[USRP-users] Possible Causes of This Spurious Signal?

Marcus Müller marcus.mueller at ettus.com
Thu Jul 27 09:34:41 EDT 2017


Dear Kyeong Su Shin,

so, when things already get "seriously overloaded" at 15dB gain, and
you're running at 0 dB, I think the likeliest explanation is that at 0
dB, you're already seeing the effects of overloading. Try with something
like 40 dB attenuation between antenna and LNA.

Best regards,

Marcus


On 07/27/2017 09:41 AM, Kyeong Su Shin via USRP-users wrote:
> Hello Marcus Müller,
>
> I have no control to the TX power, as it is attached to an outdoor
> discone antenna (sorry, I forgot to mention that). I am cross-checking
> the measured spectrum against an Aglient spectrum analyzer. I am also
> using 0dB RX gain now, although maybe I can go and attach more
> attenuators to achieve lower gain levels.
>
> I tried increasing the RX gain (instead of decreasing it), and the
> relationship was apparently linearly dependent. (5dB increase in gain
> -> approx. 5dB increase in the power level in dBFS, 10dB increase in
> gain -> approx. 11dB increase in the power level in dBFS). The LNA got
> seriously overloaded when I tried 15dB RX gain (noise increase). The
> strength of the spurious signal was quickly varying, so I could have
> had mismeasured it, though.
>
> Attached is the PSD estimates collected using a Agilent SA (somewhat
> coarse version).
>
> Regards,
> Kyeong Su Shin
>
> On Wed, Jul 26, 2017 at 1:07 PM, Marcus Müller via USRP-users
> <usrp-users at lists.ettus.com <mailto:usrp-users at lists.ettus.com>> wrote:
>
>     Hello Kyeong Su Shin,
>
>     what's the RF power reaching the USRP? Does this get better when
>     you reduce the RX gain or the TX power?
>
>     Best regards,
>
>     Marcus
>
>
>     On 26.07.2017 16:52, Kyeong Su Shin via USRP-users wrote:
>>     Hello,
>>
>>     I observe a spurious signal in the PSD estimates that are
>>     obtained using a USRP, which I cannot identify the cause. Could
>>     someone help me identifying the cause of this?
>>
>>     Plots are shown below.
>>
>>     _Hardware_:
>>     -USRP N200 r4 + UBX-40 v1
>>     -Agilent FieldFox SA, as the reference.
>>
>>     _Software_:
>>     -Windows Server 2012
>>     -UHD 3.9.3
>>     -GNURadio 3.7.10.1
>>
>>     ..and also on a UHD-based (non-GNU Radio) spectrum scanner script
>>     that I am working on.
>>
>>     _Setting_:
>>     Gain : 0
>>     lo offset : 0
>>     Samp rate : 25MS/s (100e6/4)
>>     Antenna: RX2
>>
>>     _The Spurious Signal:_
>>
>>     @ fc = 641.25 MHz (reproducible at 642 / 641 MHz)
>>
>>>>     Goes away @ fc = 630MHz.
>>
>>>>     _+- 25MHz_:_
>>
>>     _No, I don't see any signal that could get aliased; I believe
>>     this is not due to the aliasing from the DDC._
>>
>>     _
>>
>>
>>>>     _1/2 fc (315MHz):_ Clean (no signal observed).
>>
>>
>>>>
>>     Regards,
>>     Kyeong Su Shin
>>
>>
>>     _______________________________________________
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