Half Duplex GMSK Modem

FX579

© 1996 Consumer Microcircuits Limited

6

D/579/4

1.4

External Components

External Components

Component

Value

Tolerance

Component

Value

Tolerance

R1

1M

Ω

±10%

C3

Note 2

±20%

R2

Note 1

± 5%

C4

Note 2

±20%

X1

Note 3

C5

Note 3

C1

100nF

±20%

C6

Note 3

C2

100nF

±20%

C7

Note 1

±5%

Notes:

1.

The RC network formed by R2 and C7 is required between the TXOP pin and the input to the 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. The ground

connection to the capacitor C7, 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 (Ohms) and the capacitance (Farads) is:

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

BT of 0.5 = 0.21/bit rate (bits/sec)

with suitable values for common bit rates being:

R2

C7

4000 bits/sec

BT = 0.3

180.0k

Ω

470pF

4800 bits/sec

BT = 0.5

91.0k

Ω

470pF

8000 bits/sec

BT = 0.3

91.0k

Ω

470pF

9600 bits/sec

BT = 0.5

47.0k

Ω

470pF

19 200 bits/sec

BT = 0.5

91.0k

Ω

120pF

32 000 bits/sec

BT = 0.3

47.0k

Ω

220pF

32 000 bits/sec

BT = 0.5

47.0k

Ω

150pF

38 400 bits/sec

BT = 0.3

47.0k

Ω

180pF

38 400 bits/sec

BT = 0.5

47.0k

Ω

120pF

Note that in all cases the value of R2 should not be less than 47.0k

Ω and that the calculated value of C7 includes calculated

parasitic capacitances.

2.

C3 and C4 should both be 15.0nF for a data rate of 8kbits/sec, and inversely proportional to the data rate for other data rates, e.g.

30.0nF at 4kbits/sec, 3.0nF at 40kbits/sec.

3.

capacitance. As a guide:

At 5 Volts, C5 = C6 = 33.0pF at 1.0MHz falling to 18pF at 6.5MHz

At 3 Volts, C5 = C6 = 33.0pF at 1.0MHz falling to 18pF at 5MHz

The equivalent series resistance of X1 should be less than 2.0k

Ω at 1MHz falling to 150Ω at the maximum frequency. Stray

capacitance on the Xtal/Clock circuit pins must be minimised.

Figure 2 Recommended External Components (SOIC Package)