WHAT IS THE DIFFERENCE BETWEEN RECEIVER OVERLOAD AND ADC OVERFLOW?

This is an excerpt from Varian VNMR News dated 11-18-2001.

Some users wonder why occasionally they observe an ADC overflow without a receiver overload, or why sometimes they see a receiver overload that is not also causing an ADC overflow. While it is obvious that with an ADC overflow the incoming (audio) signal could not be digitized properly (which usually causes severe spectral distortions), it is often unclear what the significance of a receiver overflow is.

The difference between receiver overload and ADC overflow is more than the fact that an ADC overflow is a "digital" measure for "too much signal", while the receiver overload is measured with an analog method: additionally, the ADC samples the signal after the programmable audio filter, while the receiver overload is derived from the signal level just before the programmable audio filter. This gives us two reasons why the two signals may appear at different levels:


The other, major difference is in the fact that ADC overflow can (obviously) only be detected when the signal is sampled, i.e., while acquiring data. Receiver overflow on the other hand may be detected whenever the receiver receives any signal, i.e., whenever the receiver gate is open. If a receiver overload is detected in an early part of the pulse sequence, it is likely to be irrelevant for the outcome of the experiment. If receiver overload is observed during acquisition, it should be taken as a warning that receiver components may be encountering excessive signal levels that may lead to spectral distortions such as intermodulations, etc.

As stated above, an ADC overflow should ALWAYS be regarded as a serious problem, requiring an adjustment of the receiver gain, see also Varian NMR News 1993-11-05.

For an article on ADC overflow in connection with oversampling see Varian NMR News 1999-07-10.

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