hi! i make an envelope detector for AM demodulation using Hilbert Transform and complex envelope... but i don't really understand about the advantages of using Hilbert Transform and complex envelope... what is the advantages of using Hilbert Transform and complex envelope? why using Hilbert Transform method is more effective than square-law? and i think i can make an envelope detector just without hilbert transform. i can use the I-phase, then multiply with exp(-jwt), then LPF. and we can multiply the output by 2, because the output will give me only half amplitude... please give me some advice... thanks...
Advantages of Envelope detector using Hilbert Transform
Started by ●February 11, 2008
Reply by ●February 11, 20082008-02-11
On Feb 11, 9:06 pm, "c1910" <c_19...@hotmail.com> wrote:> > i make an envelope detector for AM demodulation using Hilbert Transform > and complex envelope... > > but i don't really understand about the advantages of using Hilbert > Transform and complex envelope...i also don't see any advantage, for the purpose of regular AM (no suppressed carrier) detection for computing the complex envelope. square-law might not be what you want, unless you like applying the square-root after filtering out the carrier (actually a frequency that is twice the carrier frequeny). some non-linearity that leave an amplitude proportional to (1+mu*x(t)) where x(t) is your message after unbiasing is what you want. maybe abs(), maybe 1/2 wave rectifier, i dunno. i think that sometimes when the signal is more complex than a modulated sinusoidal carrier (what AM is), such as a broadbanded signal, that the complex envelope might be desired because it gives you an envelope without creating new frequency components. it *does* eliminate the negative frequency components but it does not introduce new frequencies like the square-law does. i dunno when that is advantageous, but i know of audio signal practioners (these guys walk around and fly around to places with expensive analytical test gear and come up with all sorts of numbers describing a room) think that the complex envelope (or the magnitude of the complex "analytic signal") is a super salient measure of stuff. dunno why, and maybe i should, since i'm sorta into audio, but i dunno. r b-j
Reply by ●February 11, 20082008-02-11
c1910 wrote:> hi! > i make an envelope detector for AM demodulation using Hilbert Transform > and complex envelope... > > but i don't really understand about the advantages of using Hilbert > Transform and complex envelope... > > what is the advantages of using Hilbert Transform and complex envelope? > > why using Hilbert Transform method is more effective than square-law? > > and i think i can make an envelope detector just without hilbert > transform. > i can use the I-phase, then multiply with exp(-jwt), then LPF. and we can > multiply the output by 2, because the output will give me only half > amplitude... > > please give me some advice... > > thanks...Square-law detectors suffer from distortion (with the rare exception of single-sideband with carrier. They have no place in digital designs that I know of. Peak detectors work with continuous signals, but there is no reason to think that most samples will be near the carrier peak in a sampled system unless the oversampling ratio is quite high relative to the carrier or IF frequency. I-Q demodulation allows you to get the magnitude at much lower sample rates. If you didn't know that, what led you to that method? Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●February 11, 20082008-02-11
On Feb 11, 10:34 pm, Jerry Avins <j...@ieee.org> wrote:> > Peak detectors work with continuous signals, but there is no > reason to think that most samples will be near the carrier peak in a > sampled system unless the oversampling ratio is quite high relative to > the carrier or IF frequency. I-Q demodulation allows you to get the > magnitude at much lower sample rates.yer right, Jerry. that's a good reason and one i didn't think about. r b-j
Reply by ●February 12, 20082008-02-12
>c1910 wrote: >> hi! >> i make an envelope detector for AM demodulation using HilbertTransform>> and complex envelope... >> >> but i don't really understand about the advantages of using Hilbert >> Transform and complex envelope... >> >> what is the advantages of using Hilbert Transform and complexenvelope?>> >> why using Hilbert Transform method is more effective than square-law? >> >> and i think i can make an envelope detector just without hilbert >> transform. >> i can use the I-phase, then multiply with exp(-jwt), then LPF. and wecan>> multiply the output by 2, because the output will give me only half >> amplitude... >> >> please give me some advice... >> >> thanks... > >Square-law detectors suffer from distortion (with the rare exception of >single-sideband with carrier. They have no place in digital designs that>I know of. Peak detectors work with continuous signals, but there is no >reason to think that most samples will be near the carrier peak in a >sampled system unless the oversampling ratio is quite high relative to >the carrier or IF frequency. I-Q demodulation allows you to get the >magnitude at much lower sample rates. > >If you didn't know that, what led you to that method? > >Jerry >-- >Engineering is the art of making what you want from things you can get. >����������������������������������������������������������������������� >i use that method because i think there is a lot of distorsion in square law...coz, square law still have the Carrier's Amplitude...
Reply by ●February 12, 20082008-02-12
On 12 Feb, 03:06, "c1910" <c_19...@hotmail.com> wrote:> what is the advantages of using Hilbert Transform and complex envelope? > why using Hilbert Transform method is more effective than square-law?I don't know much about envelope detectors in AM systems, but generally speaking, when you have more than one way of implementing the same operation, there are a few trade- offs involved: - One method may require fewer components or FLOPS - One method may be more accurate - One method may be faster/simpler to design - One method may be faster/simpler to implement - One method may be better at suppressing artifacts - One method may provide more, more useful internal results and so on. Depending on the constraints on what you try to do, one method may score better on this sort of overview and would thus be the preferred one. It seems to me that processing methods often falls into two groups: Those that are cheap to implement or runs very fast (but which may give inaccurate results), and those that give accurate results but which are expensive to implement and/or cost more time to run. If cost and speed is all that matters, then the 'simpler' (or 'naive' or 'obvious') methods often come out on top in the evaluation. If you need high accuracy or error control, or other processing steps require information that can be squeezed out of the data by more elaborate algorithms, that would be deciding factors what methods to choose. So no methods can be said to be 'best' for any particular purpose. One needs to know one's own priorities (speed/cost or accuracy). Then one needs to know what can be obtained by each method -- and what the cost is. Rune
Reply by ●February 12, 20082008-02-12
c1910 wrote:>> c1910 wrote: >>> hi! >>> i make an envelope detector for AM demodulation using Hilbert > Transform >>> and complex envelope... >>> >>> but i don't really understand about the advantages of using Hilbert >>> Transform and complex envelope... >>> >>> what is the advantages of using Hilbert Transform and complex > envelope? >>> why using Hilbert Transform method is more effective than square-law? >>> >>> and i think i can make an envelope detector just without hilbert >>> transform. >>> i can use the I-phase, then multiply with exp(-jwt), then LPF. and we > can >>> multiply the output by 2, because the output will give me only half >>> amplitude... >>> >>> please give me some advice... >>> >>> thanks... >> Square-law detectors suffer from distortion (with the rare exception of >> single-sideband with carrier. They have no place in digital designs that > >> I know of. Peak detectors work with continuous signals, but there is no >> reason to think that most samples will be near the carrier peak in a >> sampled system unless the oversampling ratio is quite high relative to >> the carrier or IF frequency. I-Q demodulation allows you to get the >> magnitude at much lower sample rates. >> >> If you didn't know that, what led you to that method? >>> i use that method because i think there is a lot of distorsion in square > law...coz, square law still have the Carrier's Amplitude...I don't understand the part the reason for the distortion. Squaring a signal inevitably distorts it. Forget square-law detectors for recovering ordinary AM. Most "simple" AM demodulators are peak detectors. A digital peak detector is not only hard to design, hard even to define what it is. You can approximately extract the envelope by ensuring that the RF or IF signal is zero mean, then taking its absolute value -- not squaring -- and low-pass filtering. That will work fairly well most of the time, but on occasion it can fail horribly. Those failures will be brief enough to go unnoticed, except when you're demonstrating your system. Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by ●February 12, 20082008-02-12
On Feb 11, 9:06�pm, "c1910" <c_19...@hotmail.com> wrote:> hi! > i make an envelope detector for AM demodulation using Hilbert Transform > and complex envelope... > > but i don't really understand about the advantages of using Hilbert > Transform and complex envelope... > > what is the advantages of using Hilbert Transform and complex envelope? > > why using Hilbert Transform method is more effective than square-law? > > and i think i can make an envelope detector just without hilbert > transform. > i can use the I-phase, then multiply with exp(-jwt), then LPF. and we can > multiply the output by 2, because the output will give me only half > amplitude... > > please give me some advice... > > thanks...Basically using a hilbert Xform is a way to find the instantaneous amplitude. As Jerry et al stated this is not needed in standard AM radios where the IF is very high compared to the modulation frequency. In that case a simple diode with lowpass filter does a great job at demodulating (finding the envelope). But as often the case with software radios, the signal is mixed to a very low frequency (to reduced the sampling rate and its attendent overhead). In this case the observed peaks aren't very likely to occur at or near the actuall peaks, so the envelope is not represented very well. So the Hilbert method is a great analytical way to get there. In practice though, all you really need is a pair of filters (flat or nearly flat magnitude in the band of interest) that differ by 90 degrees. Call the outpouts A(t) and B(t) and your amplitude is simply sqrt( A(t)^2 + B(t)^2 ). IHTH, Clay
Reply by ●February 12, 20082008-02-12
clay@claysturner.com wrote:> On Feb 11, 9:06 pm, "c1910" <c_19...@hotmail.com> wrote: >> hi! >> i make an envelope detector for AM demodulation using Hilbert Transform >> and complex envelope... >> >> but i don't really understand about the advantages of using Hilbert >> Transform and complex envelope... >> >> what is the advantages of using Hilbert Transform and complex envelope? >> >> why using Hilbert Transform method is more effective than square-law? >> >> and i think i can make an envelope detector just without hilbert >> transform. >> i can use the I-phase, then multiply with exp(-jwt), then LPF. and we can >> multiply the output by 2, because the output will give me only half >> amplitude... >> >> please give me some advice... >> >> thanks... > > > Basically using a hilbert Xform is a way to find the instantaneous > amplitude. As Jerry et al stated this is not needed in standard AM > radios where the IF is very high compared to the modulation frequency. > In that case a simple diode with lowpass filter does a great job at > demodulating (finding the envelope). > > But as often the case with software radios, the signal is mixed to a > very low frequency (to reduced the sampling rate and its attendent > overhead). In this case the observed peaks aren't very likely to occur > at or near the actuall peaks, so the envelope is not represented very > well. So the Hilbert method is a great analytical way to get there. In > practice though, all you really need is a pair of filters (flat or > nearly flat magnitude in the band of interest) that differ by 90 > degrees. Call the outpouts A(t) and B(t) and your amplitude is simply > sqrt( A(t)^2 + B(t)^2 ).Or just A(t)^2 + B(t)^2 if you're dead set on a square-law detector. :-) Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●February 13, 20082008-02-13
"Jerry Avins" <jya@ieee.org> wrote in message news:GLGdnTjnkfQWVyzanZ2dnUVZ_g2dnZ2d@rcn.net...> c1910 wrote: >>> c1910 wrote: >>>> hi! >>>> i make an envelope detector for AM demodulation using Hilbert >> Transform >>>> and complex envelope... >>>> >>>> but i don't really understand about the advantages of using Hilbert >>>> Transform and complex envelope... >>>> >>>> what is the advantages of using Hilbert Transform and complex >> envelope? >>>> why using Hilbert Transform method is more effective than square-law? >>>> >>>> and i think i can make an envelope detector just without hilbert >>>> transform. >>>> i can use the I-phase, then multiply with exp(-jwt), then LPF. and we >> can >>>> multiply the output by 2, because the output will give me only half >>>> amplitude... >>>> >>>> please give me some advice... >>>> >>>> thanks... >>> Square-law detectors suffer from distortion (with the rare exception of >>> single-sideband with carrier. They have no place in digital designs that >> >>> I know of. Peak detectors work with continuous signals, but there is no >>> reason to think that most samples will be near the carrier peak in a >>> sampled system unless the oversampling ratio is quite high relative to >>> the carrier or IF frequency. I-Q demodulation allows you to get the >>> magnitude at much lower sample rates. >>> >>> If you didn't know that, what led you to that method? >>> > > >> i use that method because i think there is a lot of distorsion in square >> law...coz, square law still have the Carrier's Amplitude... > > I don't understand the part the reason for the distortion. Squaring a > signal inevitably distorts it. Forget square-law detectors for recovering > ordinary AM. Most "simple" AM demodulators are peak detectors. A digital > peak detector is not only hard to design, hard even to define what it is. > You can approximately extract the envelope by ensuring that the RF or IF > signal is zero mean, then taking its absolute value -- not squaring -- and > low-pass filtering. That will work fairly well most of the time, but on > occasion it can fail horribly. Those failures will be brief enough to go > unnoticed, except when you're demonstrating your system. > > Jerry > -- > Engineering is the art of making what you want from things you can get. > ���������������������������������������������������������������������� Perhaps I'm missing something. I *think* this is a "digital peak detector" that behaves like a diode feeding an RC network y(0) = 0; y(n+1) = x(n) > y(n) ? x(n) : A * y(n); A should be a bit smapper than 1... Best wishes, --Phil Martel
