----- Start Original Message ----- Sent: Wed, 11 Jul 2007 12:38:41 +0200 From: "Trond Danielsen" <[EMAIL PROTECTED]> To: [email protected] Subject: Re: [Discuss-gnuradio] FPGA Gain?
> 2007/7/10, Eric Cottrell <[EMAIL PROTECTED]>: > > 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 _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
