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

Kyeong Su Shin ksshin at uw.edu
Thu Jul 27 03:41:35 EDT 2017


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> 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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>
>
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