GMSK Packet Data Modem and RF Transceiver
CMX990
4.4
Receive
The receiver relies on external LNA, filtering and T/R switch; details can be found in the following
sections. The 1st mixer in the CMX990 has a differential input. To ensure optimum performance a balun
is required when driving from typical LNAs or filters. The balun may be a transformer type, which is
implemented using LC networks. Suitable matching components around the balun, selected for the
desired operating frequency, provide matching to 50W. Figure 6 shows a typical configuration for 800 -
840MHz operation.
Figure 6 Recommended External Components - Receive
C18 5 pF
C21 1 nF
C25 5 pF
C41 1 nF
C46 10 nF
C47 15 pF
C48 5.6 pF
L2
L17
T1
R29
R36
RXX
RYY
F1
1.2 mH
15 nH
2.7 kW
330 W
39 kW
100 W
100 W
45G7B1
Notes:
1
Resistors ±2%, capacitors ±5% unless otherwise stated.
2
F1 is a 4 pole crystal filter with a ±3.5kHz pass-band, implemented as a matched pair. The recommended
part is from Golledge, however other parts may be equally suitable although matching arrangements will
vary. When selecting and matching a crystal filter care should be taken to ensure a flat pass-band.
ã 2004 CML Microsystems Plc
12
D/990/1
CMLMICRO [ CML MICROCIRCUITS ]
