Hi, all I want to demodulate an IF real signal and it seems that there are some different methods on it. I can employ hilbert transform to construct the analytic signal and downconvert it to baseband. Otherwise, I can get the signal multiplied by a sine and a cosine which oscillate at IF carrier frequency and then lowpass-filter unwanted parts. These two methods now also deal with complex numbers. What is the difference between these two methods? I can't tell which is superior. Thanks for answering.

# Hilbert transform vs quadrature processing

Started by ●September 17, 2009

Reply by ●September 17, 20092009-09-17

alomar wrote:> Hi, all > > I want to demodulate an IF real signal and it seems that there are some > different methods on it. I can employ hilbert transform to construct the > analytic signal and downconvert it to baseband. Otherwise, I can get the > signal multiplied by a sine and a cosine which oscillate at IF carrier > frequency and then lowpass-filter unwanted parts. These two methods now > also deal with complex numbers. > > What is the difference between these two methods?As long as you can separate the mirror images, both methods are equvalent.> I can't tell which is superior.You have to use Hilbert transform if the image spectrum overlaps with the downconverted spectrum. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com

Reply by ●September 17, 20092009-09-17

alomar wrote:> Hi, all > > I want to demodulate an IF real signal and it seems that there are some > different methods on it. I can employ hilbert transform to construct the > analytic signal and downconvert it to baseband. Otherwise, I can get the > signal multiplied by a sine and a cosine which oscillate at IF carrier > frequency and then lowpass-filter unwanted parts. These two methods now > also deal with complex numbers. > > What is the difference between these two methods?As long as you can separate the mirror images, both methods are equvalent.> I can't tell which is superior.You have to use Hilbert transform if the image spectrum overlaps with the downconverted spectrum. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com

Reply by ●September 17, 20092009-09-17

On Sep 17, 2:52�am, "alomar" <alomar.w...@gmail.com> wrote:> Hi, all > > I want to demodulate an IF real signal and it seems that there are some > different methods on it. I can employ hilbert transform to construct the > analytic signal and downconvert it to baseband. Otherwise, I can get the > signal multiplied by a sine and a cosine which oscillate at IF carrier > frequency and then lowpass-filter unwanted parts. These two methods now > also deal with complex numbers. > > What is the difference between these two methods? I can't tell which is > superior. > > Thanks for answering.Maybe you will find this helpful: http://www.claysturner.com/dsp/ASG.pdf Clay

Reply by ●September 17, 20092009-09-17

>Hi, all > >I want to demodulate an IF real signal and it seems that there are some >different methods on it. I can employ hilbert transform to construct the >analytic signal and downconvert it to baseband. Otherwise, I can get the >signal multiplied by a sine and a cosine which oscillate at IF carrier >frequency and then lowpass-filter unwanted parts. These two methods now >also deal with complex numbers. > >What is the difference between these two methods? I can't tell which is >superior. > >Thanks for answering. > >If you multiply by a sine and a cosine which oscillate at IF carrier frequency you loss phase information. If you dont need thats information you can use that method. Otherwise, you need hilbert transform.

Reply by ●September 17, 20092009-09-17

>>Hi, all >> >>I want to demodulate an IF real signal and it seems that there are some >>different methods on it. I can employ hilbert transform to constructthe>>analytic signal and downconvert it to baseband. Otherwise, I can getthe>>signal multiplied by a sine and a cosine which oscillate at IF carrier >>frequency and then lowpass-filter unwanted parts. These two methods now >>also deal with complex numbers. >> >>What is the difference between these two methods? I can't tell which is >>superior. >> >>Thanks for answering. >> >> > >If you multiply by a sine and a cosine which oscillate at IF carrier >frequency you loss phase information. If you dont need thats information >you can use that method. Otherwise, you need hilbert transform. >thank you all, your advice really help me a lot

Reply by ●September 18, 20092009-09-18

On 9/17/2009 5:07 PM, JAlbertoDJ wrote:>> Hi, all >> >> I want to demodulate an IF real signal and it seems that there are some >> different methods on it. I can employ hilbert transform to construct the >> analytic signal and downconvert it to baseband. Otherwise, I can get the >> signal multiplied by a sine and a cosine which oscillate at IF carrier >> frequency and then lowpass-filter unwanted parts. These two methods now >> also deal with complex numbers. >> >> What is the difference between these two methods? I can't tell which is >> superior. >> >> Thanks for answering. >> >> > > If you multiply by a sine and a cosine which oscillate at IF carrier > frequency you loss phase information. If you dont need thats information > you can use that method. Otherwise, you need hilbert transform.How so? PSK modems, where the information is in the phase, use quadrature mixing for up and downconversion routinely. -- Eric Jacobsen Minister of Algorithms Abineau Communications http://www.abineau.com

Reply by ●September 18, 20092009-09-18

JAlbertoDJ wrote:>> Hi, all >> >> I want to demodulate an IF real signal and it seems that there are some >> different methods on it. I can employ hilbert transform to construct the >> analytic signal and downconvert it to baseband. Otherwise, I can get the >> signal multiplied by a sine and a cosine which oscillate at IF carrier >> frequency and then lowpass-filter unwanted parts. These two methods now >> also deal with complex numbers. >> >> What is the difference between these two methods? I can't tell which is >> superior....> If you multiply by a sine and a cosine which oscillate at IF carrier > frequency you loss phase information.Did Horus, the god of mathematics, tell you that? According to the math I know, phase is preserved. ... Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������

Reply by ●September 18, 20092009-09-18

>JAlbertoDJ wrote: >>> Hi, all >>> >>> I want to demodulate an IF real signal and it seems that there aresome>>> different methods on it. I can employ hilbert transform to constructthe>>> analytic signal and downconvert it to baseband. Otherwise, I can getthe>>> signal multiplied by a sine and a cosine which oscillate at IFcarrier>>> frequency and then lowpass-filter unwanted parts. These two methodsnow>>> also deal with complex numbers. >>> >>> What is the difference between these two methods? I can't tell whichis>>> superior. > > ... > >> If you multiply by a sine and a cosine which oscillate at IF carrier >> frequency you loss phase information. > >Did Horus, the god of mathematics, tell you that? According to the math >I know, phase is preserved. > > ... > >Jerry >-- >Engineering is the art of making what you want from things you can get. >����������������������������������������������������������������������� >I have wrote that because multiplying by a sine and a cosine i cannot do run an Automatic Frecuency Control for my modem using atan(Im/Re) in the FFT output. Otherwise, with Hilbert transform instead of multiplying, AFC run perfectly. why? i dont know, and i would like something explain to me. In the example of PSK demodulation you dont have any problem with phase. But....try to apply an AFC to your PSK demodulator and then tell me if converge.

Reply by ●September 19, 20092009-09-19

On 9/18/2009 4:18 PM, JAlbertoDJ wrote:>> JAlbertoDJ wrote: >>>> Hi, all >>>> >>>> I want to demodulate an IF real signal and it seems that there are > some >>>> different methods on it. I can employ hilbert transform to construct > the >>>> analytic signal and downconvert it to baseband. Otherwise, I can get > the >>>> signal multiplied by a sine and a cosine which oscillate at IF > carrier >>>> frequency and then lowpass-filter unwanted parts. These two methods > now >>>> also deal with complex numbers. >>>> >>>> What is the difference between these two methods? I can't tell which > is >>>> superior. >> ... >> >>> If you multiply by a sine and a cosine which oscillate at IF carrier >>> frequency you loss phase information. >> Did Horus, the god of mathematics, tell you that? According to the math >> I know, phase is preserved. >> >> ... >> >> Jerry >> -- >> Engineering is the art of making what you want from things you can get. >> ����������������������������������������������������������������������� >> > > I have wrote that because multiplying by a sine and a cosine i cannot do > run an Automatic Frecuency Control for my modem using atan(Im/Re) in the > FFT output. > > Otherwise, with Hilbert transform instead of multiplying, AFC run > perfectly. why? i dont know, and i would like something explain to me. > > In the example of PSK demodulation you dont have any problem with phase. > But....try to apply an AFC to your PSK demodulator and then tell me if > converge.Yes, the AFC will converge, as will a loop locked onto the carrier phase. -- Eric Jacobsen Minister of Algorithms Abineau Communications http://www.abineau.com