[USRP-users] B100 + WBX Aliasing

Marcus Müller marcus.mueller at ettus.com
Tue Oct 14 05:47:33 EDT 2014


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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