CMX972 Datasheet, PDF (13/41 Pages) CML Microcircuits – Small 32-lead VQFN Package

Quadrature Demodulator with IF PLL/VCO
CMX972
4.5.2 Low IF Output
The quadrature demodulator output bandwidth is at least 5MHz, (see section 8.1.3.3), so the output of
each quadrature demodulator mixer can be configured to mix down to a low IF and use one of the
differential amplifiers to provide gain. A possible configuration for the Q channel is shown in Figure 8.
RXQN
RXQP
CMX972
C2
Bias
F1
Voltage
R2
OA2P
R1
C1
R3
OA2N
BBAmp2
OA2O
12.5k or 6.25k
Ceramic Filter
R4
C3
Figure 8 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 and IF frequencies.
A different external IF filter, e.g. 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 Rx Mode register ($1D), section 6.4.1, the unused channel being powered-
down.
 2015 CML Microsystems Plc
13
D/972/2