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RF Sampling

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Hello, I have a quick question regarding sampling RF. Say I have an ADC referenced to 3.3v and am receiving a signal from a satellite at -130dBm. My thinking is that I need 144dB of gain to bring it up to 14dBm, which should be about 3v into a 50ohms. Is this correct thinking, or are there other things I haven't considered? My worry is that the thermal noise floor is somewhere around -111dBm, so would I need less gain to fit the noise into the range?

You'll be getting the SNR back by processing, not analog filtering.  Give it enough gain to improve the noise figure of the ADC (related to its ENOB, noise performance and bandwidth -- you'll have to look up the data and figure this out) and do the rest in DSP.

Too much gain and not enough filtering in the front end, and you'll be screwed out of dynamic range.  Just a few dB more of a nearby interfering signal and *splat* goes the SNR of your desired signal (mostly due to IMD)...



--- Quote from: evb149 on July 02, 2016, 11:36:06 pm ---That's a very low level signal and probably unrealistic to process due to noise and the high gain required unless there are particular characteristics of the signal modulation and receiver architecture that facilitate the acquisition.  Some signals can be received at levels below the noise floor if redundancy in the signal characteristics make it possible to apply some kind of simple (or complex) averaging/correlation techniques to effectively improve the the signal to  noise ratio.  But usually for such signals with mainstream relevance with complex modulations there are dedicated receiver ICs that have digital and analog filtering and so on to recover them.

--- End quote ---

So this is for experimenting with a GPS front end, which as you suggested, uses Gold Codes to pull the signal out of the noise in the DSP side of things. From what I've found, you can use a very low resolution (4-1 bits) adc to decode the signal. What is confusing me is how the amplification stage before the adc is calculated.

--- Quote from: T3sl4co1l on July 02, 2016, 11:50:36 pm ---Give it enough gain to improve the noise figure of the ADC (related to its ENOB, noise performance and bandwidth -- you'll have to look up the data and figure this out) and do the rest in DSP.

--- End quote ---

Could you elaborate on this a little more. Say for example I use a TI THS1009 ADC (datasheet attached, relevant info on page 4), which has a SNR of 58-60dB. I've calcualted the NF of the ADC to be 62dB using the equation on page 4 of the attached MT-006 application note on the subject with SNR=60 B=4e^6 and Pfs = 14.1 (not sure if this is correct). How does increasing the gain improve the ADC's NF? I assume I'm using the wrong Pfs?

I think Pfs = 14.1dBm looks completely wrong.  The THS1009 doesn't seem to present a AINP load other than 1) 10pF input capacitance and 2) 10uA peak leakage current.   Pfs is very small for FS @ 3.2V; 32uW is the FS leakage power (so Pfs is more like -45dBm) and I doubt the 10pF is a factor at 4MHz, but it depends on your source impedance.  If you buffer the input to drop the source impedance to some fraction of an ohm the 10pF disappears at your frequencies.  (But then you add another NF to the chain, so balance the effect of a buffer on the Friis sum vs the benefit of dropping Pfs; if you already have a low source impedance gratuitously adding another buffer doesn't help.)
So, NF=-45+174-60-66 = 3dB

That looks a lot more reasonable, no?

That does look a lot more reasonable, but the Analog Devices app note does say that it should be higher than you would expect and calculated 30dB in their calculation example.


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