Hello all, I am involved in the development of an underwater modem. Right now I am dealing with the simulation stage with simulink. The modulation scheme I am using is a non-coherent FSK. The carrier frequencies are 20kHz and 22kHz. At the demodulator part, I have designed a matched filters scheme, with both bandpass FIR filters at 20 and 22kHz respectively. The question is, which should be a typical number of coefficients for those filters? I am not sure whether I make the filters too selective... Otherwise, for non coherent detection, which scheme should be more robust, matched filters or a quadrature receiver? Thanks in advance
FSK demodulation
Started by ●January 8, 2009
Reply by ●January 8, 20092009-01-08
On Jan 8, 6:21�am, "Tomeu" <to...@marexi.com> wrote:> Hello all, > > I am involved in the development of an underwater modem. Right now I am > dealing with the simulation stage with simulink. > > The modulation scheme I am using is a non-coherent FSK. The carrier > frequencies are 20kHz and 22kHz. > > At the demodulator part, I have designed a matched filters scheme, with > both bandpass FIR filters at 20 and 22kHz respectively. The question is, > which should be a typical number of coefficients for those filters? I am > not sure whether I make the filters too selective... > > Otherwise, for non coherent detection, which scheme should be more robust, > matched filters or a quadrature receiver? > > Thanks in advanceWhether you do the matched filter at 20 and 22 or -1 and 1 kHz, the BER vs Eb/No curve is the same. John
Reply by ●January 8, 20092009-01-08
On Thu, 08 Jan 2009 05:21:14 -0600, Tomeu wrote:> Hello all, > > I am involved in the development of an underwater modem. Right now I am > dealing with the simulation stage with simulink. > > The modulation scheme I am using is a non-coherent FSK. The carrier > frequencies are 20kHz and 22kHz. > > At the demodulator part, I have designed a matched filters scheme, with > both bandpass FIR filters at 20 and 22kHz respectively. The question is, > which should be a typical number of coefficients for those filters? I am > not sure whether I make the filters too selective... > > Otherwise, for non coherent detection, which scheme should be more > robust, matched filters or a quadrature receiver? > > Thanks in advanceFor any given transmitted signal and receiver sampling rate, there can be one and only one matched filter. So either you're not clear on the concept of matched filters or you're making some almost-matched filters in an attempt to make the system more robust to the path filtering the sound. As pointed out elsewhere, a quadrature receiver is just another way to implement the same old thing. It may have advantages in system cost, but that's about it. -- Tim Wescott Control systems and communications consulting http://www.wescottdesign.com Need to learn how to apply control theory in your embedded system? "Applied Control Theory for Embedded Systems" by Tim Wescott Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
Reply by ●January 8, 20092009-01-08
On Jan 9, 12:21�am, "Tomeu" <to...@marexi.com> wrote:> Hello all, > > I am involved in the development of an underwater modem. Right now I am > dealing with the simulation stage with simulink. > > The modulation scheme I am using is a non-coherent FSK. The carrier > frequencies are 20kHz and 22kHz. > > At the demodulator part, I have designed a matched filters scheme, with > both bandpass FIR filters at 20 and 22kHz respectively. The question is, > which should be a typical number of coefficients for those filters? I am > not sure whether I make the filters too selective... > > Otherwise, for non coherent detection, which scheme should be more robust, > matched filters or a quadrature receiver? > > Thanks in advanceWhat's wrong with a PLL?
Reply by ●January 8, 20092009-01-08
HardySpicer wrote:> On Jan 9, 12:21 am, "Tomeu" <to...@marexi.com> wrote: > >>Hello all, >> >>I am involved in the development of an underwater modem. Right now I am >>dealing with the simulation stage with simulink. >> >>The modulation scheme I am using is a non-coherent FSK. The carrier >>frequencies are 20kHz and 22kHz. >> >>At the demodulator part, I have designed a matched filters scheme, with >>both bandpass FIR filters at 20 and 22kHz respectively. > > What's wrong with a PLL?PLL requires thinking. VLV
Reply by ●January 8, 20092009-01-08
Tomeu wrote:> Hello all, > > I am involved in the development of an underwater modem. Right now I am > dealing with the simulation stage with simulink. > > The modulation scheme I am using is a non-coherent FSK. The carrier > frequencies are 20kHz and 22kHz. > > At the demodulator part, I have designed a matched filters scheme, with > both bandpass FIR filters at 20 and 22kHz respectively. The question is, > which should be a typical number of coefficients for those filters? I am > not sure whether I make the filters too selective... > > Otherwise, for non coherent detection, which scheme should be more robust, > matched filters or a quadrature receiver?If those matters are the questions for you, you'd better use a frequency counter as the demodulator. Simple, stupid and it works. VLV
Reply by ●January 9, 20092009-01-09
> > >Tomeu wrote: > >> Hello all, >> >> I am involved in the development of an underwater modem. Right now Iam>> dealing with the simulation stage with simulink. >> >> The modulation scheme I am using is a non-coherent FSK. The carrier >> frequencies are 20kHz and 22kHz. >> >> At the demodulator part, I have designed a matched filters scheme,with>> both bandpass FIR filters at 20 and 22kHz respectively. The questionis,>> which should be a typical number of coefficients for those filters? Iam>> not sure whether I make the filters too selective... >> >> Otherwise, for non coherent detection, which scheme should be morerobust,>> matched filters or a quadrature receiver? > >If those matters are the questions for you, you'd better use a frequency>counter as the demodulator. Simple, stupid and it works. > > >VLV >Ok VLV, no need to be an ass... Anyways. I had a misconception on matched filters. What I am doing right now is: two bandpass filters, one at 20 and one at 22kHz, then envelope detection. My question was: are this type of filters feasible? they have to be too selective. Any suggestions are greatly appreciated.
Reply by ●January 9, 20092009-01-09
Tomeu <tomeu@marexi.com> wrote:>Anyways. I had a misconception on matched filters. What I am doing right >now is: two bandpass filters, one at 20 and one at 22kHz, then envelope >detection. My question was: are this type of filters feasible? they have to >be too selective.That can work if the bauds are long enough. I'm not sure you have said how fast you are signalling, or maybe I missed it. Steve
Reply by ●January 9, 20092009-01-09
On Jan 9, 5:23�am, spop...@speedymail.org (Steve Pope) wrote:> Tomeu <to...@marexi.com> wrote: > >Anyways. I had a misconception on matched filters. What I am doing right > >now is: two bandpass filters, one at 20 and one at 22kHz, then envelope > >detection. My question was: are this type of filters feasible? they have to > >be too selective. > > That can work if the bauds are long enough. �I'm not sure > you have said how fast you are signalling, or maybe I missed it. > > SteveX-no-archive: You may want to research the old ham radio system called RTTY. For a low baud rate system like that, it was seen that using two separate filters for mark and space can work better becasue it rejects the interference in between the two tones. That only works if the frequency spacing is high relative to the baud rate. I could not find much on the web about this other than here: http://lists.contesting.com/_rtty/2001-09/msg00027.html Mark
Reply by ●January 9, 20092009-01-09
Tomeu wrote:> Anyways. I had a misconception on matched filters. What I am doing right > now is: two bandpass filters, one at 20 and one at 22kHz, then envelope > detection. My question was: are this type of filters feasible? they have to > be too selective.Those filters are definitely not for the spoon-fed students. They should use one filter from 20 to 22kHz, with a frequency counter after that.> Any suggestions are greatly appreciated.How much is the "greatly appreciation"? VLV