CMX991 Datasheet, PDF (14/56 Pages) CML Microcircuits – Two-mode demodulator

RF Quadrature Transceiver / RF Quadrature Receiver
CMX991/CMX992
4.2.3.2 Low IF Output
The I/Q demodulator output bandwidth has a minimum of 1MHz, typically 1.4MHz (see section 8.1.3.2), so
the output of each I and Q demodulator mixer can be configured to mix down to a low 2^nd IF and use a
demodulator output amplifier to provide gain. A typical configuration for the Q channel is shown in Figure 9.
RXQP
R1
Pin 20
F1
VBIAS
C1
25k Ceramic
Filter
C2
R2
QAMPP
Pin 21
R3
QAMPN
Pin 22
QAMP
R4
C3
QAMPO
Pin 23
Figure 9 Example External Components – Receive Low IF Output
C1 100nF
C2 47nF
C3 33pF
F1 CFWL455KEFA-B0
R1 1.5kΩ
R2 1.5kΩ
R3 1.5kΩ
R4 4.7kΩ
Table 7 Rx Low IF (455kHz) Components
The components above specify, as an example, a particular ceramic filter (F1) that would typically be used
in a 25kHz channel application in a system with an IF frequency of 455kHz. The other component values
specified (e.g. R1, R3) are determined by the input/output impedance of the filter used. The filter and other
components can be easily changed to allow for other bandwidths or any 2^nd IF output up to 1MHz.
A different external 2^nd IF filter, of different bandwidth, could similarly be connected to the I channel output
to support a second modulation bandwidth mode, e.g. to receive a 6.25kHz channel signal. The channel to
be used is selectable via the general control register ($11), section 6.2, the unused channel being
powered-down.
 2015 CML Microsystems Plc
14
D/991_992/20