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Re: [Discuss-gnuradio] Re: Mode S and ADS-B

From: Martin Dvh
Subject: Re: [Discuss-gnuradio] Re: Mode S and ADS-B
Date: Sun, 15 Apr 2007 21:58:35 +0200
User-agent: Debian Thunderbird 1.0.2 (X11/20070113)

Eric A. Cottrell wrote:
> Martin Dvh wrote:
>>Cory Papenfuss wrote:
>>>>I have a digital-wip branche which is work in progress for digital QAM
>>>>general-wip which is my work in progress for general gnuradio things
>>>>(latest addition is gr-cxadc, a driver for cx2388x capture cards to
>>>>use as a general purpose high-speed ADC frontend as a cheap
>>>>alternative for
>>>>an USRP)
>>>    Has there been some progress on that front then?  I've been lurking
>>>here for awhile, but last time I looked into this I don't think the TV
>>>-capture cards were viable.  The BT8xx chips still have holes in the
>>>data, right?  CX2388x can stream baseband without VBI holes?
>>Yes they can.
>>Look in the mailinglist for TV card SDR:
>>How Chee page on this:
>>My page on this:
>>SVN files are in
>>cxadc is the kernel driver
>>gr-cxadc is the gnuradio driver/glue
>>gnuradio-examples/cxadc  are the example scripts
> Hello,
> What are the sampling rates for these cards?  Can they go up to 8 or 10
> MSPS?  If so I might be able to try them out with Mode S.
You could try them out.

Standard is 28.63636.0 MSPS which is normally converted using a built-in 
samplerateconverter to 27 MSPS.
(The samplerateconversion is configurable but this is not implemented in the 
driver yet)
So you can capture up to 14 Mhz wide signals.

I "overclocked" mine to run at 32.0 MSPS to be able to use a tuner with a 44 
Mhz IF freq.
(with 28.63636 MSPS  you have a niquist boundary at 42.95 Mhz which is right in 
the 40 - 48 Mhz bandpass of the  44 Mhz IF)

You must also know that the ADC is 10 bits of which you standard only get the 
high 8 bits, so you don't have a very high dynamic range.
(using some tricks it should be possible to get the full 10 bits at halve the 

I must say this is still work-in-progress quality.

A lot of these tv-cards do not have a tuner with a suitable IF-output (if any) 
or only a "narrow-band" 200 kHz If at 10.7 MHz.
The IF-output level usually also is much too low (1 to 100 mv) for the 
video-input (1 V) to use the complete dynamic range. Amplification needs
top be added to get better sensitivity and dynamic range.

To have the most fun, you also really want a tuner with a 8 Mhz wide IF at the 
right IF-freq.

For now I get the best results when using an external microtune tuner with a 
5.75 Mhz IF.

I am working on getting the built-in thomson tuner of my pcHDTV3000 cx2388xx 
tv-card usable.

So the main work ahead is finding out which tuners are suitable, how to connect 
them to the video-input (amplification) and how to tune them.

The end-product will not be equivalent to an usrp but it will be a low-budget 
way to do SDR with gnuradio on quite a broad frequency range.
(0-50 Mhz using direct sampling and 50-860 Mhz using tuner)

If there is interest I am willing to supply premodified cards with the tuner 
replaced with a suitable model.
(As soon as we have a good working setup)


> 73 Eric

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