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Re: [Discuss-gnuradio] FPGA Gain?
From: |
Eric Cottrell |
Subject: |
Re: [Discuss-gnuradio] FPGA Gain? |
Date: |
Wed, 11 Jul 2007 18:01:01 -0400 (EDT) |
----- Start Original Message -----
Sent: Wed, 11 Jul 2007 12:38:41 +0200
From: "Trond Danielsen" <address@hidden>
To: address@hidden
Subject: Re: [Discuss-gnuradio] FPGA Gain?
> 2007/7/10, Eric Cottrell <address@hidden>:
> > Hello,
> >
> > The LFRX daughterboard on the USRP has 20 dB of gain. I seem to remember
> > that 10 dB of that gain is from the FPGA. I assume that this gain is after
> > the ADC? If so, is it really useful?
> >
> > I relate digital gain to zoom on a digital camera. The analog gain is the
> > optical zoom and the digital gain is the digital zoom. Digital cameras
> > have 10x or more digital zoom but it is not really useful. At high digital
> > zooms you get lower resolution and the picture is blocky. Optical zoom is
> > more useful as it comes before the digitizer.
> >
> > So if I have noise values of 35 and signal values of 40 then I get values
> > of 350 and 400 after the 10 dB digital amplifier. I may see a higher
> > amplitude signal but how do I gain anything? Am I missing something?
> >
> > 73 Eric
>
> Hi Eric,
>
> The ADC has a programmable gain stage prior to sampling. Could it be
> that one you are thinking of?
>
> Regards,
> --
> Trond Danielsen
Hello,
It must be. I thought there was a digital amplifier implemented in the FPGA.
Also I realized that in my example that the problem would be the lsb error
would grow from +/- 1 bit to +/- 10 bits and the resolution would decrease.
The values would be the same for analog or digital.
With the oscilloscope and fft programs I can experiment with gain settings and
see the practical effect. I tend to set the gain to where the noise floor
rises up slightly. I put in a 20 dB amplifier inline to use the AR5000 with
the LFRX so I could keep the gain at 10. Now I can try a USRP Gain Setting of
11 or even 20 to see what happens. Nigel would be pleased.
To you RF Engineers, Noise Figure and Gain Distribution are common mundae
design issues. I am still figuring them out.
73 Eric