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Today's Topics:
1. weird FFT behavior (Federico Larroca)
2. Re: weird FFT behavior (Matt Ettus)
3. Re: weird FFT behavior (Matt Ettus)
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Message: 1
Date: Thu, 14 Apr 2016 11:37:07 -0300
From: Federico Larroca <[email protected]>
To: [email protected]
Subject: [USRP-users] weird FFT behavior
Message-ID:
<CAHe2E1J4kdaZpp=h1-G-xZ_Z6Lbu91AJd2QzCjFWEA_9emmM=g...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Dear all,
We have been using USRPs for some years now, mostly for teaching. One of
the first things we show is naturally the spectrum. Some time ago we have
seen a weird behavior in certain frequencies.
For instance, when using a b200 centered at 300MHz (sampling rate 20MS/s)
obtains the following:
[image: Im?genes integradas 1]
Apparently, there is something interesting around 307MHz. When we re-center
the frequency we obtain the following:
[image: Im?genes integradas 2]
The interesting signal is gone. This is just an example. We get similar
behaviors at different frequencies (e.g. 180MHz).
Maybe it's a known phenomena which we ignore. Interestingly, it's present
also in our b100, and using the simple FFT (the examples are made with the
cool gr-fosphor).
Can anybody give us a pointer?
Best regards,
Federico
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Message: 2
Date: Thu, 14 Apr 2016 10:26:21 -0700
From: Matt Ettus <[email protected]>
To: Federico Larroca <[email protected]>
Cc: "[email protected]" <[email protected]>
Subject: Re: [USRP-users] weird FFT behavior
Message-ID:
<CAN=1kn-gJWRqCLK=l1wtdqsbicavneaydsrqxs0iam3dryx...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
What you are seeing is most likely not at exactly the frequency it looks
like, and you may be looking at a subharmonic. Instead of shifting from
300 to 307, try moving to 300.25. Does the signal of interest move towards
the middle of the display, or away? Does it move exactly 0.25 MHz, or a
multiple of that?
Matt
On Thu, Apr 14, 2016 at 7:37 AM, Federico Larroca via USRP-users <
[email protected]> wrote:
> Dear all,
> We have been using USRPs for some years now, mostly for teaching. One of
> the first things we show is naturally the spectrum. Some time ago we have
> seen a weird behavior in certain frequencies.
>
> For instance, when using a b200 centered at 300MHz (sampling rate 20MS/s)
> obtains the following:
> [image: Im?genes integradas 1]
>
> Apparently, there is something interesting around 307MHz. When we
> re-center the frequency we obtain the following:
>
> [image: Im?genes integradas 2]
>
> The interesting signal is gone. This is just an example. We get similar
> behaviors at different frequencies (e.g. 180MHz).
>
> Maybe it's a known phenomena which we ignore. Interestingly, it's present
> also in our b100, and using the simple FFT (the examples are made with the
> cool gr-fosphor).
>
> Can anybody give us a pointer?
> Best regards,
> Federico
>
> _______________________________________________
> USRP-users mailing list
> [email protected]
> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
>
>
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------------------------------
Message: 3
Date: Thu, 14 Apr 2016 11:38:06 -0700
From: Matt Ettus <[email protected]>
To: Federico Larroca <[email protected]>
Cc: "[email protected]" <[email protected]>
Subject: Re: [USRP-users] weird FFT behavior
Message-ID:
<CAN=1kn8xgaf8zoyqvowuosefkjp1ztlnjvgfkyjznunxb6i...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
The B200-series has a direct antenna connection without any analog
filtering. Any quadrature mixer device, like that used in the B200, will
have responses like this to strong signals. The normal way to handle this
is to use filters, which in this case would need to be external. The
E300-series has a complete set of RF filters which would eliminate this
issue.
Matt
On Thu, Apr 14, 2016 at 11:34 AM, Federico Larroca <[email protected]>
wrote:
> Hi Matt,
> Thank you for the quick answer. Here's what I've obtained. When using a
> central frequency 300.25, the weird signal has moved up by (approximately)
> 1 MHz (or maybe 0.75MHz?).
>
> [image: Im?genes integradas 1]
>
> I've looked around a bit the spectrum for the "original" signal, and
> here's what I've found (at 893 MHz) which looks similar:
> [image: Im?genes integradas 1]
>
> I have two questions then:
> 1 - What generates this phenomena? This is probably an answer I should
> know already, but maybe there is a reference I may read (or keyword I may
> look up).
> 2- How can I make sure that what I see is actually a signal at that
> frequency, and not an harmonic of another signal? The example I mentioned
> at 180MHz shows what seems like a DTV channel, and it sometimes masks the
> analog TV channels, which may be much more disturbing than the figures I
> show here.
>
> best,
> Federico
>
>
> 2016-04-14 14:26 GMT-03:00 Matt Ettus <[email protected]>:
>
>>
>> What you are seeing is most likely not at exactly the frequency it looks
>> like, and you may be looking at a subharmonic. Instead of shifting from
>> 300 to 307, try moving to 300.25. Does the signal of interest move towards
>> the middle of the display, or away? Does it move exactly 0.25 MHz, or a
>> multiple of that?
>>
>> Matt
>>
>> On Thu, Apr 14, 2016 at 7:37 AM, Federico Larroca via USRP-users <
>> [email protected]> wrote:
>>
>>> Dear all,
>>> We have been using USRPs for some years now, mostly for teaching. One of
>>> the first things we show is naturally the spectrum. Some time ago we have
>>> seen a weird behavior in certain frequencies.
>>>
>>> For instance, when using a b200 centered at 300MHz (sampling rate
>>> 20MS/s) obtains the following:
>>> [image: Im?genes integradas 1]
>>>
>>> Apparently, there is something interesting around 307MHz. When we
>>> re-center the frequency we obtain the following:
>>>
>>> [image: Im?genes integradas 2]
>>>
>>> The interesting signal is gone. This is just an example. We get similar
>>> behaviors at different frequencies (e.g. 180MHz).
>>>
>>> Maybe it's a known phenomena which we ignore. Interestingly, it's
>>> present also in our b100, and using the simple FFT (the examples are made
>>> with the cool gr-fosphor).
>>>
>>> Can anybody give us a pointer?
>>> Best regards,
>>> Federico
>>>
>>> _______________________________________________
>>> USRP-users mailing list
>>> [email protected]
>>> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
>>>
>>>
>>
>
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