CMX7163 QAM Modem
CMX7163
The CMX998 Cartesian Feed-back Loop Transmitter datasheet and an application note “CMX998
Cartesian Feedback Loop dc Calibration” are both available from the CML website (www.cmlmicro.com)
and should be referred to for a more in-depth understanding of the need for dc offset calibration.
The CMX7163 performs automatic dc offset calibration as either part of a transmit sequence or in a
separate calibration stage. DC offset calibration determines the dc offset that should be applied to the I
Output and Q Output signals by the CMX7163 to minimise carrier leakage. The results of calibration will be
held by the CMX7163 for use in later transmissions and are made available to the host. The interface is
required to be as shown in Figure 16 CMX998 DC Calibration Interfaces.
I Input
I Output
A
A
Q Input
Q Output
AuxADC2
DCMEAS
B
CMX998
CMX7163
SPI Thru-Port
(Chip Sel = SSOUT1)
C-BUS In
PA Cntrl
C
D
AuxDAC1 (RAMDAC)
A. The CMX7163 I and Q Outputs are used to provide dc levels, which are adjusted to make the error I/Q
measurements equal to the Reference I/Q measurements
B. AuxADC2 is used to sample DCMEAS – To measure Reference signals and error signals
C. The SPI Thru-Port is used to control the CMX998 – selecting Reference I/Q and Error I/Q as measurements,
as well as high gain/low gain modes of the CMX998
D. The RAMDAC is typically used to ramp the PA Control voltage up after calibration is complete. This is not a
part of the calibration sequence, but may be active as part of the transmit sequence.
Figure 16 CMX998 DC Calibration Interfaces
During calibration the CMX998 is controlled by the CMX7163 using the SPI Thru-Port (The CMX998 is
assumed to be device 1) to select one of the following to be output to the CMX998 DCMEAS output:
I Reference
Q Reference
I Error (Low/high gain )
The CMX998 dc reference for the In-phase signal path
The CMX998 dc reference for the Quadrature signal path
The CMX998 measure of the dc produced by the input signal on the In-
phase signal path
2
Q Error (Low/high gain)
The CMX998 measure of the dc produced by the input signal on the
Quadrature signal path
During calibration the CMX7163 uses AuxADC2 to measure Reference I and Reference Q. It then outputs
a dc level on the I Output, Q Output signals. AuxADC2 is used to measure the DCMEAS I and Q Error and
I Output, Q Output are adjusted to make the DCMEAS I and Q errors equal to the DCMEAS I Reference
and Q Reference measurements.
2
The low and high gain states are created by adjusting the gain of the error amplifiers in the CMX998, see
the CMX998 datasheet for more information.
2014 CML Microsystems Plc
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