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

Kyeong Su Shin ksshin at uw.edu
Thu Jul 27 09:39:26 EDT 2017


Hello Marcus Müller,

Thank you, I will give it a shot.

Regards,
Kyeong Su Shin

On Thu, Jul 27, 2017 at 6:34 AM, Marcus Müller via USRP-users <
usrp-users at lists.ettus.com> wrote:

> 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> 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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>
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