and the potential for

conflict with other unwanted sources of RF is very real. Despite careful design,

all RF products are at risk from interference.

Interference can manifest itself in many ways. Low-level interference will produce

noise and hashing on the output and reduce the link’s overall range. Thanks to

the capture properties of an FM system, the transceiver will still function when an

intended signal is present at a higher level than the interference.

Another type of interference can be caused by higher-powered devices such as

hopping spread-spectrum devices. Since these devices move rapidly from

frequency to frequency they will usually cause short, intense losses of

information. Such errors are referred to as bursting errors and will generally be

dealt with through protocol.

High-level interference is caused by products sharing the same frequency or

from near-band high-power devices. Fortunately, this type of interference is less

common than those mentioned previously, but in severe cases can prevent all

useful function of the affected device.

Although technically it is not interference, multipath is also a factor to be

understood. Multipath is a term used to refer to the signal cancelation effects that

occur when RF waves arrive at the receiver in different phase relationships. This

is particularly a factor in interior environments where objects provide many

different reflection paths. Multipath results in lowered signal levels at the receiver

and thus shorter useful distances for the link.

A spectrum analyzer is the primary tool of the RF engineer for assessing

interference sources and levels, but is often not available due to its cost.

Fortunately, the RSSI voltage output can also be used to determine the presence

and strength of interference during the development process.