# Re: FSK demodulation **URL:** \ **Category:** GNU Radio\ **Created:** [June 19, 2014, 6:01am UTC](https://www.ruby-forum.com/t/re-fsk-demodulation/230461 "2014-06-19T06:01:01Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![jason\_sam](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/7993a0/32.png) [@jason\_sam](https://www.ruby-forum.com/u/jason_sam)\ **Post date:** [June 19, 2014, 6:01am UTC](https://www.ruby-forum.com/t/re-fsk-demodulation/230461/1 "2014-06-19T06:01:01Z")
> Hi Marcus! Thanks for your detailed reply. To what I have studied yet polar discriminator will be affected more by the amplitude variations during transmission. Currently I do not know how to cater these variations, I am reading literature on it. Currently, I am trying to implement correlation receiver using FIR filters. I am looking into polyphase filterbank channelizer to implement correlation, but currently unable to do so because I currently do not know how to assign taps for channelizer. > practical system, if your FSK shift is sufficiently larger than your > frequency offset. Oversampling at baseband ? I am currently sampling at 1Msps (usrp sink sampling rate) for a tone spacing of 125KHz. You mean after the USRP sink block I should upsample again and then do the correlation? Or you mean something else ? Please forgive me for asking too many question, I am still learning. > All in all, the question which demodulator architecture to use is an > academic/research one and not related to the implementation in GNU Radio > modulation. This is a highly interesting field and I can but encourage > you to read a lot of books [1] and papers on this. Thanks for the link, nice stuff. > modulation: > > Assuming you don’t want phase jumps between symbols, why not just use > the VCO block to generate frequencies out of your input values? Wow! modulation is easy using VCO. A question, why would someone want phase jump between symbols? Marc ---
**Author:** ![Marc\_Miller](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/c5a1d2/32.png) [@Marc\_Miller](https://www.ruby-forum.com/u/Marc_Miller)\ **Post date:** [June 19, 2014, 9:37am UTC](https://www.ruby-forum.com/t/re-fsk-demodulation/230461/2 "2014-06-19T09:37:13Z")
Hi Marc, > Currently, I am trying to implement correlation receiver using FIR > filters. I am looking into polyphase filterbank channelizer to implement > correlation, but currently unable to do so because I currently do not > know how to assign taps for channelizer. I should have pointed you at Tom’s article on PFB channelizers and synthesizers [1]. > Oversampling at baseband ? I am currently sampling at 1Msps (usrp sink > sampling rate) for a tone spacing of 125KHz. You mean after the USRP sink > block I should upsample again and then do the correlation? Or you mean > something else ? Please forgive me for asking too many question, I am still > learning. > don’t worry; this list is meant for GNU Radio discussion, I think things > like exploring possibilities to do certain tasks in GNU Radio fit > nicely, especially because you are considering using something “cool and > new” like the PFBs in your approach. GNU Radio can always use more > people actually trying to implement stuff! So: Oversampling is what you do when getting more samples out of your ADC than nyquist forces you to do. If you do 125k spacing 4FSK, then you have a total bandwidth of 750kHz + X, so 1MS/s is “just enough”; anyway, if you sample with let’s say 3.125MS/s, than you’ll have more samples. Within that received bandwidth, you will have the same signal as before, but now correlating against your known tone will give you a higher value than with only 1MS/s, and because noise stays uncorrelated, this will increase SNR. This is processing gain! > > … encourage you to read a lot of books [1] and papers on this. > Thanks for the link, nice stuff. > If you find something interesting, don’t hesitate to add a new heading > like “FSK demodulator technology” and add the references there; I think > this list is something that could use broader usage 🙂 > > > Assuming you don’t want phase jumps between symbols, why not just use > > the VCO block to generate frequencies out of your input values? > > Wow! modulation is easy using VCO. A question, why would someone want phase > > jump between symbols? > > In FSK systems, you’d usually want to avoid that, because phase jumps > > lead to spectral components that are not at the frequency you shifted > > to. > > With PSK, phase jumps is your only way to encode data, and with QAM > > “half” of the information is encoded in phase. However, you usually see > > a pulse shaper after these, because transmitting a real phase > > discontinuity is somewhat physically impossible[2]. So these modulations > > get the spectrum of a phase-jumping system, smoothed by the spectral > > shape of the pulse shaping. Greetings, Marcus [1] > **[PFB Channelizers and Synthesizers — Rondeau Research](http://www.trondeau.com/examples/2014/1/23/pfb-channelizers-and-synthesizers.html)** > > Building the Filters > Based on the paper presented at the  Karlsruhe Institute of Technology's  Workshop on Software Radio (WSR). > The paper starts off by explaining how to build the prototype filters and how these lead directly to the concept of... [2] Basically, imagine the baseband signal as a sequence of rectangles in time; now, the fourier transform of a rectangle is a sin(x)/x, and that has a support that doesn’t fit into a limited bandwidth.