[USRP-users] Daughter board with a log response

Ian Buckley ianb at ionconcepts.com
Fri Apr 10 17:21:07 EDT 2015


Just one other point: Throttle blocks. You should't have any in a flow graph that contains *real* hardware…the sample rate of the USRP will pace the flow graph. Throttle blocks are generally intended for flow graphs that contain no real H/W, where otherwise the flow graph will consume all available system CPU.

On Apr 10, 2015, at 1:40 PM, "Marcus D. Leech via USRP-users" <usrp-users at lists.ettus.com> wrote:

> On 04/10/2015 04:17 PM, Venkatesh Sandilya wrote:
>> 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.
> Turn the gain down on the DBSRX2.
> 
> Also you say "when the observed output voltage was around 1340mV".  Since you don't get to see any absolute voltage values on the digital side,
>   just magnitude values that are *proportional* to the input voltage at the antenna, I'm curious as to how you derived your 1340mV value?
> 
> The ADCs on the USRP motherboard are set with 1.2V references, so the highest value that can be "seen" at the ADC input would be 1200mV, but this
>   is *AFTER* downconversion and gain and baseband filtering.  You typically have a *lot* of gain ahead of the ADCs.
> 
> So, I'd start with an input power level of around -110dBm, and work your way up from there.  It doesn't surprise me that -30dBml, with some non-trivial
>   amount of gain on your DBSRX2 produces an output from the DBSRX2 that saturates the ADCs.
> 
> 
>> 
>> 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 list
>>> USRP-users at lists.ettus.com
>>> http://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.
>> 
>> 
>> 
>> 
>> _______________________________________________
>> USRP-users mailing list
>> USRP-users at lists.ettus.com
>> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
>> 
>> 
> 
> _______________________________________________
> USRP-users mailing list
> USRP-users at lists.ettus.com
> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.ettus.com/pipermail/usrp-users_lists.ettus.com/attachments/20150410/b7c32ec0/attachment-0002.html>


More information about the USRP-users mailing list