Tim Wescott <tim@seemywebsite.com> wrote:>As long as the ADC is reasonably linear then you would get all kinds of >processing gain in the digital domain.Isn't this only true if it is dithered? (Odds are, you may not require an explicit dithering signal, but it is something to keep in mind.) Steve
A question on practical receiver
Started by ●September 23, 2010
Reply by ●September 24, 20102010-09-24
Reply by ●September 24, 20102010-09-24
Tim Wescott <tim@seemywebsite.com> wrote: (snip)> Mark was kind enough to let me know off the group, but I'd rather the > error get corrected here -- I'm rusty with my calculations, and > neglected to account for the fact that dBm is milliwatts, not watts. So > his number is correct.> Still not bad, but definitely getting down to where you'd like to do > some band pass filtering before the ADC if you can (and you'd like to > double check the noise performance of the ADC -- most of them really suck).As far as I know, most noise sources decrease reasonably with increasing frequency, so a high pass filter could be pretty important. Maybe 500kHz if you want the AM broadcast band. Then you need the anti-alias filter. For a 3.6Gs/s, ADC that might come naturally in the RF amplifier. I suppose amplifiers are getting faster all the time, but it can't be that easy to build one up to 1.8GHz. Though if you do the processing in an FPGA, it seems that Software isn't the right word for it. So, a small board with antenna, RF amplifier, ADC, FPGA, DAC, audio amplifier, and speaker, could be built to be used in undergrad lab classes. Add an LED display, and you can build a clock radio. -- glen
Reply by ●September 24, 20102010-09-24
On 09/24/2010 01:16 PM, Steve Pope wrote:> Tim Wescott<tim@seemywebsite.com> wrote: > >> As long as the ADC is reasonably linear then you would get all kinds of >> processing gain in the digital domain. > > Isn't this only true if it is dithered? > > (Odds are, you may not require an explicit dithering signal, > but it is something to keep in mind.)If RMS thermal noise is equal to or greater than one LSB of the ADC, then that's enough "dithering". I suspect that such an ADC would have a healthy amount of self noise as is, making the last few bits more marketing than actual adders of precision to the device. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●September 24, 20102010-09-24
On 09/24/2010 01:23 PM, glen herrmannsfeldt wrote:> Tim Wescott<tim@seemywebsite.com> wrote: > (snip) > >> Mark was kind enough to let me know off the group, but I'd rather the >> error get corrected here -- I'm rusty with my calculations, and >> neglected to account for the fact that dBm is milliwatts, not watts. So >> his number is correct. > >> Still not bad, but definitely getting down to where you'd like to do >> some band pass filtering before the ADC if you can (and you'd like to >> double check the noise performance of the ADC -- most of them really suck). > > As far as I know, most noise sources decrease reasonably with > increasing frequency, so a high pass filter could be pretty > important. Maybe 500kHz if you want the AM broadcast band. > > Then you need the anti-alias filter. For a 3.6Gs/s, ADC that > might come naturally in the RF amplifier. I suppose amplifiers > are getting faster all the time, but it can't be that easy > to build one up to 1.8GHz. > > Though if you do the processing in an FPGA, it seems that Software > isn't the right word for it. > > So, a small board with antenna, RF amplifier, ADC, FPGA, DAC, > audio amplifier, and speaker, could be built to be used in > undergrad lab classes. Add an LED display, and you can build > a clock radio.I have to say, I was mostly riffing on what _could_ you do with such an ADC, not what _should_ you do. What you _should_ do, in all but some really oddball circumstances, is to precede the ADC with a bandpass filter of some sort (and hence anti-alias as well), probably some amplification, and maybe even some AGC. But it would depend heavily on the service to which the receiver was going to be put -- receiving long-distance space communications would be vastly different than something that might go into a cell phone tower. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●September 25, 20102010-09-25
Thanks Tim.>If you know the frequency range that you want to receive in, it's much >better to go > >Antenna -> Bandpass -> AGC -> ADC -> etc. >Just one que: Do we not need anti-aliasing before ADC?> >Receiver design is a complicated subject. There's certainly a lot there >to study. >I agree with you and hopefully, I will learn the trade. Cheers, Chintan
Reply by ●September 25, 20102010-09-25
cpshah99 <cpshah99@n_o_s_p_a_m.rediffmail.com> wrote: (snip)> Just one que: Do we not need anti-aliasing before ADC?That is what I was wondering. With a 3.6GHz ADC, it might be fast enough compared to the amplifier bandwidth that you don't need anything more. There is now a TV commercial for a car radio with a 40GB disk drive in it. The commercial mentions that you can now pause and rewind radio. It seems to me that with 40GB it should be able to record all the radio stations at once. Then you could pause, switch stations, and rewind. If you record in MP3 format all the AM and FM stations you can receive, how long before the 40GB disk is full? -- glen
Reply by ●September 25, 20102010-09-25
On 09/25/2010 03:17 AM, cpshah99 wrote:> Thanks Tim. > >> If you know the frequency range that you want to receive in, it's much >> better to go >> >> Antenna -> Bandpass -> AGC -> ADC -> etc. >> > > Just one que: Do we not need anti-aliasing before ADC?Just one question for you, then: how might a bandpass filter and an anti-alias filter be related? (hint: http://www.wescottdesign.com/articles/Sampling/sampling.html) -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●September 26, 20102010-09-26
On Sep 25, 6:56�am, glen herrmannsfeldt <g...@ugcs.caltech.edu> wrote:> cpshah99 <cpshah99@n_o_s_p_a_m.rediffmail.com> wrote: > > (snip) > > > Just one que: Do we not need anti-aliasing before ADC? > > That is what I was wondering. > > With a 3.6GHz ADC, it might be fast enough compared to the > amplifier bandwidth that you don't need anything more. > > There is now a TV commercial for a car radio with a 40GB > disk drive in it. �The commercial mentions that you can > now pause and rewind radio. > > It seems to me that with 40GB it should be able to record > all the radio stations at once. �Then you could pause, > switch stations, and rewind. > > If you record in MP3 format all the AM and FM stations > you can receive, how long before the 40GB disk is full? > > -- glenFor 100 of each, roughly 6 hours.
Reply by ●September 27, 20102010-09-27
>On 09/25/2010 03:17 AM, cpshah99 wrote: >> Thanks Tim. >> >>> If you know the frequency range that you want to receive in, it's much >>> better to go >>> >>> Antenna -> Bandpass -> AGC -> ADC -> etc. >>> >> >> Just one que: Do we not need anti-aliasing before ADC? > >Just one question for you, then: how might a bandpass filter and an >anti-alias filter be related? (hint: >http://www.wescottdesign.com/articles/Sampling/sampling.html) > >-- > >Tim Wescott >Wescott Design Services >http://www.wescottdesign.com > >Do you need to implement control loops in software? >"Applied Control Theory for Embedded Systems" was written for you. >See details at http://www.wescottdesign.com/actfes/actfes.html >%%%% Thanks Tim for the link, I will have a look at the material. Cheers Chintan
Reply by ●September 28, 20102010-09-28
On Sep 27, 6:22�am, "cpshah99" <cpshah99@n_o_s_p_a_m.rediffmail.com> wrote:> >On 09/25/2010 03:17 AM, cpshah99 wrote: > >> Thanks Tim. > > >>> If you know the frequency range that you want to receive in, it's much > >>> better to go > > >>> Antenna -> �Bandpass -> �AGC -> �ADC -> �etc. > > >> Just one que: Do we not need anti-aliasing before ADC? > > >Just one question for you, then: how might a bandpass filter and an > >anti-alias filter be related? �(hint: > >http://www.wescottdesign.com/articles/Sampling/sampling.html) > > >-- > > >Tim Wescott > >Wescott Design Services > >http://www.wescottdesign.com > > >Do you need to implement control loops in software? > >"Applied Control Theory for Embedded Systems" was written for you. > >See details athttp://www.wescottdesign.com/actfes/actfes.html > > %%%% > > Thanks Tim for the link, I will have a look at the material. > > Cheers > > Chintan- Hide quoted text - > > - Show quoted text -I�ve come to this discussion late, but in skimming through the messages I didn�t see where the OP specified the analysis bandwidth. RF frequency is one issue, and has to do with the radio and translation of the modulated signal down to baseband. But the final analysis bandwidth and required dynamic range determines stuff just prior to the A/D and the A/D it self. This of course assumes that the RF portions, ie, NF and OIP has been designed to accommodate the desired dynamic range. If I�ve missed something, I apologize. Regards.
