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