[USRP-users] Questions on set_rx_gain

James Humphries james.humphries at ettus.com
Sat Mar 12 19:02:46 EST 2016


Yes, it was a simple offset to calibrate the USRP to a known input power.

-Trip

On Wed, Mar 9, 2016 at 5:08 AM, Crozzoli Maurizio <
maurizio.crozzoli at telecomitalia.it> wrote:

> Thank you Trip!
>
>
>
> As far as I can understand, the same “quick and dirty” approach was used
> to derive the fudge factor: it is right?
>
>
>
> Maurizio.
>
>
>
> *Da:* James Humphries [mailto:james.humphries at ettus.com]
> *Inviato:* mercoledì 9 marzo 2016 02:29
> *A:* Crozzoli Maurizio
> *Cc:* USRP-users at lists.ettus.com
> *Oggetto:* Re: Re: [USRP-users] Questions on set_rx_gain
>
>
>
> Yup, just a visual approach to see distortion of frequency domain data.
> Like I said, it was a very quick and dirty measurement just to get a rough
> idea.
>
>
>
> -Trip
>
>
>
> On Mon, Mar 7, 2016 at 6:15 PM, Crozzoli Maurizio <
> maurizio.crozzoli at telecomitalia.it> wrote:
>
> "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> 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]
> *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
>
>
>
>
>>
>
>
> On Mon, Mar 7, 2016 at 8:32 AM, Maurizio Crozzoli via USRP-users <
> usrp-users at lists.ettus.com> wrote:
>
> Martin Braun via USRP-users <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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