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:

• there may be large signals outside the audio filter bandwidth; these signals may cause a receiver overload, but will not be sampled by the ADC.
• the receiver / audio filter signal path is a well-balanced chain of (mostly fixed gain) amplifiers on one hand, and gain adjustments (programmable signal attenuators), mixers and filters (all of which attenuate the signal amplitude) on the other hand. Even if there is no signal outside the filter bandwidth, the signal levels prior to the programmable audio filter and at the ADC may therefore be different, and it would be a coincidence if for all filter bandwidths the receiver overload would be observed at EXACTLY the point where the ADC is full.

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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