[USRP-users] Daughter board with a log response

Venkatesh Sandilya vsandilya at gmail.com
Fri Apr 10 17:25:58 EDT 2015


I understand. I realize that now about the throttle blocks. Will experiment
without them and see if that makes a difference.
On Apr 10, 2015 5:21 PM, "Ian Buckley" <ianb at ionconcepts.com> wrote:

> 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
>>
>>
>>  _______________________________________________
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>>
>>  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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