CMX7163 QAM Modem
CMX7163
vs time) or other diagnostic modes in receive. In some cases it is advisable to obtain a trigger pulse that is
synchronised to the transmitting modem symbol rate, for example if the transmitted signal comes from a
signal generator.
Rx Diagnostics
A diagnostic mode is provided that produces channel filtered I/Q signals and an optional dc offset
correction indication. This aids in diagnosing reception issues that may be related to I/Q dc offsets in the
CMX7163 input signal. This diagnostic mode can still be of use when there is a frequency error present in
the received signal. As shown in Figure 21 and Figure 22, the estimated I/Q dc offset correction is an extra
dot in the centre of the constellation.
Figure 21 Sample at symbol
timing with I/Q dc offset
diagnostic mode (no frequency
error)
Figure 22 Sample at symbol
timing with I/Q dc offset
diagnostic mode (with
frequency error)
A normalised received constellation diagnostic output is provided. It relies on having detected a frame sync
and therefore being able to output the signal level measured at the symbol timing instant, with the
frequency error removed and amplitude corrected. So long as the CMX7163 remains locked to a suitable
signal the normalised constellation output will remain static regardless of frequency error and amplitude of
the input signal (within limits – see section 9.1.4 CMX7163 FI-4.x Parametric Performance). If the signal
becomes noisy or its amplitude small then the constellation points will spread as shown in Figure 23 and
Figure 24.
2014 CML Microsystems Plc
Page 39
D/7163_FI-4.x/12
CMLMICRO [ CML MICROCIRCUITS ]
