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ADJUSTING THE OUTPUT POWER

Depending on the type of antenna being used, the output power of the

transmitter may be higher than FCC regulations allow. It is intentionally set high

to compensate for losses resulting from inefficient antennas. Since attenuation is

often required, it is generally wise to provide for its implementation so that the

FCC test lab can easily attenuate the transmitter to the maximum legal limit.

A T-pad is a network of three

resistors that allows for variable

attenuation while maintaining the

correct match to the antenna. An

example layout is shown in the

adjacent figure. For more details

on T-pad attenuators, please

see Application Note AN-00150.

INPUTTING DIGITAL DATA

The DATA line may be directly connected to virtually any digital peripheral,

including microcontrollers, encoders, and UARTs. It has an impedance of 200k

Ω

and can be used with any data that transitions from 0V to a 3 to 5V peak

amplitude within the specified data rate of the module. While it is possible to send

data at higher rates, the internal filter will cause severe roll off and attenuation.

Many RF products require a fixed data rate or place tight constraints on the mark

/ space ratio of the data being sent. The HP3 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.

The HP3 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, then send out a digital control command. Such mixed mode

systems can greatly enhance the function and versatility of many products.

INPUTTING ANALOG SIGNALS

Analog signals from 50Hz to 28kHz may be connected directly to the

transmitter’s DATA line. The HP3 is a single supply device and, as such, is not

capable of operating in the negative voltage range. Analog sources should be

to achieve the best performance. The size of the coupling capacitor should be

large enough to ensure the passage of all desired frequencies and small enough

to allow the start-up time desired. Since the modulation voltage applied to the

DATA line determines the carrier deviation, distortion can occur if the DATA line

is over-driven. The actual level of the input waveform should be adjusted to

achieve optimum in-circuit results for your application.

The HP3 is capable of providing audio quality comparable to a radio or intercom.

In applications where higher quality audio is required, a compandor may be

employed to increase dynamic range and reduce noise. If true high-fidelity audio

is required, the HP3 is probably not the best choice, as it is optimized for data.

TIMING CONSIDERATIONS

Timing plays a key role in link reliability, especially when the modules are being

rapidly turned on and off or hopping channels. Unlike a wire, allowance must be

made for the programming and settling times of both the transmitter and

receiver, or portions of the signal will be lost. There are two major timing

considerations the engineer must consider when designing with the HP3 Series

transmitter. These are shown in the table below. The stated timing parameters

assume a stable supply of 2.8 volts or greater. They do not include the charging

times of external capacitance on the module’s supply lines, the overhead of

external software execution, or power supply rise times.

T1 is the maximum time required for the transmitter to power-up and lock on-

transitioning high.

T2 is the worst-case time needed for a powered-up module to switch between

channels after a valid channel selection. This time does not include external

overhead for loading a desired channel in Serial Channel Select Mode.

Normally, the transmitter will be turned off after each transmission. This is

courteous use of the airwaves and reduces power consumption. The transmitter

may be shut down by switching its supply or the PDN line. When the transmitter

is again powered up, allowance must be made for the requirements above.

In many cases, the transmitter will lock more quickly than the times indicated.

When turn-around time or power consumption are critical, the CTS line can be

monitored so data may be sent immediately upon transmitter readiness.

TRANSMITTING DATA

Once an RF link has been established, the challenge becomes how to effectively

transfer data across it. While a properly designed RF link provides reliable data

transfer under most conditions, there are still distinct differences from a wired link

that must be addressed. Since the modules do not incorporate internal encoding

or decoding, the user has tremendous flexibility in how data is handled.

It is important to separate the types of transmissions that are technically possible

from those that are legally allowed in the country of operation. Application Notes

AN-00126, AN-00140 and Part 15, Section 249 of the FCC rules should be

reviewed for details on acceptable transmission content in the U.S.

If you want to transfer simple control or status signals (such as button presses)

and your product does not have a microprocessor or you wish to avoid protocol

development, consider using an encoder / decoder IC set. These chips are

available from several manufacturers, including Linx. They take care of all

encoding and decoding functions and provide a number of data lines to which

switches can be directly connected. Address bits are usually provided for

security and to allow the addressing of multiple receivers independently. These

ICs are an excellent way to bring basic remote control products to market quickly

and inexpensively. It is also a simple task to interface with inexpensive

microprocessors or one of many IR, remote control, DTMF, or modem ICs.

Parameter

Description

Max.

T1

Transmitter turn-on time

10.0mS

T2

Channel change time (time to valid data)

1.5mS

GROUND PLANE

ON LOWER LAYER

ANTENNA

RF

MODULE

R1

R1

R2

GROUND

Figure 12: T-Pad Attenuator Example Layout