Newbie questions

[Dave Lawrence]

Dear GQRX,

I have a couple of questions regarding the spectral output of GQRX (and related gr applications). I have an airspy mini and tuning to 95.8MHz with an FM antenna attached gives me

I would like to better understand the apparent "shoulders" at the lower and upper ends of the [6MHz] range.

If I retune away from the FM radio range I get 

and there there seem to be carriers at the top end of the plot, but they are obviously phantoms as they apppear at any tuned centre frequency.

Thank you,

[Lee Donaghy]

I see them on my airspy too, i think it's just the design of the hardware iirc on sdrsharp the edges are cut off to remove them but other sdr apps show then entire bandwidth.

[Dave Lawrence]

Hi Lee,

Thanks for the confirmation. Do you know what the usable bandwidth is? Do you have an Airspy Mini or Airspy2?

Thanks

[Alexandru Csete]

Hi Dave,

The "shoulders" you see at the edges of the spectrum is the roll-off
of the anti-aliasing filter in the device (or gqrx if you use input
decimation). It is normal and you will see it everywhere where there
is sampling or resampling involved. There purpose is to prevent out of
band signals from folding into the passband. That is indeed what you
observe when you see a strong singal at the edge, which then becomes
weaker as it moves in.

Wikipedia has probably a better explanation than what I can give:
https://en.wikipedia.org/wiki/Aliasing

As Lee points out, some SDR applications do not show this region by
showing a narrower spectrum, but in gqrx I chose to show the full
spectrum.

Alex

On Fri, Nov 18, 2016 at 8:52 PM, Dave Lawrence
<mail.dave...@gmail.com> wrote:
>
> Dear GQRX,
>
> I have a couple of questions regarding the spectral output of GQRX (and related gr applications). I have an airspy mini and tuning to 95.8MHz with an FM antenna attached gives me
>
>

>,

> I would like to better understand the apparent "shoulders" at the lower and upper ends of the [6MHz] range.
>
>
> If I retune away from the FM radio range I get
>
>
>

[Lee Donaghy]

I have an R2, when set to 10MSPS i see 8MHz of bandwidth.

[Dave Lawrence]

Hi Alex,

Thanks for the explanation. Is there a defined way of calculating the available bandwidth? Lee says he gets .8x which seems about right for my Airspy Mini too.

I am wanting to produce a wide spectral plot by retuning the hardware and pasting together multiple FFTs, so I need to ensure there's no odd effects at the joins.

Chers,

[Alexandru Csete]

I depends on the device. For the Airspy Mini the website says "Up to 6
MHz panoramic spectrum view"
http://airspy.com/airspy-mini/

Alex

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[Dave Lawrence]

Hi Alex,

I see the shoulders with the Mini set to 6M, so the actual useable range is somewhat less than that. I will just make a user-selectable option in my app.

Thanks,