DB929B datasheet(42/51 Pages) CMLMICRO

42 / 51 page

4-Level FSK Modem Data Pump

Page 42 of 50

MX929B PRELIMINARY INFORMATION

www.mxcom.com Tel: 800 638 5577 336 744 5050

Fax: 336 744 5054

Doc. # 20480171.003

4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA

All trademarks and service marks are held by their respective companies.

5.5

Radio Performance

The maximum data rate that can be transmitted over a radio channel using this modem depends on:

RF channel spacing.

Allowable adjacent channel interference.

Symbol rate.

Peak carrier deviation (modulation index).

Tx and Rx reference oscillator accuracy.

Modulator and demodulator linearity.

Receiver IF filter frequency and phase characteristics.

Use of error correction techniques.

Acceptable error rate.

As a guide, 4800 symbols/sec (9600bps) can be achieved (subject to local regulatory requirements) over a

system with 12.5kHz channel spacing if the transmitter frequency deviation is set to ±2.5kHz peak for a

repetitive ' +3 +3 -3 -3 ... ' pattern and the maximum difference between transmitter and receiver 'carrier'

frequencies is less than 2400Hz.

The modulation scheme employed by these modems is designed to achieve high data throughput by

exploiting as much as possible of the RF channel bandwidth. However, this does place constraints on the

performance of the radio. Particular attention must be paid to:

Linearity, frequency, and phase response of the Tx Frequency Modulator. For a 4800 symbols/sec

(9600bps) system, the frequency response should be within ±2dB over the range 3Hz to 5kHz, relative to

2400Hz.

The bandwidth and phase response of the receiver's IF filters.

Accuracy of the Tx and Rx reference oscillators, as any difference will shift the received signal towards the

skirts of the IF filter response and cause a DC offset at the discriminator output.

Viewing the equalized received signal eye diagram, using the Mode Register RXEYE function, provides a good

indication of the overall RF transmitter/receiver performance.

CIRCUITS

D0 - D7

A0 - A1

IRQ

D0 - D7

A0 - A1

IRQ

TXOUT

SIGNAL AND

DC LEVEL

ADJUSTMENT

SIGNAL LEVEL

ADJUSTMENT

RXIN

RXAMPOUT

Rx FREQUENCY

DISCRIMINATOR

µC

FREQUENCY

MODULATOR

DC LEVEL

ADJUSTMENT

MX929A MODEM

Figure 26: Typical Connections between Radio and MX929B

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