[USRP-users] interpretation of received signal

Marcus D. Leech mleech at ripnet.com
Sat Oct 21 12:39:25 EDT 2017

On 10/21/2017 11:59 AM, Kevin McGuire via USRP-users wrote:
> My knowledge is limited, therefore, read this with a grain of salt. 
> However, I wanted to try to help and if something I say does not make 
> sense then double-check it or someone else may come along and correct me.
> I had this same problem when I started with these types of systems. I 
> had trouble understanding what the numbers meant in terms of a 
> physical measurement. People would give me a short summary of it but I 
> still failed to completely understand until I dug down into what the 
> system actually does from the time the RF energy is presented to the 
> time it hands back a sequence of complex numbers.
> Yes, Marcus is correct not only because he is an expert/professional 
> but because what he says aligns with what I have learned. The USRP 
> does not provide dBm. All it provides is a complex vector of 32-bits 
> (16-bit) or 16-bits (8-bit). This is then normalized and scaled 
> between 0.0 and 1.0. That 16-bit I and Q are what the ADC outputs - 
> although I know it is manipulated by the FPGA with filters and 
> decimation but for what it is worth that is where they come from. The 
> ADC and FPGA really have no idea what the original signal 
> power/voltage level was before being amplified unless they calculate 
> it. You /can/ do your own calculation but then the accuracy is 
> questionable unless you calibrate it. I also know the FPGA can control 
> various amplifiers so it is not just a single component, therefore, 
> that must be what makes it difficult to know for certain the accuracy.
> I think calibration is difficult because of lots of complex factors. I 
> could only suspect this would be in relation to distortions and 
> interactions between components with different gains, temperatures, 
> and ... well that is what I think.
> But, if you used a loop back you could see why you get units less than 
> makes sense. As in, you could see that the antenna had little effect 
> at the same gain and such, or maybe the antenna is making a big 
> difference.
Kevin, thanks for your input.

But I see below that you made the recommendation to DIRECTLY CONNECT  a 
receiver to a transmitter.   This is NEVER a good idea, as you can 
easily exceed
   the maximum safe input levels for the receiver.    When you are doing 
such direct loopback tests, ALWAYS have at least 40dB of attenuation 

The amplifiers used in receivers are sensitive.  Structurally, they are 
usually GaAsFET transistors with an exceedingly-thin gate region (a few 
molecules thick). It's very easy to destroy that gate region with 
too-much input power.

A receiver is, after all, designed to receive signals from an antenna 
"through the air".   A quick look at standard path-loss models means 
that levels that one might reasonably describe as "a flea sneezing" are 
more-than-adequate to drive a receiver. Transmitters, on the other hand, 
produce power that ranges from "can drive a tiny electric motor" to 
"boil a mug of coffee".

Here's a f'rexample.   Consider a satellite in low-earth orbit, 
producing, let's say, +20dBm into a dipole, and transmitting at 2.3GHz.  
Let's say its in an orbit 180km above the earth.   We ascribe 3dB gain 
to the antennae on each end, although in reality, at least the ground 
segment will have a high-gain antenna. Plugging this in to a path-loss 
calculator, there's 139dB of path-loss between the satellite and ground 
station.     So, that +20dBm signal is now at about -121dBm coming into 
your receiver.   That's within reach of a typical SDR receiver.  Add 
some gain on the ground end, and you have an even better signal. Now, 
-121dBm is 1.0e-15 *watts*.  Putting, let's say, +10dBm into that same 
receiver means that it is trying to process a signal that is 130dB 
louder than the faintest "reasonable" signal that it can process.  The 
very *best* outcome is that the receiver will become non-linear.  The 
very-worst is that it will become damaged.

Folks who have heretofore "grown up digital" may have almost no 
intuitive feel for how the analog electronics world works, and that it 
is dominated by the physics of the real-world, and thus governed by laws 
that you cannot easily "get around".   This takes some getting used to....


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