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Historically, QAM (Quadrature
Amplitude Modulation) signals have presented significant challenges to tape-based
data recorders since very low signal noise and group delay are both critical to successful,
error free recording. Fortunately, these problems can be
overcome with a carefully designed disk recorder. For example, the IF
channel of Avalon's new AE8400 disk recorder offers a SFDR in excess of 60 dB, a
frequency response of +/-0.5 dB and a group delay of better than +/-2 ns across
the entire 80 MHz pass-band. This level of performance has been demonstrated to be more
than adequate for recording QAM signals up to at least level 256. An
important concern for many users is whether the disk recorder will itself
contribute to the error rate when recording a signal with a poor
Carrier-to-Noise ratio (CNR). A typical test for this situation is illustrated
by Figures 1 thru 4 showing
the effect of intentionally adding noise to the input signal until errors occur.
These experiments demonstrate that the Avalon disk recorder had more than
sufficient SNR to support error free recording of poor QAM 64 and QAM 256
signals. QAM 64 , 70MHz
Centre Frequency, 6MHz Symbol Rate The
following graphs were made with no noise added at the input of the recorder while
recording a QAM 64 signal. When noise was applied to
the input, the replayed QAM 64 signal was found to be Error Free up to a CNR of 34dB (recorder
spec >46 dB). In other words, the QAM 64 test signal system itself
required a CNR of about 34 dB for error-free performance, while the CNR of the
recorder itself is around 12 dB better. So it can be confidently stated
that the recorder is making no significant contribution to the error rate when
recording the QAM 64 test signal.
256 QAM, 70MHz
Centre Frequency, 6MHz Symbol Rate
The following graphs were made with no noise added at the input of the recorder while
recording a QAM 256 signal. When noise was applied to
the input, the replayed signal was found to be Error Free up to a CNR ratio of 38dB (recorder
spec >46 dB). Comparing these two particular tests (QAM 256 vs. QAM 64)
it can be seen that the higher order signal requires a higher CNR for error-free
operation (38 dB vs. 34 dB). (The theoretical 'rule-of-thumb' found in the
literature is that the CNR requirement goes up by 6dB when quadrupling the
number of points in the constellation. As with the QAM 64 test, the
performance of the Avalon disk recorder is more than adequate for recording this
particular QAM 256 test signal.
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