Half Duplex GMSK Modem

FX579

© 1996 Consumer Microcircuits Limited

8

D/579/4

Note:

(1)

Device operation is not guaranteed for operation above 40kbits/sec or below

4kbits/sec.

(2)

If a suitable Xtal or clock signal is not supplied, device supply current may

increase.

Transmit Signal Path

The binary data applied to the DATAIO input is retimed within the chip on each rising edge of the Tx

connected to the input of the low pass Tx filter, and the output connected to the TXOP pin.

The Tx filter has a low pass frequency response, which is approximately Gaussian in shape, as shown

in Figure 4. This minimises 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 versus frequency response of the transmit filtering provided by

this chip has been designed to meet the specifications for most GMSK modem systems, having a -3dB

bandwidth switchable between 0.3 and 0.5 times the data bit rate.

Note that an external RC network is required between the TXOP pin and the input to the frequency

modulator. This network, which can form part of any dc level shifting and gain adjustment circuitry,

forms an important part of the transmit signal filtering, and the ground connection to the capacitor

should be positioned to give maximum attenuation of high frequency noise into the modulator. The

component values should be chosen so that the product of the resistance (in Ohms) and the

capacitance (in Farads) is:

0.34 / bit rate for BT of 0.3

0.21 / bit rate for BT of 0.5

(bit rate in bits per second)

Suitable values for common bit rates are given in Table 2

Data Rate

BT

R2

C7

4800 bits/sec

0.5

91.0 k

Ω

470pF

8000 bits/sec

0.3

91.0 k

Ω

470pF

9600 bits/sec

0.5

47.0 k

Ω

470pF

19 200 bits/sec

0.5

91.0 k

Ω

120pF

32 000 bits/sec

0.3

47.0 k

Ω

220pF

32 000 bits/sec

0.5

47.0 k

Ω

150pF

38 400 bits/sec

0.3

47.0 k

Ω

180pF

38 400 bits/sec

0.5

47.0 k

Ω

120pF

Table 2 Filter Bandwidth Selection

and negative for logic "0" inputs.

Note that when in receive mode the input of the internal buffer

Figure 5 shows typical transmit eye patterns (measured after the external RC network) for BT values of

0.3 and 0.5

Figure 6 shows the transmit output spectrum (measured after the external RC network) again for BT

values of 0.3 and 0.5.