[USRP-users] R: Re: Questions on set_rx_gain

Crozzoli Maurizio maurizio.crozzoli at telecomitalia.it
Mon Mar 7 18:15:00 EST 2016


"The lines end where I started to see distortion": just a "visual" approach or did you measure somehow the distortion?


---- James Humphries ha scritto ----

Hi Maurizio,

As a disclaimer, it is discouraged to input power levels this high to your USRP as it can easily cause damage. I recommend at least a 20dB attenuator if you wire directly.

For an input, I used an RF signal generator (Agilent 8648C) with a simple sine output at 915MHz. I used this to control the input power to the USRP. I did as you suggested to calculate the magnitude of the signal, and then used a fudge factor to 'calibrate' it to the known input power (Although, you can see that the fudge factor is slightly off in this data). This was only a quick test to get a rough estimate of saturation points, noise floor, and linearity.

-Trip



On Mon, Mar 7, 2016 at 10:56 AM, Crozzoli Maurizio <maurizio.crozzoli at telecomitalia.it<mailto:maurizio.crozzoli at telecomitalia.it>> wrote:
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<mailto:james.humphries at ettus.com>]
Inviato: lunedì 7 marzo 2016 16:01
A: Crozzoli Maurizio
Cc: USRP-users at lists.ettus.com<mailto: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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