[USRP-users] B100 + WBX Aliasing

Germano germanocapela at gmail.com
Wed Oct 15 05:59:23 EDT 2014


Thanks for the detailed answer.
I tested with two different antennas, Ettus' VERT400 and a DVB-T antenna 
(not one of those that come with the rtl dongles) and the results are 
similar (more prominent w/ the DVBT antenna).
I attached some screenshots of my measurements (I also tried with lower 
gain, 10dB, and the results are equivalent).



On 14-10-2014 17:00, usrp-users-request at lists.ettus.com wrote:
> Date: Tue, 14 Oct 2014 11:47:33 +0200 From: Marcus M?ller 
> <marcus.mueller at ettus.com> To: usrp-users at lists.ettus.com Subject: Re: 
> [USRP-users] B100 + WBX Aliasing Message-ID: 
> <543CF135.1000208 at ettus.com> Content-Type: text/plain; 
> charset="iso-8859-1" Hi Germano, usually, 600 MHz should really be far 
> enough to strongly suppress signals; the filters on the WBX are 
> "rather good". Do you have any numbers for us, ie. can you give us an 
> impression on how strong your "powerful replica" is, maybe in relation 
> to a known signal or the noise floor? What is your gain? How strong is 
> the "original" GSM900 downlink compared to the image? As a short 
> explanation: the WBX [1] is centered around the AD5387 IQ mixer [2] 
> which, if I remember correctly, has a downconversion bandwidth of up 
> to 240 MHz, which the WBX then filters down to 40 MHz of baseband 
> bandwidth using a multistage analog filter. If you're seeing strong 
> intermodulation, then maybe you're driving the WBX with a *very* 
> strong GSM signal, and even the best amplifiers and mixers do have 
> nonlinear behaviour when driven too close to their maximum power. 
> Nonlinearity leads to quadratic and higher order terms, which lead to 
> mixing... So the answer is: no, I don't believe that the filtering on 
> the WBX is not sufficient to suppress usual communication signals, but 
> that depends on the power of the signals. Also, you're doing spectrum 
> measurements around 312 MHz, but you also seem to receive GSM900 
> downlink (should be somewhere around 925-960 MHz); unless you have a 
> broadband [3] antenna, one of these should be attenuated. Greetings, 
> Marcus [1] schematics to all our d'boards can be found at 
> http://files.ettus.com/schematics [2] 
> http://www.analog.com/en/rfif-components/modulatorsdemodulators/adl5387/products/product.html 
> [3] Ok, this is just guesswork, but given the numbers I have: let's 
> assume your antenna's working range is really just 300 - 1000 MHz, 
> closely fitting both your observed frequency and GSM, giving it a 700 
> MHz bandwidth at a 650 MHz center frequency, which is quite a nice 
> broadband antenna. Our LP0410 comes close to that, with its 
> logperiodic design, but I don't have numbers on how it works below its 
> lower 400 MHz boundary. On 12.10.2014 18:20, Germano Capela via 
> USRP-users wrote:
>> >Hi,
>> >
>> >When I run a simple spectrum measurement around 312MHz, I observe a
>> >powerful replica of the GSM900 downlink (arround 935 MHz) spectra. Is it
>> >possible that the anti-aliasing filters are not enough to prevent this
>> >effect?
>> >
>> >Regards,
>> >
>> >Germano
>> >
>> >
>> >
>> >_______________________________________________
>> >USRP-users mailing list
>> >USRP-users at lists.ettus.com
>> >http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com

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