GMSK Packet Data Modem and RF Transceiver
CMX990
4.2
Synthesiser and TCXO
The CMX990 synthesiser section provides two independent synthesisers. The PLLs implement a Type II
loop with a phase / frequency phase detector providing an output of a charge pump current. Various
types of loop filter can be used and should be optimised for VCO gain of a particular design. Figure 4
gives typical configuration and values.
Figure 4 Recommended External Components - Synthesiser and TCXO
C4
1 nF
1 nF
1 nF
27 nF
27 nF
680 nF
1 nF
C114
C116
T2
R79
R80
R87
R91
U1
1 nF
C92
C93
C99
C100
C101
C113
150 nF
TCM4-25
2.7 kW
1.2 kW
39 kW
1.5 kW
See notes
Notes:
1
Resistors ±2%, capacitors ±5% unless otherwise stated.
2
For optimum lock time / phase noise it is recommended C113 and C116 use a low piezo type such as PPS
film; optimum performance is not guaranteed with X7R or Y5V types.
3
U1 Should be a VCTCXO or TCXO depending on application requirements. A typical device is the
Golledge GTXO-81. The CMX990 has a high impedance input suitable for use with oscillators with clipped
sine wave output. An external DC blocking capacitor (as shown, C4) is required.
ã 2004 CML Microsystems Plc
10
D/990/1
CMLMICRO [ CML MICROCIRCUITS ]
