[USRP-users] Daughter board with a log response

Venkatesh Sandilya vsandilya at gmail.com
Fri Apr 10 16:17:47 EDT 2015


Hello

We are feeding a pulse modulated signal at 1090 MHz to the input of the
N200 and looking at the output using Gnuradio companion. The GRC flowgraph
is really simple and it contains a USRP source, few throttle blocks, a
complex to magnitude block (which is essentially doing sqrt(I*I + Q*Q)) and
looking at the output voltage using a scope sink. We start with an input
power of -80 dbm and increase it in steps of 5 and observe the output
voltage (in mv). We take these values of input power vs output voltage and
plot it in Excel. I think the receiver was saturating around -30 dbm when
the observed output voltage was around 1340mv.

On Fri, Apr 10, 2015 at 1:12 PM, Marcus D. Leech via USRP-users <
usrp-users at lists.ettus.com> wrote:

>  On 04/10/2015 12:33 PM, John Keller via USRP-users wrote:
>
>
> Hello
>
>     We are using a USRP N200 that has a DBSR2X daughter board. We are
> decoding signals at 1090 MHz that have a very large dynamic range Our old
> system used receivers that had log amps with a dynamic range of over 80 db
> and are linear when you plot input power vs output amplitude.
>
>  We plotted the output of the DBSR2X using a signal generator as an input
> source. We then plotted the input power vs output amplitude and it is very
> non linear. Do you have a daughter board with a log characteristic or could
> you suggest another way.
>
>   Thank you.
>
> Regards,
>
> John Keller
>
>
> _______________________________________________
> USRP-users mailing listUSRP-users at lists.ettus.comhttp://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
>
>  How are you plotting this?
>
> The power of a signal is proportional to the square of the voltage.  The
> DBSRX2 receiver should be very linear, except at the very upper end,
> perhaps.
>
> Logarithmic amplifier chains are generally *ONLY* used in
> high-dynamic-range power detection, they aren't general-purpose RF
> subsystems.
>
> The DBSRX2 on an N200 should have more than 80dB of dynamic range,
> depending on sample rates, and bandwidth settings of the DBSRX2 card.
>
> The instantaneous power of a signal can be calculated using:
>
> P = (I*I)+(Q*Q)
>
> Square-root and average to taste.
>
> So, what is it that you're actually measuring and plotting?  The DBSRX2
> should be quite linear over a fairly large dynamic range.
>
>
>
>
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>
>
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