Could some DSP guru please provide some hints to my problem - my DSP is a bit rusty ? I am experimenting with a QPSK design for which the input data streams (I, Q) are synchronized with a 1 MHz clock. In this case what should be the carrier frequency ? I understand that the Nyquist criteria must hold, and so in this case what should be the carrier signal frequency ? Any hints would be valuable - thnks in advance.
QPSK carrier frequency ??
Started by ●December 17, 2010
Reply by ●December 17, 20102010-12-17
On 12/17/2010 08:11 AM, Daku wrote:> Could some DSP guru please provide some hints to > my problem - my DSP is a bit rusty ? I am experimenting with a QPSK > design for which the input data streams (I, Q) are synchronized with a > 1 MHz clock. In this case > what should be the carrier frequency ? I understand that the Nyquist > criteria must hold, and so in this case what should be the carrier > signal frequency ? Any hints would be valuable - thnks in advance.Do you mean the I and Q streams are _sampled_ at 1MHz, or that the sampling is done at some lower rate and the samples happen at the clock edges? With I, Q data, the carrier frequency after sampling can be just about anything you want* -- you've got enough information in your pair of signals to change the frequency from anything to anything within the interval [-Fs/2 Fs/2]. For that matter, with decent down-conversion, you're going to capture the carrier and it's close-by spectrum with good fidelity, as long as you don't alias far-away spectrum onto your signal. So the question you _should_ be asking is how fast can your data rate be, and what sort of pulse shaping should you be using. * Although wanting to keep the after-sampling carrier frequency = 0, to avoid making your brain hurt, is a laudable desire. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●December 17, 20102010-12-17
On 12/17/2010 08:11 AM, Daku wrote:> Could some DSP guru please provide some hints to > my problem - my DSP is a bit rusty ? I am experimenting with a QPSK > design for which the input data streams (I, Q) are synchronized with a > 1 MHz clock. In this case > what should be the carrier frequency ? I understand that the Nyquist > criteria must hold, and so in this case what should be the carrier > signal frequency ? Any hints would be valuable - thnks in advance.Oops -- I forgot: http://www.wescottdesign.com/articles/Sampling/sampling.html. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●December 17, 20102010-12-17
Daku wrote:> Could some DSP guru please provide some hints to > my problem - my DSP is a bit rusty ? I am experimenting with a QPSK > design for which the input data streams (I, Q) are synchronized with a > 1 MHz clock. In this case > what should be the carrier frequency ?Try the frequencies of 0, -1, and +/- j> I understand that the Nyquist > criteria must hold, and so in this case what should be the carrier > signal frequency ? Any hints would be valuable - thnks in advance.Don't poke your nose into the things far beyond your comprehension. VLV
Reply by ●December 17, 20102010-12-17
On Dec 17, 1:36�pm, Vladimir Vassilevsky <nos...@nowhere.com> wrote:> Daku wrote: > > Could some DSP guru please provide some hints to > > my problem - my DSP is a bit rusty ? I am experimenting with a QPSK > > design for which the input data streams (I, Q) are synchronized with a > > 1 MHz clock. In this case > > what should be the carrier frequency ? > > Try the frequencies of 0, -1, and +/- j > > > I understand that the Nyquist > > criteria must hold, and so in this case what should be the carrier > > signal frequency ? Any hints would be valuable - thnks in advance. > > Don't poke your nose into the things far beyond your comprehension.Oh come on, Vlad. It's called "jumping in with both feet." I learned electronics (after building a superregenerative receiver from plans) by tearing out the guts of a Magnavox (or was the first one a Capehart?) and reorganizing the old components, cutting distortion and improving frequency in the process. Although built in quantity in the middle 1940s, those console radios had been designed before WWII. By 1950, enough more had been learned so it was easy to do better. Jerry -- Engineering is the art of making something decent out of what came in the box.
Reply by ●December 17, 20102010-12-17
On Fri, 17 Dec 2010 12:36:10 -0600, Vladimir Vassilevsky <nospam@nowhere.com> wrote:> > >Daku wrote: > >> Could some DSP guru please provide some hints to >> my problem - my DSP is a bit rusty ? I am experimenting with a QPSK >> design for which the input data streams (I, Q) are synchronized with a >> 1 MHz clock. In this case >> what should be the carrier frequency ? > >Try the frequencies of 0, -1, and +/- j > >> I understand that the Nyquist >> criteria must hold, and so in this case what should be the carrier >> signal frequency ? Any hints would be valuable - thnks in advance. > >Don't poke your nose into the things far beyond your comprehension. > >VLVThat sort of comment has no apparent redeeming value and has been causing the entire group harm by way of damaged reputation. It'd be appreciated if you could dial it back a bit. Eric Jacobsen Minister of Algorithms Abineau Communications http://www.abineau.com
Reply by ●December 17, 20102010-12-17
On 12/17/2010 12:53 PM, Eric Jacobsen wrote:> On Fri, 17 Dec 2010 12:36:10 -0600, Vladimir Vassilevsky > <nospam@nowhere.com> wrote: > >> >> >> Daku wrote: >> >>> Could some DSP guru please provide some hints to >>> my problem - my DSP is a bit rusty ? I am experimenting with a QPSK >>> design for which the input data streams (I, Q) are synchronized with a >>> 1 MHz clock. In this case >>> what should be the carrier frequency ? >> >> Try the frequencies of 0, -1, and +/- j >> >>> I understand that the Nyquist >>> criteria must hold, and so in this case what should be the carrier >>> signal frequency ? Any hints would be valuable - thnks in advance. >> >> Don't poke your nose into the things far beyond your comprehension. >> >> VLV > > That sort of comment has no apparent redeeming value and has been > causing the entire group harm by way of damaged reputation. It'd be > appreciated if you could dial it back a bit.Thanks Eric -- for the sentiment, and for wording it diplomatically. Vladimir: he's right. You weren't born knowing everything; you seem to forget that. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●December 18, 20102010-12-18
>On 12/17/2010 12:53 PM, Eric Jacobsen wrote: >> On Fri, 17 Dec 2010 12:36:10 -0600, Vladimir Vassilevsky >> <nospam@nowhere.com> wrote: >> >>> >>> >>> Daku wrote: >>> >>>> Could some DSP guru please provide some hints to >>>> my problem - my DSP is a bit rusty ? I am experimenting with a QPSK >>>> design for which the input data streams (I, Q) are synchronized witha>>>> 1 MHz clock. In this case >>>> what should be the carrier frequency ? >>> >>>i hope you want to make aloopback sampling >>>>>mod>>>up conversion >>>>>>down conversion>>>>>>demod>>>samplingyou can start with generating a carrier ,,,,your concern should be the carrier freq and sampling rate by this i mean that suppose you have a carrier of 64 khz the max samples you can have in your sine/cos wave generation for carrier will be 16(16x64=1 MHz)but you can also use less samples ...it all depends on you ,,,,,oyu can use different combinations like 128 khz with 8 samples ,,,,\\ the first thing you should do is to make a whole system design interms of sampling rate ,,,keeping in view your dsp specs ,,,,,and then defining your system
Reply by ●December 18, 20102010-12-18
On 12/17/2010 11:11 AM, Daku wrote:> Could some DSP guru please provide some hints to > my problem - my DSP is a bit rusty ? I am experimenting with a QPSK > design for which the input data streams (I, Q) are synchronized with a > 1 MHz clock. In this case > what should be the carrier frequency ? I understand that the Nyquist > criteria must hold, and so in this case what should be the carrier > signal frequency ? Any hints would be valuable - thnks in advance.Impossible to say given the information you've provided. Often times just the fact that you've got complex IQ samples and not real samples means you've already downconverted to a lowpass signal, but that's not always the case. Why not examine the spectrum of a few thousand samples and see? -- Randy Yates % "My Shangri-la has gone away, fading like Digital Signal Labs % the Beatles on 'Hey Jude'" yates@digitalsignallabs.com % http://www.digitalsignallabs.com % 'Shangri-La', *A New World Record*, ELO
