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THE DATA OUTPUT

The DATA line outputs recovered digital data. It is an open collector output with

an internal 4.7k

Ω pull-up. When an RF transmission is not present, or when the

received signal strength is too low to ensure proper demodulation, the data

output is squelched continuous high. This feature supports direct operation with

UARTs, which require their input to be continuously high. An HP3 transmitter and

receiver can be directly connected between two UARTs without the need for

buffering or logical inversion. It should be noted that the squelch level is set just

over the receiver’s internal noise threshold. Any external RF activity above that

threshold will “break squelch” and produce hashing on the line. While the DATA

line will be reliably squelched in low-noise environments, the designer should

always plan for the potential of hashing.

AUDIO OUTPUT

The HP3 Series is optimized for the transmission of serial data; however, it can

also be used very effectively to send a variety of analog signals, including audio.

The ability of the HP3 to send combinations of audio and data opens new areas

of opportunity for creative design.

The analog output of the AUDIO line is valid from 50 Hz to 28 kHz, providing an

AC signal of about 1V peak-to-peak. This is a high impedance output and not

suitable for directly driving low-impedance loads, such as a speaker. In

applications where a low impedance load is to be driven, a buffer circuit should

always be used. For example, in the case of a speaker, a simple op-amp circuit

such as the one shown below can be used to act as an impedance converter.

The transmitter’s modulation voltage is critical, since it determines the carrier

deviation and distortion. The transmitter input level should be adjusted to

achieve the optimum results for your application in your circuit. Please refer to

the transmitter data guide for full details.

When used for audio, the analog output of the receiver should be filtered and

buffered to obtain maximum sound quality. For voice, a 3-4kHz low-pass filter is

often employed. For broader-range sources, such as music, a 12-17kHz cutoff

may be more appropriate. In applications that require high-quality audio, a

compandor may be used to further improve SNR. The HP3 is capable of

providing audio quality comparable to a radio or intercom. For applications where

true high fidelity audio is required, the HP3 will probably not be the best choice,

and a device optimized for audio should be utilized.

HP Analog Out

10k

6

2

3

+

–

4

LM386

5

0.05uF

1uF

250uF

10 ohm

VCC

Figure 12: Audio Buffer Amplifier

TIMING CONSIDERATIONS

There are four major timing considerations to be aware of when designing with

the HP3 Series receiver. These are shown in the table below.

T1 is the maximum amount of time that can elapse without a data transition. Data

must always be considered in both the analog and the digital domain. Because

the data stream is asynchronous and no particular format is imposed, it is

possible for the data to meet the receiver’s data rate requirement yet violate the

analog frequency requirements. For example, if a 255 (0FF hex) were sent

continuously, the receiver would view the data as a DC level. It would hold that

level until a transition was required to meet the minimum frequency specification.

If no transition occurred, data integrity could not be guaranteed. While no

particular structure or balancing requirement is imposed, the designer must

ensure that both analog and digital signals meet the transition specification.

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 the serial channel-selection mode.

T3 is the time to receiver readiness from the PDN line going high. Receiver

readiness is determined by valid data on the DATA line. This assumes an

readiness is determined by valid data on the DATA line. This assumes an

incoming data stream and the PDN line is high or open.

RECEIVING 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

Time between DATA output transitions

20.0mS

T2

Channel change time (time to valid data)

1.5mS

T3

Receiver turn-on time via PDN

3.0mS

T4

7.0mS