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Re: [Discuss-gnuradio] Re: FUNCube dongle

From: Moeller
Subject: Re: [Discuss-gnuradio] Re: FUNCube dongle
Date: Fri, 25 Feb 2011 20:38:37 +0100
User-agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv: Gecko/20101207 Thunderbird/3.1.7

On 25.02.2011 18:45, Alexandru Csete wrote:

> There is still an audio codec - the difference is that it is in the Funcube 
> Dongle rather than in the host computer. Moreover, the IQ imbalance is not
> only due to audio hardware but also due to the qudrature demodulator, so DC 
> offset and phase error is pretty much a "feature" of all direct conversion
> type receivers (and transmitters) where you process the I and Q channels in 
> separate hardware chains. The USRP & daughterboards have it and the
> Funcube Dongle has it too.

It's just an ADC sampler. I didn't say anything against the audio ADC.
But the audio transmission line is much more: 2 audio jacks with analog contact
imperfactions (IQ imbalance), lots of "digital noise" within the PC,
amplifiers, analog filters, audio mixers etc.
The frequency response of the overall analog part is not linear.
I think the sub bass region <20Hz is critical, not so important for the
human ear, but a very relevant region in the digital baseband spectrum.

>     Yes, I think it's a nice abstract interface.
>     Do you know the theoretical limits for the sample rate?
> If you ask about the Funcube Dongle then it has a fixed sample rate of 96 
> kHz. It uses signed 16 bit samples, so the required USB bandwidth is ~ 3
> Mbps. It can actually work on USB 1.1 hosts, though I don't know who may have 
> such things lying around today.

I know, the Funcube Dongle has a low sample rate.
But maybe this virtual sound interface could be "misused" for higher rates in 
low-cost SDR designs. If there is no limit in the interface specification.

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