9

Application Information ......

Tx Signal Path Description

The binary data applied to the ‘Tx Data’ input is

retimed within the chip on each rising edge of the ‘Tx

Clock’ and then converted to a 1-volt peak-to-peak

binary signal centred about V

If the ‘Tx Enable’ input is ‘high,’ then this internal

binary signal will be connected to the input of the

lowpass Tx Filter, and the output of the filter connected

to the ‘Tx Out’ pin.

data bit-rate (BT).

Note that an external RC network is required

between the ‘Tx Out’ pin and the input to the

Frequency Modulator (see Figures 2 and 3). This

network, which can form part of any d.c. level shifting

and gain adjustment circuitry, forms an important part

of the transmit signal filtering, and the ground

connection to the capacitor C

give maximum attenuation of high-frequency noise into

the modulator.

The component values should be chosen so that

the product of the resistance (Ohms) and the

capacitance (Farads) is:

BT of 0.3 = 0.34/bit rate (bits/second)

BT of 0.5 = 0.22/bit rate (bits/second)

with suitable values for common bit rates being:

RC

8000 bits/sec, BT = 0.3

91.0k

Ω

470pF

4800 bits/sec, BT = 0.5

100k

Ω

470pF

9600 bits/sec, BT = 0.5

47.0k

Ω

470pF

The signal at ‘Tx Out’ is centred around V

positive for logic “1” (high) level inputs to the ‘Tx Data’

input and negative for logic “0” (low) inputs.

When the transmit circuits are put into a

‘powersave’ mode (by a logic “1” to the ‘Tx PS’ pin) the

output voltage of the Tx Filter will go to V

When power is subsequently restored to the Tx Filter,

its output will take several bit-times to settle. The ‘Tx

Enable’ input can be used to prevent these abnormal

voltages from appearing at the ‘Tx Out’ pin.

A ‘low’ input to the ‘Tx Enable’ will connect the input of

the Tx Filter to V

from the filter, connecting it instead to V

high resistance (nominally 500k

Ω).

The Tx Filter has a lowpass frequency response,

which is approximately gaussian in shape as shown in

Figure 9, to minimise amplitude and phase distortion of

the binary signal while providing sufficient attenuation

of the high frequency-components which would

otherwise cause interference into adjacent radio

channels. The actual filter bandwidth to be used in any

particular application will be determined by the overall

system requirements. The attenuation-vs-frequency

response of the transmit filtering provided by the

FX489 have been designed to meet the specifications

for most GMSK modem systems, having a -3dB

bandwidth switchable between 0.3 and 0.5 times the

Tx Enable

Tx Filter Input

Tx Out Pin

‘1’ (high)

1 volt p-p Data In

Filtered Data

“0” (low)

V

BIAS

V

1 BIT PERIOD

1.0µSMin.

1.0µSMin.

1.0µSMax.

1.0µSMax.

Tx DATA SAMPLED BY

THE FX489 AT THESE

INSTANCES

Tx Data

Rx Data

Rx Clock

Tx Clock

DON’T CARE

DATA INVALID

DATA VALID

DATA MUST

BE VALID

EXTERNAL CIRCUITS SHOULD

SAMPLE Rx DATA AT THIS TIME

Tx CLOCK AND Rx CLOCK OUTPUTS

(MARK/SPACE) DUTY CYCLE NOMINALLY 50%

Fig.8 Rx and Tx Clock Data Timings