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An on-board micro-controller reads the channel-selection lines and programs the

PLL to the desired channel frequency. The micro-controller also monitors the

status of the PLL and indicates when the transmitter is stable and ready to

transmit data by asserting the CTS line high.

A buffer amplifier is used to isolate the VCO from the antenna and to increase

the output power of the transmitter. The output of the buffer amplifier is connected

to a LPF which is used to suppress harmonic emissions. All harmonic

specifications are based on a 50-ohm load.

BOARD LAYOUT CONSIDERATIONS

If you are at all familiar with RF devices you may be concerned about specialized

layout requirements. Fortunately, because of the care taken by Linx in the layout

of the module’s PCB, integrating an HP Series-II transmitter into your design is

very straightforward.By adhering carefully to a few basic design and layout rules,

you can enjoy a trouble-free path to RF success.

A ground-plane (as large as possible) should be placed directly under the HP-II

transmitter. This ground-plane can also be critical to the performance of your

antenna.

The HP-II transmitter should, as much

as reasonably possible, be isolated

from all other components on your

PCB.

Specifically,

high-frequency

circuitry such as crystal oscillators

should be kept as far away as possible

from the transmitter module.

If the transmitter must be mounted

horizontally, it should be laid over so that the side with the crystal is closest to

the user’s PC board and the RF side is facing away from the PC board.

The trace from the receiver to the antenna should be kept as short as possible.

A simple trace is suitable for runs up to 1/8 inch for monopole antennas with wide

bandwidth characteristics. For longer runs or to avoid detuning a narrow

bandwidth antenna such as a helical, use a 50-ohm coax or 50-ohm microstrip

transmission line as shown in Figure 14.

Observant designers will notice that the output power of the HP Series-II is set

5-6dB above Part 15 limits (assuming the use of a unity antenna).This is done

purposefully to allow designers with less efficient antennas to achieve the

maximum output power allowed by law.

In cases where an efficient antenna

style such as a whip is utilized, it is

often necessary to make provisions for

external attenuation. This is easily

accomplished by providing pads for a

three-resistor attenuation network as

shown in fig. 13A. The inline pads may

be bridged if the network is not needed.

The resistors should be a surface-

Figure 13: HP Series-II transmitter footprint

Figure 13A: Attenuation pad layout