[USRP-users] interpretation of received signal

Nirmala Soundararajan nsound1 at students.towson.edu
Sat Oct 21 20:10:05 EDT 2017


Thanks for a very good explanation on power levels Marcus. Actually I think
I got the answer to what I was looking for. The "faintest reasonable signal
level" that a typical SDR can process!  (Typically  around -120  to  -130
dBm).!

In my application, I have a bunch of channels through which I can transmit
and receive. I wanted to do a quick health check of transmit and receive
and wanted to eliminate the overhead of creating a packet. So thought I
would send some tones through the channels of "particular power level",
then if I receive it within a "particular power level", then I could
declare that a particular channel is 'Pass'!  Of course this is only for
simulation purpose. It cannot be in real time!  But given all the
explanation, the "power based approach" seems to "not stand in favor"!
Perhaps best would be to send and receive known data and then declare the
channel as healthy??

regards

Nirmala



On Sat, Oct 21, 2017 at 12:39 PM, Marcus D. Leech via USRP-users <
usrp-users at lists.ettus.com> wrote:

> 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
> in-line.
>
> 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....
>
> Cheers
> Marcus
>
>
>
>
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