Hi, I have a problem. If anyone may help , I would be appreciated. There are signals at 70 MHz IF output of a DDC. and there are equispace signals which should be demodulated. I am trying to find the best sampling frequency for this purpose. I know about the bandpass sampling theorem which says: Fs >= 2*B (1) (2Fc-B)/m >= fs >= (2*Fc + B)/(m+1) (2) Where Fs = Sampling frequency Fc = Center Frequency B = Bandwidth M any integer which satisfies the condition (1) But that is for one signal which is at Fc with BW = B But what about many signals around the Signal of Interest ? If you could not find the correct Fs, There may occur aliasing between the other signals as well.

# Bandpass signal sampling

Started by ●February 10, 2008

Reply by ●February 10, 20082008-02-10

"Dedicated" <dedicated_glory@hotmail.com> wrote in message news:xI-dnYOHL5K8EzLanZ2dnUVZ_rOqnZ2d@giganews.com...> Hi, > > I have a problem. > If anyone may help , I would be appreciated. > > There are signals at 70 MHz IF output of a DDC. > and there are equispace signals which should be demodulated. > I am trying to find the best sampling frequency for this purpose. > > I know about the bandpass sampling theorem which says: > > Fs >= 2*B (1) > > (2Fc-B)/m >= fs >= (2*Fc + B)/(m+1) (2) > > Where Fs = Sampling frequency > Fc = Center Frequency > B = Bandwidth > M any integer which satisfies the condition (1) > > > But that is for one signal which is at Fc with BW = B > > But what about many signals around the Signal of Interest ? > > If you could not find the correct Fs, There may occur aliasing between the > other signals as well.Just change the bandwidth to the *total* bandwidth of all signals of interest. Fred

Reply by ●February 11, 20082008-02-11

On 11 Feb, 03:40, "Fred Marshall" <fmarshallx@remove_the_x.acm.org> wrote:> "Dedicated" <dedicated_gl...@hotmail.com> wrote in message > > news:xI-dnYOHL5K8EzLanZ2dnUVZ_rOqnZ2d@giganews.com... > > > > > > > Hi, > > > I have a problem. > > If anyone may help , I would be appreciated. > > > There are signals at 70 MHz IF output of a DDC. > > and there are equispace signals which should be demodulated. > > I am trying to find the best sampling frequency for this purpose. > > > I know about the bandpass sampling theorem which says: > > > Fs >= 2*B (1) > > > (2Fc-B)/m >= fs >= (2*Fc + B)/(m+1) (2) > > > Where Fs = Sampling frequency > > Fc = Center Frequency > > B = Bandwidth > > M any integer which satisfies the condition (1) > > > But that is for one signal which is at Fc with BW = B > > > But what about many signals around the Signal of Interest ? > > > If you could not find the correct Fs, There may occur aliasing between the > > other signals as well. > > Just change the bandwidth to the *total* bandwidth of all signals of > interest....or demodulate before sampling. Rune

Reply by ●February 11, 20082008-02-11

Dedicated wrote:> Hi, > > I have a problem. > If anyone may help , I would be appreciated. > > There are signals at 70 MHz IF output of a DDC. > and there are equispace signals which should be demodulated. > I am trying to find the best sampling frequency for this purpose. > > I know about the bandpass sampling theorem which says: > > Fs >= 2*B (1) > > (2Fc-B)/m >= fs >= (2*Fc + B)/(m+1) (2) > > Where Fs = Sampling frequency > Fc = Center Frequency > B = Bandwidth > M any integer which satisfies the condition (1) > > > But that is for one signal which is at Fc with BW = B > > But what about many signals around the Signal of Interest ? > > If you could not find the correct Fs, There may occur aliasing between the > other signals as well.I'm trying to sort out what you mean, and I'm not getting it. I _can_ tell that you are trying to go beyond a rule without understanding the reasoning behind it. When you find yourself doing this, the best thing to do is to study the rule, understand what goes into it, then take the pieces and make your own rule. This article may help, in a tangential way: http://www.wescottdesign.com/articles/Sampling/sampling.html. It seems that you want to sample a signal and interference without filtering out the interference before sampling, and you want to know how fast to sample? If that's the case then take Fred's advise -- or do some filtering before you hit your sampler. When I hit a problem like this I make sure to make a little diagram, like the ones you see in textbooks, that show the spectrum of the signal before sampling, all the folding frequencies due to sampling, then a separate sketch of the signal (fortuitously sampled to within my sampling rate, I hope) and all of it's aliases (which don't interfere with the part of the signal that I want, I hope). This aids in my understanding, so I have confidence I'm not blindly (and possibly incorrectly) applying a rule that I don't understand. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" gives you just what it says. See details at http://www.wescottdesign.com/actfes/actfes.html

Reply by ●February 11, 20082008-02-11

"Tim Wescott" <tim@seemywebsite.com> wrote in message news:nuydnfRdPIqYEy3anZ2dnUVZ_qOknZ2d@web-ster.com...> Dedicated wrote: >> Hi, >> >> I have a problem. >> If anyone may help , I would be appreciated. >> >> There are signals at 70 MHz IF output of a DDC. >> and there are equispace signals which should be demodulated. >> I am trying to find the best sampling frequency for this purpose. >> >> I know about the bandpass sampling theorem which says: >> >> Fs >= 2*B (1) >> >> (2Fc-B)/m >= fs >= (2*Fc + B)/(m+1) (2) >> >> Where Fs = Sampling frequency >> Fc = Center Frequency B = Bandwidth >> M any integer which satisfies the condition (1) >> >> >> But that is for one signal which is at Fc with BW = B >> >> But what about many signals around the Signal of Interest ? >> >> If you could not find the correct Fs, There may occur aliasing between >> the >> other signals as well. > > I'm trying to sort out what you mean, and I'm not getting it. > > I _can_ tell that you are trying to go beyond a rule without understanding > the reasoning behind it. When you find yourself doing this, the best > thing to do is to study the rule, understand what goes into it, then take > the pieces and make your own rule. This article may help, in a tangential > way: http://www.wescottdesign.com/articles/Sampling/sampling.html. > > It seems that you want to sample a signal and interference without > filtering out the interference before sampling, and you want to know how > fast to sample? If that's the case then take Fred's advise -- or do some > filtering before you hit your sampler. > > When I hit a problem like this I make sure to make a little diagram, like > the ones you see in textbooks, that show the spectrum of the signal before > sampling, all the folding frequencies due to sampling, then a separate > sketch of the signal (fortuitously sampled to within my sampling rate, I > hope) and all of it's aliases (which don't interfere with the part of the > signal that I want, I hope). This aids in my understanding, so I have > confidence I'm not blindly (and possibly incorrectly) applying a rule that > I don't understand. > > -- > > Tim Wescott > Wescott Design Services > http://www.wescottdesign.com >I sure agree with the diagrams ... I call 'em "cartoons". Fred