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user16324
user16324

It's a compromise.

You can easily make thermal noise dominate by adding an LNA stage with enough gain that the thermal noise exceeds the quantisation noise.

Then strong signals will exceed the ADC's input range and clip, even if the LNA is linear enough to keep intermodulation products from strong inputs below the quantisation noise (which is not always the case)

Or you can reduce the gain allowing good linearity for strong input signals; then quantisation noise will predominate.

Or you can use a higher resolution ADC; reducing the quantisation noise but adding cost.

Or you can add front end selectivity ahead of the ADC, to reduce the strength of signals outside the desired front end BW.

If this is ahead of the LNA it also guards against LNA non-linearity, but attenuates the input (thereby worsening thermal noise).

What you get is whatever compromise the designer chose, from their knowledge of the system requirements including response to unwanted interfering signals.

Second question : if the receiver includes an ADC, its noise figure ought to include not just the quantisation noise, but the ADC's own excess noise above that (from INL, DNL, sampling jitter or phase noise).

It's a compromise.

You can easily make thermal noise dominate by adding an LNA stage with enough gain that the thermal noise exceeds the quantisation noise.

Then strong signals will exceed the ADC's input range and clip, even if the LNA is linear enough to keep intermodulation products from strong inputs below the quantisation noise (which is not always the case)

Or you can reduce the gain allowing good linearity for strong input signals; then quantisation noise will predominate.

Or you can use a higher resolution ADC; reducing the quantisation noise but adding cost.

Or you can add front end selectivity ahead of the ADC, to reduce the strength of signals outside the desired front end BW.

If this is ahead of the LNA it also guards against LNA non-linearity, but attenuates the input (thereby worsening thermal noise).

What you get is whatever compromise the designer chose, from their knowledge of the system requirements.

Second question : if the receiver includes an ADC, its noise figure ought to include not just the quantisation noise, but the ADC's own excess noise above that (from INL, DNL, sampling jitter or phase noise).

It's a compromise.

You can easily make thermal noise dominate by adding an LNA stage with enough gain that the thermal noise exceeds the quantisation noise.

Then strong signals will exceed the ADC's input range and clip, even if the LNA is linear enough to keep intermodulation products from strong inputs below the quantisation noise (which is not always the case)

Or you can reduce the gain allowing good linearity for strong input signals; then quantisation noise will predominate.

Or you can use a higher resolution ADC; reducing the quantisation noise but adding cost.

Or you can add front end selectivity ahead of the ADC, to reduce the strength of signals outside the desired front end BW.

If this is ahead of the LNA it also guards against LNA non-linearity, but attenuates the input (thereby worsening thermal noise).

What you get is whatever compromise the designer chose, from their knowledge of the system requirements including response to unwanted interfering signals.

Second question : if the receiver includes an ADC, its noise figure ought to include not just the quantisation noise, but the ADC's own excess noise above that (from INL, DNL, sampling jitter or phase noise).

Source Link
user16324
user16324

It's a compromise.

You can easily make thermal noise dominate by adding an LNA stage with enough gain that the thermal noise exceeds the quantisation noise.

Then strong signals will exceed the ADC's input range and clip, even if the LNA is linear enough to keep intermodulation products from strong inputs below the quantisation noise (which is not always the case)

Or you can reduce the gain allowing good linearity for strong input signals; then quantisation noise will predominate.

Or you can use a higher resolution ADC; reducing the quantisation noise but adding cost.

Or you can add front end selectivity ahead of the ADC, to reduce the strength of signals outside the desired front end BW.

If this is ahead of the LNA it also guards against LNA non-linearity, but attenuates the input (thereby worsening thermal noise).

What you get is whatever compromise the designer chose, from their knowledge of the system requirements.

Second question : if the receiver includes an ADC, its noise figure ought to include not just the quantisation noise, but the ADC's own excess noise above that (from INL, DNL, sampling jitter or phase noise).