Which noise sources dominate in a receiver?

Question

For a radio receiver that digitizes over-the-air RF signals (such as an SDR), my assumption (as a DSP guy; pardon my hardware ignorance!) has been that thermal noise dominates total receiver noise, and quantization noise is a slightly distant second.

One reason for this is I assume you would put an LNA as close to the beginning of the signal chain as possible to reduce the effect of noise sources later in the receiver. Thus, SNR would largely be determined by thermal noise (probably mainly from the antenna). Is this accurate, or does it depend on the bit depth of the ADC?

Second question: Is quantization noise typically captured in the reported receiver noise figure, F, or does F only account for thermal noise?

Edit

Obviously the answer always depends on the designer's choices. But it seems likely there would be something typical, or a couple of standard cases (perhaps at different price points; I'm primarily interested in higher end hardware).

Answer 1

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).

Answer 2

Thermal noise tends to dominate in an RF receiver. The bit-depth can be large enough so that the corresponding quantization noise becomes less significant w.r.t thermal noise. Quantization noise can be expressed as a noise figure and added to the noise figures of the other devices in cascade using Friis' formula.

Answer 3

I appreciate the other answers given, but I'm adding this answer to my own question because I think I found a more detailed answer here.

According to the author, it is typical to design for thermal noise to dominate rather than quantization noise. The main trade-off is dynamic range vs. noise figure degradation due to increased significance of quantization noise. If you place the quantization noise far below the input thermal noise, you give up dynamic range (I assume because to do this you have to boost your input signal power higher up in the range of the ADC).

This is all for a fixed ADC bit depth. If you increase the number of bits, quantization noise goes down, and/or dynamic range increases (my guess is you would typically use the extra bits to increase dynamic range and use an LNA to make sure the quantization noise doesn't dominate).