Randy Yates <yates@digitalsignallabs.com> writes:> your signal of interest is concerned, your only using 10 bits of that....you're only using... (I hate it when I make these stupid grammar errors...) -- Randy Yates Digital Signal Labs http://www.digitalsignallabs.com
Effect of Input ADC Bandwidth
Started by ●September 18, 2013
Reply by ●September 23, 20132013-09-23
Reply by ●September 24, 20132013-09-24
>On Thu, 19 Sep 2013 09:49:10 -0500, "bradsdr" <95105@dsprelated> >wrote: > >>>radams2000@gmail.com writes: >>> >>>> If you have large interfering signal components above your 2mhzsignal>>>> range then it's best to remove them in the analog domain, otherwise >>>> any ADC nonlinearity could produce in-band spurs that cannot be >>>> removed by subsequent digital filtering. But if the out-of-bandenergy>>>> is just low-level noise then it probably won't make much difference >>>> either way. >>> >>>Bob, >>> >>>You bring up (indirectly) a point I failed to address in my response >>>yesterday. Even if the ADC had no such nonlinearities, you still have >>>the problem that, since the ADC's peak input is limited, such a signal >>>would require the signal-of-interest to be greatly attenuated, which in >>>effect throws away a good bit of your ADC resolution. >>> >>>If you add the possible (and practical) nonlinearity problem in, then >>>between the two it can be a serious issue. >>>-- >>>Randy Yates >>>Digital Signal Labs >>>http://www.digitalsignallabs.com >>> >> >>Lets say my signal-of-interest is X dB, then what could be the >>power/strength of interfering signal that is tolerable i.e it doesn't >>affect my SNR? Sorry if I am asking wrong questions > >Mostly you want enough dynamic range so that the ADC output isn't >pushed into saturation or other distortion. So the number of bits >has to be enough to handle both the maximum expected signal plus the >maximum expected sum of interference energy. > >Since you indicate you have an AGC, this can often be used to bound >the number of bits needed for the desired signal. The number of bits >needed for interference can then be determined if the interference >energy is specified relative to the desired signal, e.g., +20dBc or >something like tha.t >Hi Eric thank you so much for your comment. What would you prefer controlling the gain of received signal by digital means or using analog AGC ?>>ADC I am going to use is quite good ads62p44: >> >>SNR(dB) 73.8 >>SFDR(dB) 86 >>ENOB (Bits) 11.8 >>SINAD (dB) 73.4 >>Input Range 2Vpp >>Maximum Sample Rate: 125 MSPS >>14-Bit Resolution with No Missing Codes >>95 dB Crosstalk >>3.5 dB Coarse Gain and Programmable Fine Gain >>up to 6 dB for SNR/SFDR Trade-Off >>Amplitude Down to 400 mVPP >> >>Regards, >>Brad >> >> >>_____________________________ >>Posted through www.DSPRelated.com > >Eric Jacobsen >Anchor Hill Communications >http://www.anchorhill.com >_____________________________ Posted through www.DSPRelated.com
Reply by ●September 24, 20132013-09-24
>"bradsdr" <95105@dsprelated> writes: > >>>radams2000@gmail.com writes: >>> >>>> If you have large interfering signal components above your 2mhzsignal>>>> range then it's best to remove them in the analog domain, otherwise >>>> any ADC nonlinearity could produce in-band spurs that cannot be >>>> removed by subsequent digital filtering. But if the out-of-bandenergy>>>> is just low-level noise then it probably won't make much difference >>>> either way. >>> >>>Bob, >>> >>>You bring up (indirectly) a point I failed to address in my response >>>yesterday. Even if the ADC had no such nonlinearities, you still have >>>the problem that, since the ADC's peak input is limited, such a signal >>>would require the signal-of-interest to be greatly attenuated, which in >>>effect throws away a good bit of your ADC resolution. >>> >>>If you add the possible (and practical) nonlinearity problem in, then >>>between the two it can be a serious issue. >>>-- >>>Randy Yates >>>Digital Signal Labs >>>http://www.digitalsignallabs.com >>> >> >> Lets say my signal-of-interest is X dB, then what could be the >> power/strength of interfering signal that is tolerable i.e it doesn't >> affect my SNR? Sorry if I am asking wrong questions > >You are not asking wrong questions at all, Brad. These are excellent >questions. By the way, I apologize for taking so long to respond. > >The thing to remember is that you will need to limit the maximum >amplitude going in to the ADC to avoid saturation. Think of the >following scenario in the frequency domain: let's say you have a large >undesired sine wave at 20 MHz (assuming you're sampling at 100 MHz). >Just for example, let's say it is 24 dB higher than your desired signal, >which is a sine wave at 1 MHz. Then that means you'll have to adjust >your AGC so that the biggest signal, which is the undesired signal, is >at or below your ADC reference That means that your desired sine wave is >then 24 dB _below_ the full scale range of the ADC. > >That's just like "throwing away" 24/6 = 4 bits of your ADC. That is, if >your ADC is 14 bits (as you state below), then effectively, as far as >your signal of interest is concerned, your only using 10 bits of that. > >Of course that may still be enough, depending on other parameters in >your system, but this is one of the potential problems you should >consider when deciding where and how you do your system filtering.Randy, That was a very nice and clear example. The picture I get is that as I have no control over the undesired signal so I must do filtering before ADC to filter out the undesired signal and maintain SNR. Because otherwise we would always be using less bits of ADC and hence SNR degrades irrespective of whether I use AGC or not . And AGC will only ensure that ADC doesn't go into saturation right? Or there is any other way around ? -- Brad> >--Randy > > >> ADC I am going to use is quite good ads62p44: >> >> SNR(dB) 73.8 >> SFDR(dB) 86 >> ENOB (Bits) 11.8 >> SINAD (dB) 73.4 >> Input Range 2Vpp >> Maximum Sample Rate: 125 MSPS >> 14-Bit Resolution with No Missing Codes >> 95 dB Crosstalk >> 3.5 dB Coarse Gain and Programmable Fine Gain >> up to 6 dB for SNR/SFDR Trade-Off >> Amplitude Down to 400 mVPP >> >> Regards, >> Brad > > >> >> >> _____________________________ >> Posted through www.DSPRelated.com > >-- >Randy Yates >Digital Signal Labs >http://www.digitalsignallabs.com >_____________________________ Posted through www.DSPRelated.com
Reply by ●September 24, 20132013-09-24
"bradsdr" <95105@dsprelated> writes:>>"bradsdr" <95105@dsprelated> writes: >> >>>>radams2000@gmail.com writes: >>>> >>>>> If you have large interfering signal components above your 2mhz > signal >>>>> range then it's best to remove them in the analog domain, otherwise >>>>> any ADC nonlinearity could produce in-band spurs that cannot be >>>>> removed by subsequent digital filtering. But if the out-of-band > energy >>>>> is just low-level noise then it probably won't make much difference >>>>> either way. >>>> >>>>Bob, >>>> >>>>You bring up (indirectly) a point I failed to address in my response >>>>yesterday. Even if the ADC had no such nonlinearities, you still have >>>>the problem that, since the ADC's peak input is limited, such a signal >>>>would require the signal-of-interest to be greatly attenuated, which in >>>>effect throws away a good bit of your ADC resolution. >>>> >>>>If you add the possible (and practical) nonlinearity problem in, then >>>>between the two it can be a serious issue. >>>>-- >>>>Randy Yates >>>>Digital Signal Labs >>>>http://www.digitalsignallabs.com >>>> >>> >>> Lets say my signal-of-interest is X dB, then what could be the >>> power/strength of interfering signal that is tolerable i.e it doesn't >>> affect my SNR? Sorry if I am asking wrong questions >> >>You are not asking wrong questions at all, Brad. These are excellent >>questions. By the way, I apologize for taking so long to respond. >> >>The thing to remember is that you will need to limit the maximum >>amplitude going in to the ADC to avoid saturation. Think of the >>following scenario in the frequency domain: let's say you have a large >>undesired sine wave at 20 MHz (assuming you're sampling at 100 MHz). >>Just for example, let's say it is 24 dB higher than your desired signal, >>which is a sine wave at 1 MHz. Then that means you'll have to adjust >>your AGC so that the biggest signal, which is the undesired signal, is >>at or below your ADC reference That means that your desired sine wave is >>then 24 dB _below_ the full scale range of the ADC. >> >>That's just like "throwing away" 24/6 = 4 bits of your ADC. That is, if >>your ADC is 14 bits (as you state below), then effectively, as far as >>your signal of interest is concerned, your only using 10 bits of that. >> >>Of course that may still be enough, depending on other parameters in >>your system, but this is one of the potential problems you should >>consider when deciding where and how you do your system filtering. > > Randy, > > That was a very nice and clear example. The picture I get is that as I have > no control over the undesired signal so I must do filtering before ADC to > filter out the undesired signal and maintain SNR. Because otherwise we > would always be using less bits of ADC and hence SNR degrades irrespective > of whether I use AGC or not . And AGC will only ensure that ADC doesn't go > into saturation right?Brad, That's right.> Or there is any other way around ?No, you've got it. Don't forget there is also potential nonlinearity as well, as Bob Adams was saying. That is, even if you do put an AGC on the front and limit the interferer's magnitude so that it doesn't saturate the ADC, you still could be some nasty (possibly in-band) spurs. Yeah, if you've got large interfering signals, it's better to filter before the ADC if you can. --Randy> > -- > Brad > >> >>--Randy >> >> >>> ADC I am going to use is quite good ads62p44: >>> >>> SNR(dB) 73.8 >>> SFDR(dB) 86 >>> ENOB (Bits) 11.8 >>> SINAD (dB) 73.4 >>> Input Range 2Vpp >>> Maximum Sample Rate: 125 MSPS >>> 14-Bit Resolution with No Missing Codes >>> 95 dB Crosstalk >>> 3.5 dB Coarse Gain and Programmable Fine Gain >>> up to 6 dB for SNR/SFDR Trade-Off >>> Amplitude Down to 400 mVPP >>> >>> Regards, >>> Brad >> >> >>> >>> >>> _____________________________ >>> Posted through www.DSPRelated.com >> >>-- >>Randy Yates >>Digital Signal Labs >>http://www.digitalsignallabs.com >> > > _____________________________ > Posted through www.DSPRelated.com-- Randy Yates Digital Signal Labs http://www.digitalsignallabs.com
Reply by ●September 24, 20132013-09-24
Note that the filter will also damage your signal (amplitude ripple, group delay variations). Maybe this doesn't matter (for example, an OFDM receiver sees the filter as part of the channel), but it can be problematic in other applications such as measurement receivers for closed-loop control algorithms. _____________________________ Posted through www.DSPRelated.com
Reply by ●September 24, 20132013-09-24
On Tue, 24 Sep 2013 12:56:01 -0500, "bradsdr" <95105@dsprelated> wrote:>>On Thu, 19 Sep 2013 09:49:10 -0500, "bradsdr" <95105@dsprelated> >>wrote: >> >>>>radams2000@gmail.com writes: >>>> >>>>> If you have large interfering signal components above your 2mhz >signal >>>>> range then it's best to remove them in the analog domain, otherwise >>>>> any ADC nonlinearity could produce in-band spurs that cannot be >>>>> removed by subsequent digital filtering. But if the out-of-band >energy >>>>> is just low-level noise then it probably won't make much difference >>>>> either way. >>>> >>>>Bob, >>>> >>>>You bring up (indirectly) a point I failed to address in my response >>>>yesterday. Even if the ADC had no such nonlinearities, you still have >>>>the problem that, since the ADC's peak input is limited, such a signal >>>>would require the signal-of-interest to be greatly attenuated, which in >>>>effect throws away a good bit of your ADC resolution. >>>> >>>>If you add the possible (and practical) nonlinearity problem in, then >>>>between the two it can be a serious issue. >>>>-- >>>>Randy Yates >>>>Digital Signal Labs >>>>http://www.digitalsignallabs.com >>>> >>> >>>Lets say my signal-of-interest is X dB, then what could be the >>>power/strength of interfering signal that is tolerable i.e it doesn't >>>affect my SNR? Sorry if I am asking wrong questions >> >>Mostly you want enough dynamic range so that the ADC output isn't >>pushed into saturation or other distortion. So the number of bits >>has to be enough to handle both the maximum expected signal plus the >>maximum expected sum of interference energy. >> >>Since you indicate you have an AGC, this can often be used to bound >>the number of bits needed for the desired signal. The number of bits >>needed for interference can then be determined if the interference >>energy is specified relative to the desired signal, e.g., +20dBc or >>something like tha.t >> >Hi Eric thank you so much for your comment. What would you prefer >controlling the gain of received signal by digital means or using analog >AGC ?That depends a lot on the system requirements and how the system is architected. We've used both methods, where the analog AGC controls the amplitude of the desired signal and some headroom is left at the ADC for a controlled amount of interfering or adjacent energy, or the analog AGC controls the composite power level so that the ADC dynamic range is always fully utilized regardless of the ratio of desired/undesired input energy. The second method requires a digital AGC that controls the amplitude of the desired signal somewhere in the digital processing chain. There are advantages and disadvantages both ways. Which way to go best depends on a lot of different things.>>>ADC I am going to use is quite good ads62p44: >>> >>>SNR(dB) 73.8 >>>SFDR(dB) 86 >>>ENOB (Bits) 11.8 >>>SINAD (dB) 73.4 >>>Input Range 2Vpp >>>Maximum Sample Rate: 125 MSPS >>>14-Bit Resolution with No Missing Codes >>>95 dB Crosstalk >>>3.5 dB Coarse Gain and Programmable Fine Gain >>>up to 6 dB for SNR/SFDR Trade-Off >>>Amplitude Down to 400 mVPP >>> >>>Regards, >>>Brad >>> >>> >>>_____________________________ >>>Posted through www.DSPRelated.com >> >>Eric Jacobsen >>Anchor Hill Communications >>http://www.anchorhill.com >> > >_____________________________ >Posted through www.DSPRelated.comEric Jacobsen Anchor Hill Communications http://www.anchorhill.com
Reply by ●September 25, 20132013-09-25
One additional comment: Any AGC in a high-performance real-world receiver uses both analog and digital scaling. It is a pretty deep topic, highly system specific. For example, in OFDM you don't want to switch gain in the middle of a symbol. Other systems may give you only a brief time window to adjust.>That depends a lot on the system requirements and how the system is >architected. We've used both methods, where the analog AGC controls >the amplitude of the desired signal and some headroom is left at the >ADC for a controlled amount of interfering or adjacent energy, or the >analog AGC controls the composite power level so that the ADC dynamic >range is always fully utilized regardless of the ratio of >desired/undesired input energy.Yes, that's an interesting question. Two completely different philosophies: - ! want to guarantee a minimum SNR. The radio standard guarantees maximum interferer levels, relative to the signal. Having substantially more SNR is effectively useless (other than marginally better BER) => Use RSSI for AGC control. or - More SNR translates into throughput, for example in LTE with adaptive modulation-and-coding. I want to generally maximize SNR over time while avoiding clipping => take the blocker level into account for AGC. Expect that the latter isn't mentioned in older textbooks (guessing, haven't checked any), "traditionally" you have a SNR target and that's it. _____________________________ Posted through www.DSPRelated.com
Reply by ●September 25, 20132013-09-25
On Wed, 25 Sep 2013 00:18:41 -0500, "mnentwig" <24789@dsprelated> wrote:>One additional comment: Any AGC in a high-performance real-world receiver >uses both analog and digital scaling. It is a pretty deep topic, highly >system specific. >For example, in OFDM you don't want to switch gain in the middle of a >symbol. Other systems may give you only a brief time window to adjust.It's also not unusual for a modern analog front end to have multiple analog AGC stages before the signal ever gets to the ADC.>>That depends a lot on the system requirements and how the system is >>architected. We've used both methods, where the analog AGC controls >>the amplitude of the desired signal and some headroom is left at the >>ADC for a controlled amount of interfering or adjacent energy, or the >>analog AGC controls the composite power level so that the ADC dynamic >>range is always fully utilized regardless of the ratio of >>desired/undesired input energy. > >Yes, that's an interesting question. Two completely different philosophies: > > >- ! want to guarantee a minimum SNR. The radio standard guarantees maximum >interferer levels, relative to the signal. Having substantially more SNR is >effectively useless (other than marginally better BER) => Use RSSI for AGC >control. > >or > >- More SNR translates into throughput, for example in LTE with adaptive >modulation-and-coding. I want to generally maximize SNR over time while >avoiding clipping => take the blocker level into account for AGC. >Expect that the latter isn't mentioned in older textbooks (guessing, >haven't checked any), "traditionally" you have a SNR target and that's it.There are additional considerations, too, like whether the ADC input bandwidth is much greater than the desired signal or not. That can happen if either a fair amount of tuning/channel selection is done digitally or if the system is bandwidth adaptive/adjustable and the desired signal can be much narrower than the IF filter or anti-alias filter. In those cases the ratio of adjacent energy to the desired signal can be quite high and vary quite a bit, which almost dictates the need for a substantial digital AGC system. System architecture issues like this can make or break a system that needs a lot of flexibility. Eric Jacobsen Anchor Hill Communications http://www.anchorhill.com
Reply by ●September 25, 20132013-09-25
Shouldn't talk too much about ADC+AGC, not endanger its status as "trivial engineering detail that is irrelevant for academic research" and accidentally wipe out the whole cognitive / software radio scene :-) _____________________________ Posted through www.DSPRelated.com
Reply by ●September 25, 20132013-09-25
>On Tue, 24 Sep 2013 12:56:01 -0500, "bradsdr" <95105@dsprelated> >wrote: > >>>On Thu, 19 Sep 2013 09:49:10 -0500, "bradsdr" <95105@dsprelated> >>>wrote: >>> >>>>>radams2000@gmail.com writes: >>>>> >>>>>> If you have large interfering signal components above your 2mhz >>signal >>>>>> range then it's best to remove them in the analog domain, otherwise >>>>>> any ADC nonlinearity could produce in-band spurs that cannot be >>>>>> removed by subsequent digital filtering. But if the out-of-band >>energy >>>>>> is just low-level noise then it probably won't make much difference >>>>>> either way. >>>>> >>>>>Bob, >>>>> >>>>>You bring up (indirectly) a point I failed to address in my response >>>>>yesterday. Even if the ADC had no such nonlinearities, you still have >>>>>the problem that, since the ADC's peak input is limited, such asignal>>>>>would require the signal-of-interest to be greatly attenuated, whichin>>>>>effect throws away a good bit of your ADC resolution. >>>>> >>>>>If you add the possible (and practical) nonlinearity problem in, then >>>>>between the two it can be a serious issue. >>>>>-- >>>>>Randy Yates >>>>>Digital Signal Labs >>>>>http://www.digitalsignallabs.com >>>>> >>>> >>>>Lets say my signal-of-interest is X dB, then what could be the >>>>power/strength of interfering signal that is tolerable i.e it doesn't >>>>affect my SNR? Sorry if I am asking wrong questions >>> >>>Mostly you want enough dynamic range so that the ADC output isn't >>>pushed into saturation or other distortion. So the number of bits >>>has to be enough to handle both the maximum expected signal plus the >>>maximum expected sum of interference energy. >>> >>>Since you indicate you have an AGC, this can often be used to bound >>>the number of bits needed for the desired signal. The number of bits >>>needed for interference can then be determined if the interference >>>energy is specified relative to the desired signal, e.g., +20dBc or >>>something like tha.t >>> >>Hi Eric thank you so much for your comment. What would you prefer >>controlling the gain of received signal by digital means or using analog >>AGC ? > >That depends a lot on the system requirements and how the system is >architected. We've used both methods, where the analog AGC controls >the amplitude of the desired signal and some headroom is left at the >ADC for a controlled amount of interfering or adjacent energy, or the >analog AGC controls the composite power level so that the ADC dynamic >range is always fully utilized regardless of the ratio of >desired/undesired input energy. > >The second method requires a digital AGC that controls the amplitude >of the desired signal somewhere in the digital processing chain. > >There are advantages and disadvantages both ways. Which way to go >best depends on a lot of different things. > >Eric, I think second method suits me more currently. Because I have relatively large IF bandwidth 15 MHz compared to bandwidth of my signal of interest (3MHz), so there could be interferer signals. Now as per your suggestion (2nd method), I can use an analog AGC that can control the composite power level (desired signal+interferer) before feeding the signal to ADC. Then in digital domain I can apply filter to select my 3MHz bandwidth and apply another digital AGC. Is that right ? What could be the possible drawbacks of this approach ? specifically on SNR ? I am using FSK demod. Bob>>>>ADC I am going to use is quite good ads62p44: >>>> >>>>SNR(dB) 73.8 >>>>SFDR(dB) 86 >>>>ENOB (Bits) 11.8 >>>>SINAD (dB) 73.4 >>>>Input Range 2Vpp >>>>Maximum Sample Rate: 125 MSPS >>>>14-Bit Resolution with No Missing Codes >>>>95 dB Crosstalk >>>>3.5 dB Coarse Gain and Programmable Fine Gain >>>>up to 6 dB for SNR/SFDR Trade-Off >>>>Amplitude Down to 400 mVPP >>>> >>>>Regards, >>>>Brad >>>> >>>> >>>>_____________________________ >>>>Posted through www.DSPRelated.com >>> >>>Eric Jacobsen >>>Anchor Hill Communications >>>http://www.anchorhill.com >>> >> >>_____________________________ >>Posted through www.DSPRelated.com > >Eric Jacobsen >Anchor Hill Communications >http://www.anchorhill.com >_____________________________ Posted through www.DSPRelated.com
