SDR for xDSL spectrum analysis

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snapcracklepop
Posts: 3
Joined: Wed Aug 15, 2018 9:31 am

SDR for xDSL spectrum analysis

Post by snapcracklepop » Wed Aug 15, 2018 9:41 am

Hi,

I am very new to SDR and radio in general but I've seen various threads and YouTube videos of people using SDR for spectrum analysis.
I have purchased an SDR dongle to have a play around using direct sampling with but am wondering how I could go about connecting the SMA connector to a standard pstn/RJ11 jack/or just 2 croc clips or if this is even possible.

Many thanks and any help is greatly appreciated.

rtlsdrblog
Site Admin
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Joined: Mon Nov 19, 2012 11:54 pm

Re: SDR for xDSL spectrum analysis

Post by rtlsdrblog » Thu Aug 16, 2018 12:47 am

What frequencies does xDSL run at?

If you found a schematic of the xDSL port you could probably rig up an adapter. But do be careful and check if there is DC voltage on those lines. Otherwise an antenna near the jack or a cable running out of it might be enough.

dlritter
Posts: 31
Joined: Mon Nov 23, 2015 1:08 am

Re: SDR for xDSL spectrum analysis

Post by dlritter » Thu Aug 16, 2018 2:29 am

from the web:
All of the DSL achieve their high speeds in the same way by sending data over previously unused frequencies in phone lines. Voice signals travel over phone lines at frequencies ranging from 0 kHz to 4 kHz. Standard modems use the same frequencies, but DSL uses frequencies between 25 kHz and 1 MHz..

So an computer sound card would be more than adequate for what you want. An RTL-sdr device by itself does not operate below 24mhz.

snapcracklepop
Posts: 3
Joined: Wed Aug 15, 2018 9:31 am

Re: SDR for xDSL spectrum analysis

Post by snapcracklepop » Fri Aug 17, 2018 6:42 am

Thanks for the replies,

The frequencies used by ADSL to VDSL2 span from 25kHz to 30MHz. This range is chopped up into 4kHz segments known as 'bins'. I was hoping to use q branch direct sampling as I've read it will get me into HF as low as 500kHz (possibly lower).
The lines will more than likely have ~50v DC on them and will be a single twisted pair (cat3).
When it comes to making an adapter this is the part I'm unsure of - would I connect the copper core of the coax to 1 leg of the pair and the shielding to other leg? Or would I need a splitter and use 2 coax cables with each copper core connected to a leg each?

dlritter
Posts: 31
Joined: Mon Nov 23, 2015 1:08 am

Re: SDR for xDSL spectrum analysis

Post by dlritter » Fri Aug 17, 2018 2:14 pm

ah so much for wiki answers..
I would us a 9:1 Balun (from the RLT-SDR store)and maybe put .1uf capacitors in the DSL side. you could 'scope the DSL side.. a Dry DSL might not have 48v.. and a ring signal can go as high as 100+volts.
in the end I think you still only get 2+ mhz bandwidth so you need someway to count tone bins.
sounds like fun. I wonder what my Uverse signal looks like..

rtlsdrblog
Site Admin
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Re: SDR for xDSL spectrum analysis

Post by rtlsdrblog » Mon Aug 20, 2018 12:24 am

You'll definitely want to add a DC blocking capacitor if there's 50V on the line. You can't let that high of a voltage enter the dongle.

The direct sampling mode should be able to receive it.

snapcracklepop
Posts: 3
Joined: Wed Aug 15, 2018 9:31 am

Re: SDR for xDSL spectrum analysis

Post by snapcracklepop » Tue Aug 21, 2018 7:20 pm

Sounds good, I'll look into trying this out soon! Many thanks for the replies and information.

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