GMSK Packet Data Modem and RF Transceiver
CMX990
4.3
Transmit
The CMX990 transmitter uses an offset phase-locked loop to accurately modulate RF signals. Details
are contained in subsequent sections of this document. The components used around the CMX990 will
depend on application requirements however a typical configuration is shown in Figure 5.
Figure 5 Recommended External Components - Transmit
C51
C86
C87
C89
6.8 pF
15 nF
TBD nF
68 nF
R62 18 W
R63 270 W
R64 270 W
R68 47 W
R69 10 W
R71 36 W
L5
270 nH
Notes:
1
Resistors ±2%, capacitors ±5% unless otherwise stated.
2
The coupler may be a packaged type (e.g. 0869CP14A090) or printed on the PCB; alternatively a sample
of the output can be obtained with a resistive or capacitive tap.
3
4
Tx loop filter components need to be optimised for selected VCO.
Components between TXIF1 and TXIF2 act as a resonant load to the mixer. They should be matched to
the selected IF frequency whilst attenuating its odd harmonics. The Q of this filter can be controlled by
changing the ratio of C51 to L5 whilst the centre frequency can be maintained by keeping the product of
C51 and L5 a constant. A higher value of C51 will give a higher Q. The Q should not be so high as to
prevent accommodation of the natural frequency range of the VCO. The values shown are approximate for
an IF of 90MHz. Approximate values for an IF of 45MHz are 18pF and 560nH.
ã 2004 CML Microsystems Plc
11
D/990/1
CMLMICRO [ CML MICROCIRCUITS ]
