Forums

Need of Eqaulization ?

Started by mite_learner September 27, 2013
On Wednesday, October 2, 2013 5:35:05 PM UTC+13, Eric Jacobsen wrote:
> On Tue, 1 Oct 2013 17:47:06 -0700 (PDT), gyansorova@gmail.com wrote: > > > > >On Tuesday, October 1, 2013 6:10:32 PM UTC+13, mite_learner wrote: > > >> >On Mon, 30 Sep 2013 07:21:06 -0500, "mite_learner" <94814@dsprelated> > > >>=20 > > >> >wrote: > > >>=20 > > >> > > > >>=20 > > >> >>>>"mite_learner" <94814@dsprelated> writes: > > >>=20 > > >> >>>> > > >>=20 > > >> >>>>> Dear Members, > > >>=20 > > >> >>>>> > > >>=20 > > >> >>>>> I am experimenting on digital 4-FSK (orthogonal) communication > > >>=20 > > >> system.=20 > > >>=20 > > >> >> > > >>=20 > > >> >>>On > > >>=20 > > >> >>>>> the demodulation side, I am using AGC and then 4 filters to detect > > >>=20 > > >> >>>tones > > >>=20 > > >> >>>>> and then using symbol timing. My question is how can I know that I > > >>=20 > > >> >>>need > > >>=20 > > >> >>>>> equalization looking at the signal if I transmit and receive the > > >>=20 > > >> >>signal > > >>=20 > > >> >>>>> over the air? How can I verify that my signal is being affected by > > >>=20 > > >> >>>>> multipaths ? > > >>=20 > > >> >>>> > > >>=20 > > >> >>>>Did you mean AGC in front of your ADC? > > >>=20 > > >> >>>> > > >>=20 > > >> >>>>By the way, ignore Vlad. He's made a career out of calling people > > >>=20 > > >> idiots > > >>=20 > > >> >>>>- doesn't mean they are. > > >>=20 > > >> >>>>--=20 > > >>=20 > > >> >>> > > >>=20 > > >> >>>Currently I don't have AGC in front of ADC, but I am keeping my signal > > >>=20 > > >> >>low > > >>=20 > > >> >>>so that ADC doesn't start clipping. I applied AGC on my baseband > > >>=20 > > >> signal, > > >>=20 > > >> >>>after digitization, before doing matched filtering for individual > > >>=20 > > >> tones. > > >>=20 > > >> >>I > > >>=20 > > >> >>>am using this AGC to keep signal level in a range for symbol timing > > >>=20 > > >> >>>synchronization.=20 > > >>=20 > > >> >>> > > >>=20 > > >> >>>But Vlad says AGC is harmful in this setup, why? I would have asked hi= > > >m > > >>=20 > > >> >>but > > >>=20 > > >> >>>searching on the group, I found that he never gave an answer that coul= > > >d > > >>=20 > > >> >>be > > >>=20 > > >> >>>*useful* for anybody in the world .=20 > > >>=20 > > >> >>> > > >>=20 > > >> >>>-- > > >>=20 > > >> >> > > >>=20 > > >> >>Randy or any body, please comment on this. > > >>=20 > > >> >> > > >>=20 > > >> >>Mite > > >>=20 > > >> > > > >>=20 > > >> >I think most people are unwilling to speculate why Vladimir thinks a > > >>=20 > > >> >lot of things that he thinks. > > >>=20 > > >> > > > >>=20 > > >> True that. > > >>=20 > > >> >AGC can be a complicated topic, especially for FM/FSK. AGC can be > > >>=20 > > >> >useful, especially to manage composite power, etc. Much depends on > > >>=20 > > >> >the system architecture and the requirements. > > >>=20 > > >> > > > >>=20 > > >> > > > >>=20 > > >> That means I need to study more :) Thanks! > > >>=20 > > >>=20 > > >>=20 > > >> Mite > > >>=20 > > >>=20 > > >>=20 > > >> >>>Mite > > >>=20 > > >> >>> > > >>=20 > > >> >>>>Randy Yates > > >>=20 > > >> >>>>Digital Signal Labs > > >>=20 > > >> >>>>http://www.digitalsignallabs.com > > >>=20 > > >> >>>> =20 > > >>=20 > > >> >>> > > >>=20 > > >> >>>_____________________________ =09 > > >>=20 > > >> >>>Posted through www.DSPRelated.com > > >>=20 > > >> >>> =20 > > >>=20 > > >> >> > > >>=20 > > >> >>_____________________________ =09 > > >>=20 > > >> >>Posted through www.DSPRelated.com > > >>=20 > > >> > > > >>=20 > > >> >Eric Jacobsen > > >>=20 > > >> >Anchor Hill Communications > > >>=20 > > >> >http://www.anchorhill.com > > >>=20 > > >> > =20 > > >>=20 > > >>=20 > > >>=20 > > >> _____________________________ =09 > > >>=20 > > >> Posted through www.DSPRelated.com > > > > > >AGC is not a complicated topic at all. It is pretty easy. All you need is a= > > > way of finding the size of your signal - say by rectification or squaring = > > >or a non-linear device. Then you filter (or integrate) and feed back to a v= > > >oltage-controlled amplifier (which can also be a multiplier). What's diffic= > > >ult in that? It requires a large amount of gain (maybe 120dB split in three= > > >) so layout is tricky. > > > > When a simple architecture like that works, it's not hard, and many > > systems have been implemented like that. > > > > But many systems aren't that simple, like if you're trying to > > demodulate multiple, independently faded signals through the same AFE, > > have a wide IF filter that can potentially let in a lot of adjacent > > energy that comes and goes, or want to use something dirt simple like > > a limiter to detect your signal but not get swamped by interferers or > > adjacents. > > > > It's all simple until you sort out something cool that gives you a big > > edge on your competition. ;) > > > > > > Eric Jacobsen > > Anchor Hill Communications > > http://www.anchorhill.com
The last thing you want is a limiter. A limiter just amplifies noise at the zero crossings. OK at very high SNRs when you don't need it but an illusion at low SNRs.
On Tue, 1 Oct 2013 22:07:33 -0700 (PDT), gyansorova@gmail.com wrote:

>On Wednesday, October 2, 2013 5:35:05 PM UTC+13, Eric Jacobsen wrote: >> On Tue, 1 Oct 2013 17:47:06 -0700 (PDT), gyansorova@gmail.com wrote: >> >> >> >> >On Tuesday, October 1, 2013 6:10:32 PM UTC+13, mite_learner wrote: >> >> >> >On Mon, 30 Sep 2013 07:21:06 -0500, "mite_learner" <94814@dsprelated> >> >> >>=20 >> >> >> >wrote: >> >> >>=20 >> >> >> > >> >> >>=20 >> >> >> >>>>"mite_learner" <94814@dsprelated> writes: >> >> >>=20 >> >> >> >>>> >> >> >>=20 >> >> >> >>>>> Dear Members, >> >> >>=20 >> >> >> >>>>> >> >> >>=20 >> >> >> >>>>> I am experimenting on digital 4-FSK (orthogonal) communication >> >> >>=20 >> >> >> system.=20 >> >> >>=20 >> >> >> >> >> >> >>=20 >> >> >> >>>On >> >> >>=20 >> >> >> >>>>> the demodulation side, I am using AGC and then 4 filters to detect >> >> >>=20 >> >> >> >>>tones >> >> >>=20 >> >> >> >>>>> and then using symbol timing. My question is how can I know that I >> >> >>=20 >> >> >> >>>need >> >> >>=20 >> >> >> >>>>> equalization looking at the signal if I transmit and receive the >> >> >>=20 >> >> >> >>signal >> >> >>=20 >> >> >> >>>>> over the air? How can I verify that my signal is being affected by >> >> >>=20 >> >> >> >>>>> multipaths ? >> >> >>=20 >> >> >> >>>> >> >> >>=20 >> >> >> >>>>Did you mean AGC in front of your ADC? >> >> >>=20 >> >> >> >>>> >> >> >>=20 >> >> >> >>>>By the way, ignore Vlad. He's made a career out of calling people >> >> >>=20 >> >> >> idiots >> >> >>=20 >> >> >> >>>>- doesn't mean they are. >> >> >>=20 >> >> >> >>>>--=20 >> >> >>=20 >> >> >> >>> >> >> >>=20 >> >> >> >>>Currently I don't have AGC in front of ADC, but I am keeping my signal >> >> >>=20 >> >> >> >>low >> >> >>=20 >> >> >> >>>so that ADC doesn't start clipping. I applied AGC on my baseband >> >> >>=20 >> >> >> signal, >> >> >>=20 >> >> >> >>>after digitization, before doing matched filtering for individual >> >> >>=20 >> >> >> tones. >> >> >>=20 >> >> >> >>I >> >> >>=20 >> >> >> >>>am using this AGC to keep signal level in a range for symbol timing >> >> >>=20 >> >> >> >>>synchronization.=20 >> >> >>=20 >> >> >> >>> >> >> >>=20 >> >> >> >>>But Vlad says AGC is harmful in this setup, why? I would have asked hi= >> >> >m >> >> >>=20 >> >> >> >>but >> >> >>=20 >> >> >> >>>searching on the group, I found that he never gave an answer that coul= >> >> >d >> >> >>=20 >> >> >> >>be >> >> >>=20 >> >> >> >>>*useful* for anybody in the world .=20 >> >> >>=20 >> >> >> >>> >> >> >>=20 >> >> >> >>>-- >> >> >>=20 >> >> >> >> >> >> >>=20 >> >> >> >>Randy or any body, please comment on this. >> >> >>=20 >> >> >> >> >> >> >>=20 >> >> >> >>Mite >> >> >>=20 >> >> >> > >> >> >>=20 >> >> >> >I think most people are unwilling to speculate why Vladimir thinks a >> >> >>=20 >> >> >> >lot of things that he thinks. >> >> >>=20 >> >> >> > >> >> >>=20 >> >> >> True that. >> >> >>=20 >> >> >> >AGC can be a complicated topic, especially for FM/FSK. AGC can be >> >> >>=20 >> >> >> >useful, especially to manage composite power, etc. Much depends on >> >> >>=20 >> >> >> >the system architecture and the requirements. >> >> >>=20 >> >> >> > >> >> >>=20 >> >> >> > >> >> >>=20 >> >> >> That means I need to study more :) Thanks! >> >> >>=20 >> >> >>=20 >> >> >>=20 >> >> >> Mite >> >> >>=20 >> >> >>=20 >> >> >>=20 >> >> >> >>>Mite >> >> >>=20 >> >> >> >>> >> >> >>=20 >> >> >> >>>>Randy Yates >> >> >>=20 >> >> >> >>>>Digital Signal Labs >> >> >>=20 >> >> >> >>>>http://www.digitalsignallabs.com >> >> >>=20 >> >> >> >>>> =20 >> >> >>=20 >> >> >> >>> >> >> >>=20 >> >> >> >>>_____________________________ =09 >> >> >>=20 >> >> >> >>>Posted through www.DSPRelated.com >> >> >>=20 >> >> >> >>> =20 >> >> >>=20 >> >> >> >> >> >> >>=20 >> >> >> >>_____________________________ =09 >> >> >>=20 >> >> >> >>Posted through www.DSPRelated.com >> >> >>=20 >> >> >> > >> >> >>=20 >> >> >> >Eric Jacobsen >> >> >>=20 >> >> >> >Anchor Hill Communications >> >> >>=20 >> >> >> >http://www.anchorhill.com >> >> >>=20 >> >> >> > =20 >> >> >>=20 >> >> >>=20 >> >> >>=20 >> >> >> _____________________________ =09 >> >> >>=20 >> >> >> Posted through www.DSPRelated.com >> >> > >> >> >AGC is not a complicated topic at all. It is pretty easy. All you need is a= >> >> > way of finding the size of your signal - say by rectification or squaring = >> >> >or a non-linear device. Then you filter (or integrate) and feed back to a v= >> >> >oltage-controlled amplifier (which can also be a multiplier). What's diffic= >> >> >ult in that? It requires a large amount of gain (maybe 120dB split in three= >> >> >) so layout is tricky. >> >> >> >> When a simple architecture like that works, it's not hard, and many >> >> systems have been implemented like that. >> >> >> >> But many systems aren't that simple, like if you're trying to >> >> demodulate multiple, independently faded signals through the same AFE, >> >> have a wide IF filter that can potentially let in a lot of adjacent >> >> energy that comes and goes, or want to use something dirt simple like >> >> a limiter to detect your signal but not get swamped by interferers or >> >> adjacents. >> >> >> >> It's all simple until you sort out something cool that gives you a big >> >> edge on your competition. ;) >> >> >> >> >> >> Eric Jacobsen >> >> Anchor Hill Communications >> >> http://www.anchorhill.com > >The last thing you want is a limiter. A limiter just amplifies noise at the zero crossings. OK at very high SNRs when you don't need it but an illusion at low SNRs.
Yes, there are times when a limiter isn't what you want. That's pretty obvious. I was addressing some example cases of where you may not always want to use the simplest AGC architecture, including some potentially relevant to the OPs situation. Eric Jacobsen Anchor Hill Communications http://www.anchorhill.com