Howdy A good response Helmut The oversampling ratio (OSR) , clock purity will dominate.
What is your planned OSR , and sampler rate ? quadruple the sample rate , gain a bit of ADC of course. With an OSR = 1, for -120dBm , and full scale of say +12dBm (roughly what you have) , and 60dB of SNR will put your noise floor at 12-60 = -48dBm, so you'll need 72dB of gain. And so it will overload on the slightest thing out there ! With an OSR of 4x, you are 6dB or 1 bit better off, and so on. I know you understand this stuff so I wont elaborate. The SFDR of the converter will dominate what it can USEFULLY hear, because below the SFDR , REGARDLESS of the OSR, there will be all sorts of funny converter artifacts, and the intermods will be there also. So the SFDR , not the OSR ultimately determines the performance capability. IE the SNR might be 60dB, say a 10 bit converter, if the OSR = 4 then you'll get 66dB SNR, BUTthe SFDR does not change, the SFDR is still 60dB. So you can improve dynamic range, but not the SFDR. The SFDR, or more likely, where the third order two tone intermods are, won't change. Some LT converters have incredibly good SFDRs via internal digital dithering (later subtracted out in your receiver) . For my commercial SDR, I use a 12 bit converter at 200 Msps. The SFDR is 96dB, approx. The converter input FSD is abotu +12dBm, so the IMD will be always 12-96 = -84dBm so if I want my IMD down at -120dBm, then I need 36dB gain in front of the receiver. With such a high OSR (200M/ 10k)=43dB , the SNR is off in the stratosphere, but the IMD dominates.... In my experience the SFDR is what will limit the sensitivity. Watch out for ALIASED noise also. don't forget your converter is also equally (almost) bringing in noise form 2fs, 3fs 4fs etc. SO important that the convertor is seeing a low pass (nyquist) or band pass filter (super nyquist sampling) NOW what you can do is vary the voltage that the converter sees by fiddlign with the termination and the nosie figure can be usefully manipulated +/- 12dB (improved at the expense of full scale level) The noise figure of the converter is approx (the input level - the SNR) - 174 IE +12dBm FSD, SNR = 70, noise floor = -58dBm. Now, that is for a 200 Msps, or 100 Msps nyquist bandwidth, that is 10log10(1e8) or 80dB so -58 - 80 = -138dBm dBcHzSNR or 174 - 138 = 36dB. cheers On 28/10/2015 6:47 AM, David Rowe wrote: > Hello Glen/Matt, > > I'm working on a VHF radio prototype for testing some of my open source > DV ideas. Could you pls explain how to work out the gain required in > front of my ADC? > > For example if I have a MDS of -120dBm (0.224uV), and an ADC with a 3Vpp > (1.06Vrms) clipping point, and SFDR of say 60dB. > > Is the gain rqd simply Av=1.06/0.224E-16? That would mean the minimum > signal would hit full scale on the ADC. Perhaps we could scale that > back by 60dB plus some margin such that the MDS is still a few dB above > the floor of the ADC. > > I'm a bit mixed up by the idea of NF and ADCs. A worked example would help. > > Anyone else on the list with receiver design skills, pls feel free to > comment. If a good reference exists I'm happy to dig that up. > > Thanks, > > David > > ------------------------------------------------------------------------------ > _______________________________________________ > Freetel-codec2 mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/freetel-codec2 > -- - Glen English RF Communications and Electronics Engineer CORTEX RF & Pacific Media Technologies Pty Ltd ABN 40 075 532 008 PO Box 5231 Lyneham ACT 2602, Australia. au mobile : +61 (0)418 975077 ------------------------------------------------------------------------------ _______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2
