David, What you're asking is not a simple question. It is the bulk of the work to make a good receiver -- figuring out the answers to these questions and making the decisions about how your design will perform. I recommend reading everything you can get your hands on from TI and Analog Devices that talk about design with an ADC. I can't tell for sure if you are looking at a wide-band design or a narrow one, but it looks like wide-band. Here's a good paper to get started that works out NF in an ADC, but there are many more:
http://www.ti.com/lit/an/slyt090/slyt090.pdf A couple of points: 1. SFDR from the manufacturer is based on the assumption that you will be looking at (demodulating) large swaths of the spectrum and that you are concerned about having a spur anywhere in that reception bandwidth. It also assumes that you will generally pick a converter and then want to receive everything in the first Nyquist zone. For example, the data sheet assumes you will pick a 125Msps converter, place your Nyquist frequency at 62.5MHz and then want to simultaneously receive everything from 0-62.5MHz (less filter skirts). As hams, this is not what we do so the requirements change. Your selection of the sampling frequency will largely place many of the larger spurs that contribute to SFDR and if you are receiving a narrow bandwidth, you can place spurs where you want them, effectively raising the SFDR. In other words, selection of sampling frequency, itself, can be very important. 2. You can make a number of significant trade-offs in receiver design that are not win-win. You must look at each decision and decide which is best for your application. Is noise figure or intermodulation performance most important in your design? Do your customers need a 1dB noise figure or is 10dB fine? In the VHF world, will your customers be "real VHFers" that put a preamp on the tower or will they expect you to have an amazing noise figure and sacrifice other parameters? If overload a concern or are you willing to sacrifice some overload performance to decrease noise figure? Very few hams really understand how to work out the answers to the questions you are asking. Ultimately when you have a design on a PCB, you will really want to understand the questions you're asking and your predicted answers. Then you can test what you've learned against your receiver. In my opinion, there are no shortcuts to reading everything you can get your hands on and asking lots of questions, being careful to validate everything you hear against everything you read. Steve On Tue, Oct 27, 2015 at 2:47 PM, David Rowe <[email protected]> wrote: > 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. > > Stephen Hicks, N5AC VP Engineering FlexRadio Systems™ 4616 W Howard Ln Ste 1-150 Austin, TX 78728 Phone: 512-535-4713 x205 Email: [email protected] Web: www.flexradio.com Click Here for PGP Public Key <https://sites.google.com/a/flex-radio.com/pgp-public-keys/n5ac> *Tune In Excitement™* PowerSDR™ is a trademark of FlexRadio Systems
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