[USRP-users] R: Questions on set_rx_gain

Crozzoli Maurizio maurizio.crozzoli at telecomitalia.it
Mon Mar 7 10:56:57 EST 2016


Trip,
thank you for your answer!

We are using an E310.

What should I do to obtain a curve similar to the one you sent to us?

Which kind of input signal did you use?

Of course I know what is the power level of the input signal (dBm) but how do I get the absolute power level of collected/acquired signals? I can calculate the average of (I^2+Q^2) and express it in dB but what is the “reference” level of the acquired signal so that “dB” becomes “dBm”?

TIA!

BR,
Maurizio.

Da: James Humphries [mailto:james.humphries at ettus.com]
Inviato: lunedì 7 marzo 2016 16:01
A: Crozzoli Maurizio
Cc: USRP-users at lists.ettus.com
Oggetto: Re: [USRP-users] Questions on set_rx_gain

Hi Maurizio,

It really depends on the application as to what you set the gain to. What kind of input power do you expect on the RX? Is this for a B200/210, if so, a good starting point is around 20-30dB.

Here is some data I took a while back (see attached photo). The lines end where I started to see distortion with too much input power. Maybe this will help as a rough guide. Note the high noise floor at 0dB gain, you need some gain to improve the noise figure.

-Trip

[cid:image002.png at 01D17890.81C28E10]
​

On Mon, Mar 7, 2016 at 8:32 AM, Maurizio Crozzoli via USRP-users <usrp-users at lists.ettus.com<mailto:usrp-users at lists.ettus.com>> wrote:
Martin Braun via USRP-users <usrp-users at ...<mailto:usrp-users at ...>> writes:

>
> That is correct; an analog gain is applied. Note it is still possible to
> "do damage" this way, e.g. by driving amps or ADC into saturation.
>

So what's the rule?
Setting the correct gain seems to be really critical but I cannot understand
what is the correct approach to decide a reasonable value: is it just a
cut&try approach?

AGC might help from a theoretical point of view. But just theoretical, I'm
afraid, as far as I have understood, because most likely you tend to get
damages when AGC is on.

Nevertheless AGC would be really needed when your signal level fluctuates
along a wide range of values. And if the best suggestion is to avoid AGC, we
go back to the initial questions.

Comments, suggestions, even tutorial from a practical point of view are more
than welcome (they might even be worth of becoming a FAQ for SDR
not-so-experienced users). Volunteers?

TIA!

Maurizio.


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