11

Application Information ......

Radio Channel Requirements

To achieve legal adjacent channel performance at high bit-rates, a radio with an accurate carrier frequency and

an accurate modulation index will be required.

To achieve optimum channel utilization, (eg. low BER and high data-rates) attention must be paid to the phase

and frequency response of both the IF and baseband circuitry.

Bitrate, BT and Bandwidth

The maximum data rate that can be transmitted over a

radio channel depends on:

- Channel spacing

- Allowable adjacent channel interference

- Tx filter bandwidth (BT)

- Peak carrier deviation (Modulation Index)

- Tx and Rx carrier frequency accuracies

- Modulator and Demodulator linearity

- Rx IF filter frequency and phase characteristics

- Use of error correction techniques

- Acceptable error-rate

FM Modulator, Demodulator and IF

For optimum performance, the ‘eye’ pattern of the

received signal (when receiving random data) applied

to the FX589 should be as close as possible to the

Transmit ‘eye’ pattern examples shown in Figure 10.

Of particular importance are general symmetry,

cleanliness of the zero-crossings, and for a BT of 0.3,

the relative amplitude of the inner eye opening.

To achieve this, attention must be paid to -

-

Linearity and frequency/phase response of the

Tx frequency modulator. Unless the transmit

data is especially encoded to remove low

frequency components, the modulator

frequency response should extend down to a

few Hertz, two-point modulation being

necessary for synthesised radios.

-

Bandwidth and phase response of the Rx IF

filters.

-

Accuracy of the Tx and Rx carrier frequencies -

any difference will shift the received signal

towards one of the skirts of the IF filter

response.

As a guide, a raw data-rate of 8,000b/s at 12.5kHz

channel spacing may be achievable -depending on

local regulatory requirements- using a BT of 0.3 +/-

2kHz maximum deviation and no more than 1.5kHz

discrepancy between Tx and Rx carrier frequencies.

Forward Error Correction (FEC) could then be used

with interleaving to reduce the effect of burst errors.

Reducing the data-rate to 4,800b/s would allow the

BT to be increased to 0.5, improving the error-rate

performance.

Ideally, the Rx demodulator should be dc coupled to

the FX589 ‘Rx Signal In’ pin (with a dc bias added to

centre the signal at the Rx Feedback pin around V

[V

that:

-

The 3dB cut-off frequency for 8kb/s is 20Hz or

below (i.e. a 0.1µF capacitor in series with

100k

Ω).

-

The data does not contain long sequences of

consecutive ones or zeroes.

-

Sufficient time is allowed after a step change at

the discriminator output (resulting from channel

changing or the appearance of an RF carrier)

for the voltage into the FX589 to settle before

the ‘RxDCacq’ line is strobed.