GMSK Modem
CMX589A
Data Rates
(kbps)
BT = 0.3
BT = 0.5
R1
C1
R1
C1
4
680pF
680pF
470pF
390pF
470pF
180pF
220pF
180pF
100pF
470pF
470pF
220pF
470pF
150pF
120pF
150pF
120pF
68pF
120kW
100kW
91kW
91kW
47kW
100kW
47kW
47kW
56kW
120kW
100kW
120kW
47kW
91kW
91kW
47kW
47kW
51kW
39kW
82kW
68kW
62kW
56kW
51kW
4.8
8
9.6
16
19.2
32
38.4 *
64 *
80 *
128 *
144 *
160 *
176 *
192 *
68pF
22pF
22pF
22pF
22pF
22pF
* VDD ³ 4.5V, external clock
Table 3: Data Rate vs. BT and Selected External Component Values
Note: In all cases, the value of R1 should not be less than 20.0kW, and that the calculated value of C1
includes calculated parasitic capacitance.
2. R3, R4 and C6 form the gain components for the Rx Input signal. R3 should be chosen as required by the
signal input level.
3. For bit rate £ 64kbps, R4 = 100kW. For bit rate > 64kbps, R4 = 10kW.
4. The values chosen for C2 and C3 (including stray capacitance), should be suitable for the applied VDD
and the frequency of X1.
As a guide: C2 = C3 = 33pF at 1.0MHz falling to 18pF at the maximum frequency.
At 3.0V, C2 = C3 = 33pF falling to 18pF at 5.0MHz the equivalent series resistance of X1 should be less
than 2.0KW falling to 150W at the maximum frequency. Stray capacitance on the Xtal/Clock circuit pins
must be minimized.
1
5. For bit rate £ 64kbps, C6 = 22pF. For bit rate > 64kbps, C6 =
3´ bit rate´ 2p ´ 10kW
e.g. for 128kbps, C6 = 41.1pF.
6. C7 and C8 should both be .015mF for a data rate of 8kbps, and inversely proportional to the data rate for
other data rates, e.g. 0.030mF at 4kbps, 1800pF at 64kbps, 680pF at 192kbps.
7. The tolerance of C9 is not very critical because it primarily serves as a dc blocking capacitor.
8. The CMX589A can operate correctly with the Xtal frequencies between 1.0MHz and 16.0MHz
(VDD = 5.0V) and 1.0MHz to 5.0MHz (VDD = 3.0V). External clock frequencies up to 25.6MHz
(VDD ³ 4.5V) are also supported (See Table 4 for examples.) For best results, a crystal oscillator design
should drive the clock inverter input with signal levels of at least 40% of VDD, peak to peak. Tuning fork
crystals generally cannot meet this requirement. To obtain crystal oscillator design assistance, consult
your crystal manufacturer. Operation of this device without a Xtal or Clock input may cause device
damage.
ã 2002 CML Microsystems Plc
7
D/589A/4
CMLMICRO [ CML MICROCIRCUITS ]
