Reply by Tim Wescott February 25, 20112011-02-25
On 02/25/2011 11:17 AM, pacman101 wrote:
(top posting fixed)
>> On Thu, 24 Feb 2011 14:07:06 -0800, Tim Wescott<tim@seemywebsite.com> >> wrote: >> >>> On 02/24/2011 12:39 PM, pacman101 wrote: >>>> Thanks. All your comments make sense. I wanted to make sure that I am > on >>>> the right track since I have no experience dealing with playing with > A/D >> snip << >>> >> In addition to Tim's points on clock jitter, etc., another parameter >> that may be useful in an RF system is the aperture jitter or aperture >> delay. This is the jitter in the Sample-and-Hold Amplifier prior to >> the ADC and comes into play mostly if you're doing IF-sampling, >> sub-sampling or super-Nyquist conversion (i.e., capturing an aliased >> image rather than energy within the traditional Nyquist sampling >> range). >> >> And if what you're really getting at is whether it is possible to >> collect and process a signal that is WAY down in the dynamic range of >> the ADC, then, yes, you can, if you're careful about what you do and >> make use of processing gain to isolate and recover it. There are >> practical examples of receivers that are pretty old designs that pull >> relatively narrow-band PSK signals that occupy about 1-LSB (or a >> little less) of a single ADC (i.e., IF sampled) and process them with >> barely measurable implementation loss. >> >> Processing gain is a nice magic trick that a lot of people don't fully >> exploit. >> >> >> Eric Jacobsen >> http://www.ericjacobsen.org >> http://www.dsprelated.com/blogs-1//Eric_Jacobsen.php >>
> If I were doing supersampling (or subsampling) how do I know what is the > highest frequency an ADC can subsample? I am talking the true frequency of > interest, not the images. > > Where can I see examples of these designs? The two most important parameters that I can think of are the bandwidth of the sample-and-hold front end within the ADC, and the total jitter (aperture and clock) both inside and outside the ADC. The one will kill your signal strength without touching your noise, the other will act like phase noise, spreading out both your signal and interfering signals. -- 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 pacman101 February 25, 20112011-02-25
If I were doing supersampling (or subsampling) how do I know what is the
highest frequency an ADC can subsample?  I am talking the true frequency of
interest, not the images.

Where can I see examples of these designs?

Thanks.

>On Thu, 24 Feb 2011 14:07:06 -0800, Tim Wescott <tim@seemywebsite.com> >wrote: > >>On 02/24/2011 12:39 PM, pacman101 wrote: >>> Thanks. All your comments make sense. I wanted to make sure that I am
on
>>> the right track since I have no experience dealing with playing with
A/D
>>> converters (except using a spectrum analyzer and some readily built >>> radios). >>> >>> So if I were to design an RF receiver, what design parameters should I
pay
>>> particular attention to when choosing the correct ADC? For instance I >>> would think for my application I need high dynamic range and low
jitter,
>>> but what else should I pay attention to so that I don't get screwed by
the
>>> manufacturer? >>> >> >> context snipped << >> >>Look for converters that specify their 2nd- and 3rd-order >>intermodulation distortion -- this is a good indication of the 'useful' >>linearity in a receiver where you may be bringing a much wider band to >>the ADC than you will be sorting out after conversion. >> >>Actually, in general, you want to look for converters that are specified
>>in 'radioish' terms, and the 3rd-order intermod is just part of that. >>This has been a frustration in the past when I was helping to select a >>good ADC for video use, and had to translate (or just plain guess at) >>what the radio-centric specifications meant in terms of a broadband >>signal that went all the way down to DC. >> >>Rate converter noise by spectral height in real frequency -- i.e., take >>the sampling rate into account. >> >>Jitter is important, but shows up as phase noise in your receiver -- >>i.e., if the sampling instant jitters, then a strong signal adjacent to >>a desired weak signal will smear, and will degrade the effective SNR of >>your desired signal. >> >>Note, too, that the ADC jitter is going to specified assuming a perfect >>clock to the ADC -- you can screw this over really quick by not paying >>strict attention to your clock signal. You need to treat the clock to >>the ADC as a precious resource: I know that digital radio guys don't let
>>their ADC clocks go through their FPGAs, because they know the FPGA >>internal noise will mess up their clocks. On the other hand, don't over
>>buy -- there's no point in getting the World's Lowest Jitter ADC if you >>_are_ corrupting your clock's phase noise generating it through an FPGA. >> >>I don't know what else specific to say -- the ADC is one of the choke >>points on performance, so there's a lot of value in modeling your >>receiver's performance parametrically, then plugging a lot of ADC >>parameters into it to see which one really is best. Then look at what >>each ADC needs for care and feeding, and decide how much it's going to >>cost to keep each one happy. >> >>-- >> >>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 > >In addition to Tim's points on clock jitter, etc., another parameter >that may be useful in an RF system is the aperture jitter or aperture >delay. This is the jitter in the Sample-and-Hold Amplifier prior to >the ADC and comes into play mostly if you're doing IF-sampling, >sub-sampling or super-Nyquist conversion (i.e., capturing an aliased >image rather than energy within the traditional Nyquist sampling >range). > >And if what you're really getting at is whether it is possible to >collect and process a signal that is WAY down in the dynamic range of >the ADC, then, yes, you can, if you're careful about what you do and >make use of processing gain to isolate and recover it. There are >practical examples of receivers that are pretty old designs that pull >relatively narrow-band PSK signals that occupy about 1-LSB (or a >little less) of a single ADC (i.e., IF sampled) and process them with >barely measurable implementation loss. > >Processing gain is a nice magic trick that a lot of people don't fully >exploit. > > >Eric Jacobsen >http://www.ericjacobsen.org >http://www.dsprelated.com/blogs-1//Eric_Jacobsen.php >
Reply by Eric Jacobsen February 24, 20112011-02-24
On Thu, 24 Feb 2011 14:07:06 -0800, Tim Wescott <tim@seemywebsite.com>
wrote:

>On 02/24/2011 12:39 PM, pacman101 wrote: >> Thanks. All your comments make sense. I wanted to make sure that I am on >> the right track since I have no experience dealing with playing with A/D >> converters (except using a spectrum analyzer and some readily built >> radios). >> >> So if I were to design an RF receiver, what design parameters should I pay >> particular attention to when choosing the correct ADC? For instance I >> would think for my application I need high dynamic range and low jitter, >> but what else should I pay attention to so that I don't get screwed by the >> manufacturer? >> > >> context snipped << > >Look for converters that specify their 2nd- and 3rd-order >intermodulation distortion -- this is a good indication of the 'useful' >linearity in a receiver where you may be bringing a much wider band to >the ADC than you will be sorting out after conversion. > >Actually, in general, you want to look for converters that are specified >in 'radioish' terms, and the 3rd-order intermod is just part of that. >This has been a frustration in the past when I was helping to select a >good ADC for video use, and had to translate (or just plain guess at) >what the radio-centric specifications meant in terms of a broadband >signal that went all the way down to DC. > >Rate converter noise by spectral height in real frequency -- i.e., take >the sampling rate into account. > >Jitter is important, but shows up as phase noise in your receiver -- >i.e., if the sampling instant jitters, then a strong signal adjacent to >a desired weak signal will smear, and will degrade the effective SNR of >your desired signal. > >Note, too, that the ADC jitter is going to specified assuming a perfect >clock to the ADC -- you can screw this over really quick by not paying >strict attention to your clock signal. You need to treat the clock to >the ADC as a precious resource: I know that digital radio guys don't let >their ADC clocks go through their FPGAs, because they know the FPGA >internal noise will mess up their clocks. On the other hand, don't over >buy -- there's no point in getting the World's Lowest Jitter ADC if you >_are_ corrupting your clock's phase noise generating it through an FPGA. > >I don't know what else specific to say -- the ADC is one of the choke >points on performance, so there's a lot of value in modeling your >receiver's performance parametrically, then plugging a lot of ADC >parameters into it to see which one really is best. Then look at what >each ADC needs for care and feeding, and decide how much it's going to >cost to keep each one happy. > >-- > >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
In addition to Tim's points on clock jitter, etc., another parameter that may be useful in an RF system is the aperture jitter or aperture delay. This is the jitter in the Sample-and-Hold Amplifier prior to the ADC and comes into play mostly if you're doing IF-sampling, sub-sampling or super-Nyquist conversion (i.e., capturing an aliased image rather than energy within the traditional Nyquist sampling range). And if what you're really getting at is whether it is possible to collect and process a signal that is WAY down in the dynamic range of the ADC, then, yes, you can, if you're careful about what you do and make use of processing gain to isolate and recover it. There are practical examples of receivers that are pretty old designs that pull relatively narrow-band PSK signals that occupy about 1-LSB (or a little less) of a single ADC (i.e., IF sampled) and process them with barely measurable implementation loss. Processing gain is a nice magic trick that a lot of people don't fully exploit. Eric Jacobsen http://www.ericjacobsen.org http://www.dsprelated.com/blogs-1//Eric_Jacobsen.php
Reply by Mark February 24, 20112011-02-24
two things,
1) if you have two = tones into the ADC, they BOTH can't be at full
scale.

2) when an ADC is oversampling, it just means the sampling rate is
higher then it needs to be to meet Nyquist.  If you rasise the
sampling rate, the TOTAL Q noise remains unchanged, but that noise is
now spread out over a wider BW, over the full Nyquist BW, so the noise
in YOUR BW of INTEREST gets lowered, i.e. the noise DENSITY is lower,
but the TOTAL noise remains the same..

Mark


Reply by Tim Wescott February 24, 20112011-02-24
On 02/24/2011 12:39 PM, pacman101 wrote:
> Thanks. All your comments make sense. I wanted to make sure that I am on > the right track since I have no experience dealing with playing with A/D > converters (except using a spectrum analyzer and some readily built > radios). > > So if I were to design an RF receiver, what design parameters should I pay > particular attention to when choosing the correct ADC? For instance I > would think for my application I need high dynamic range and low jitter, > but what else should I pay attention to so that I don't get screwed by the > manufacturer? >
>> context snipped << Look for converters that specify their 2nd- and 3rd-order intermodulation distortion -- this is a good indication of the 'useful' linearity in a receiver where you may be bringing a much wider band to the ADC than you will be sorting out after conversion. Actually, in general, you want to look for converters that are specified in 'radioish' terms, and the 3rd-order intermod is just part of that. This has been a frustration in the past when I was helping to select a good ADC for video use, and had to translate (or just plain guess at) what the radio-centric specifications meant in terms of a broadband signal that went all the way down to DC. Rate converter noise by spectral height in real frequency -- i.e., take the sampling rate into account. Jitter is important, but shows up as phase noise in your receiver -- i.e., if the sampling instant jitters, then a strong signal adjacent to a desired weak signal will smear, and will degrade the effective SNR of your desired signal. Note, too, that the ADC jitter is going to specified assuming a perfect clock to the ADC -- you can screw this over really quick by not paying strict attention to your clock signal. You need to treat the clock to the ADC as a precious resource: I know that digital radio guys don't let their ADC clocks go through their FPGAs, because they know the FPGA internal noise will mess up their clocks. On the other hand, don't over buy -- there's no point in getting the World's Lowest Jitter ADC if you _are_ corrupting your clock's phase noise generating it through an FPGA. I don't know what else specific to say -- the ADC is one of the choke points on performance, so there's a lot of value in modeling your receiver's performance parametrically, then plugging a lot of ADC parameters into it to see which one really is best. Then look at what each ADC needs for care and feeding, and decide how much it's going to cost to keep each one happy. -- 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 pacman101 February 24, 20112011-02-24
I agree Dirk.

I believe what you are saying is the dynamic range equals to the
20*log(Signal) - 20*log(FFT) - Noise Figure. 

Is this formula more or less correct?

>> Theoretically my SNR is 6 dB * 14 =3D 84 dB. =A0Which means >> the SNR from the highest tone to the noise floor is 84 dB, ... > >By noise floor, if you mean on a spectrum analyzer or FFT, then that >last statement is not true. > >Dirk >
Reply by Dirk Bell February 24, 20112011-02-24
> Theoretically my SNR is 6 dB * 14 = 84 dB. &#2013266080;Which means > the SNR from the highest tone to the noise floor is 84 dB, ...
By noise floor, if you mean on a spectrum analyzer or FFT, then that last statement is not true. Dirk
Reply by pacman101 February 24, 20112011-02-24
Thanks.  All your comments make sense.  I wanted to make sure that I am on
the right track since I have no experience dealing with playing with A/D
converters (except using a spectrum analyzer and some readily built
radios).  

So if I were to design an RF receiver, what design parameters should I pay
particular attention to when choosing the correct ADC?  For instance I
would think for my application I need high dynamic range and low jitter,
but what else should I pay attention to so that I don't get screwed by the
manufacturer?


>On Thu, 24 Feb 2011 19:27:42 +0000, glen herrmannsfeldt wrote: > >> Tim Wescott <tim@seemywebsite.com> wrote: (snip on SNR and
oversampling)
>> >>> Something that will change your confusion level (either up or down
:-))
>>> is that with most ADCs, the conversion noise is white at the ADC >>> output, and has roughly the same magnitude, no matter how fast you >>> sample the ADC. So _in the discrete time domain_ the noise always >>> looks the same. >> >>> The difference is that in that same discrete time domain, when you >>> sample faster, all of your desired signals become narrower in >>> bandwidth. So you can filter tighter, from the perspective of that >>> discrete-time domain, and get rid of more noise. From a >>> frequency-domain perspective, _that_ is why oversampling helps. >> >> Otherwise, there is a long tradition of "signal averaging" to reduce
the
>> SNR. It is the basis for the "margin of error" on many statistical >> reports. For Gaussian distributed (and sometimes even non-Gaussian) >> noise, the noise averages out at 1/sqrt(N). Many measurements are >> inherently statistical (radioactive half-life, as one example), and the >> only way to get a more accurate measurement is to measure many samples. > >Which is really looking at the same thing from yet another angle again. >It's good to have lots of different angles, too look at hard problems >from. > >> Oversampling is slightly different, but mostly you get the same result >> from multple sampling and dithering. > >Most ADCs with bit counts above 12 or so provide the dithering free of >charge, in the form of random noise. > >-- >http://www.wescottdesign.com >
Reply by Tim Wescott February 24, 20112011-02-24
On Thu, 24 Feb 2011 19:27:42 +0000, glen herrmannsfeldt wrote:

> Tim Wescott <tim@seemywebsite.com> wrote: (snip on SNR and oversampling) > >> Something that will change your confusion level (either up or down :-)) >> is that with most ADCs, the conversion noise is white at the ADC >> output, and has roughly the same magnitude, no matter how fast you >> sample the ADC. So _in the discrete time domain_ the noise always >> looks the same. > >> The difference is that in that same discrete time domain, when you >> sample faster, all of your desired signals become narrower in >> bandwidth. So you can filter tighter, from the perspective of that >> discrete-time domain, and get rid of more noise. From a >> frequency-domain perspective, _that_ is why oversampling helps. > > Otherwise, there is a long tradition of "signal averaging" to reduce the > SNR. It is the basis for the "margin of error" on many statistical > reports. For Gaussian distributed (and sometimes even non-Gaussian) > noise, the noise averages out at 1/sqrt(N). Many measurements are > inherently statistical (radioactive half-life, as one example), and the > only way to get a more accurate measurement is to measure many samples.
Which is really looking at the same thing from yet another angle again. It's good to have lots of different angles, too look at hard problems from.
> Oversampling is slightly different, but mostly you get the same result > from multple sampling and dithering.
Most ADCs with bit counts above 12 or so provide the dithering free of charge, in the form of random noise. -- http://www.wescottdesign.com
Reply by glen herrmannsfeldt February 24, 20112011-02-24
Tim Wescott <tim@seemywebsite.com> wrote:
(snip on SNR and oversampling)

> Something that will change your confusion level (either up or down :-)) > is that with most ADCs, the conversion noise is white at the ADC output, > and has roughly the same magnitude, no matter how fast you sample the > ADC. So _in the discrete time domain_ the noise always looks the same.
> The difference is that in that same discrete time domain, when you sample > faster, all of your desired signals become narrower in bandwidth. So you > can filter tighter, from the perspective of that discrete-time domain, > and get rid of more noise. From a frequency-domain perspective, _that_ > is why oversampling helps.
Otherwise, there is a long tradition of "signal averaging" to reduce the SNR. It is the basis for the "margin of error" on many statistical reports. For Gaussian distributed (and sometimes even non-Gaussian) noise, the noise averages out at 1/sqrt(N). Many measurements are inherently statistical (radioactive half-life, as one example), and the only way to get a more accurate measurement is to measure many samples. Oversampling is slightly different, but mostly you get the same result from multple sampling and dithering. -- glen