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INPUTTING DIGITAL DATA
The data input pin may be directly connected to virtually any digital peripheral
including microcontrollers, encoders, and UART’s. The data input has an
impedance of 200kΩ and can be used with any data that transitions from 0V to
a 3V-5V peak amplitude within the specified bandwidth of the module. While it is
possible to send data at rates higher than specified, the internal data filter will
cause severe roll off and attenuation.
Many RF products require a fixed data transition rate or place tight constraints
on the mark/space ratio of the data being sent. Thankfully, the HP-3 transmitter
architecture eliminates such considerations and allows virtually any signal,
including PWM, Manchester and NRZ data to be sent at rates from 100bps to
56kbps. This is accomplished by directly modulating the PLL’s frequency
reference within the loop filter bandwidth. By doing so, the loop filter can be
optimized for rapid startup while allowing near DC modulation.
Unlike a radio modem the HP-3 does not encode or packetize the data in any
manner. This transparency gives the designer great freedom in software and
protocol development. A designer may also find creative ways to utilize the ability
of the transmitter to accept both digital and analog signals. For example, an
application might transmit voice in analog then send out a digital control
command. Such mixed mode systems, which combine analog signals and data
can greatly enhance the function and versatility of many products without a
significant increase in implementation cost.
It is always important to think of an RF link as a total system taking into account
both the transmitter and receiver characteristics. The incoming data must not
only be compatible with the transmitter but also within the capability of the
receiver to reproduce it. For example, if the transmitter were sending a 255 (0FF
hex) continuously the receiver would view the stream of high bits as a DC level.
The receiver would hold that level until a transition was required to meet its
minimum transition frequency requirement. If no transition occurred, data
integrity could not be guaranteed. The HP-3 transmitter has been designed for
compatibility with all generations of HP receivers. While it can potentially be used
with receivers from other manufacturers we do not recommend it. The easiest
application and field reliability will be obtained when HP family components are
used for the entire link.
PROXIMITY OPERATION
Multiple transmitters may be active on separate channels so long as an adjacent
channel's signal does not enter the receiver at a level exceeding the rejection
capability of the receiver. In serial mode the channels are closely spaced and will
not all be useable in proximity. The large number of channels is not meant to imply
that all can be successfully used in close proximity. The high channel count is
provided to accommodate hopping, allow compatibility with a broad range of
receiver frequencies, and allow agility in avoiding other interference sources. In
cases where the modules are combined to form a transceiver they should be
operated in half-duplex, meaning that only the transmitter or receiver is active at
any time. Full-duplex operation is possible but will result in reduced range due to
receiver desensing from the closely adjacent transmitter..
