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发表于 2009-9-29 18:04:18
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It may be readily concluded that the addition of dither noise at the A/D input makes a significant improvement to the quantisation spurs. It most systems, the natural system noise will produce the required effect but in situations where high signal levels cause the AGC to drive down the system noise floor below that of the A/D then spurs will occur. A typical example of this might be in instrumentation where high level CW signals may be used for test and calibration purposes.
The addition of wideband noise can certainly help but, to reduce the spurs below noise level, the amount of noise present will add to the inherent A/D noise and reduce dynamic range by at least 3 dB. The advantage of this method, however, is its simplicity since it only requires the addition of a noise diode of the correct level and bandwidth at the A/D input. NoiseCom produce a range of such diodes and the method is to be recommended if adequate performance can be achieved in this way.
For ultimate performance, narrowband dither is highly recommended. Spurs may be driven down to close to system noise level without adding to the average noise level. The main disadvantage is the requirement for additional low-pass filtering and amplification since filtering the noise also reduces its level. It must also be remembered that placing an additional filter at the output of the main anti-alias filter also affects the load impedance. It is normal, therefore, to design such a structure, carefully taking account of these effects. |
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