Hmm... Good point Crystal filter should have given it away. I need more coffee or glasses one of the two.
73 Matthew VK5ZM On 29 October 2015 at 10:30, glen english <[email protected]> wrote: > Not direct conversion Matthew- that is a superhet- David will undersample > the IF. > > > On 29/10/2015 10:42 AM, Matthew Cook wrote: > > David, > > Ok that's just a straight direct conversion receiver. As Helmut has said > this is a different beast. The majority of your design decisions will be > made in the analogue world. > > Here's a few good worked examples for you to look at, it doesn't matter > these are for HF the same rule applies you just need too substitute for the > components you can source at your frequency of interest. > > > http://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/qst/1993/02/page23/ > > http://www.arrl.org/files/file/QEX_Next_Issue/Jul-Aug_2013/QEX_7_13_Horrabin.pdf > > Somewhere I've got a copy of Doug DeMaw (W1FB) receiver design book, I > must try to find it and let you have a read. Dougs book discusses the > trade off's moving amps and filters around the mixer etc. > > Oh and with your ring diode mixer, look at putting a diplexer in there to > terminate all the unwanted energy in a resistor and pick off the band of > interest before feeding to your post mixer amp. Lots of references on the > net for doing this. > > 73 > > Matthew > VK5ZM > > On 29 October 2015 at 07:30, David Rowe <[email protected]> wrote: > >> Thank you Helmut, Glen, and Steve, >> >> 1/ Ok it's starting to make sense, lets see if I can work through a >> contrived example: >> >> We have a 16 bit sound blaster ADC, with a SNR of 6*15 = 90dB. Its FSD >> is 1Vrms (10dBm). It samples at 48 kHz: >> >> The ADC noise floor is at 10-96=-86dBm. This is in a Nyquist BW of >> 48kHz, so the noise power normalised to a 1Hz BW is -86-10*log10(24E3) = >> -129.8dBm/Hz. The thermal noise floor is -174dBm/Hz so our ADC NF is >> 174-129.8 = 44.2dB. >> >> Help me understand what NF is too, and why a filter has a negative NF - >> it moves the signal closer to the thermal noise floor. >> >> 2/ I'm actually working on narrow band constant envelope radio, So: >> >> BPF- LNA - Mixer - Xtal filter - limiting amp - ADC >> >> If I am sampling a constant envelope signal (FM, FSK, GMSK), what are >> the SNR/SFDR requirements for the ADC? The limiting amp has removed all >> amplitude information. So do we just need the sign bit of the ADC? >> Could we just sample the signal with a flip flop? >> >> Thanks, >> >> David >> >> >> On 28/10/15 17:03, glen english wrote: >> > 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 >> >> >> > >> >> >> ------------------------------------------------------------------------------ >> _______________________________________________ >> Freetel-codec2 mailing list >> [email protected] >> https://lists.sourceforge.net/lists/listinfo/freetel-codec2 >> > > > > ------------------------------------------------------------------------------ > > > > _______________________________________________ > Freetel-codec2 mailing > [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 > >
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