Quadrature low pass filter question
in [DSP]
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From: Zhi.Shen on 11 May 2010 05:21 If you just don't want to implement the LPF twice, you can double your LPF system clock to complete the I and Q's LP process in only one filter. "jacobfenton" <jacob.fenton(a)n_o_s_p_a_m.gmail.com> д����Ϣ����:DrKdnXQKFfXUrnXWnZ2dnUVZ_qCdnZ2d(a)giganews.com... >I have I and Q data that I want to low pass filter. From what I have read I > would impliment the same LPF twice, one on I and one on Q. Is that the > only > way to do this, or is there some trick out there I have not found out > about? Also I would assume my LPF must have linear phase? > > Thanks. > > -JF
From: Jerry Avins on 11 May 2010 12:34 On 5/11/2010 5:09 AM, Rune Allnor wrote: ... > The fact that the I and Q channels are processed individually as > real-valued data, also have some implictaions. Quadrature filters > are actually restricted by Nyquist's limit, Fs> 2B, whereas > complex-valued filters are restricted by the far more forgiving > Fs> B. > > Or have I misunderstood or missed out on something? It's the same thing. For complex signals, there are two samples per sample period: one real, the other imaginary. That is effectively sampling twice as fast, so the bandwidth is doubled. If one wants to get mathematically hoity-toity, one can say that real sampling covers the range from -fs/2 to +fs/2, while I/Q sampling covers the range from 0 to fs. Personally, I think that obscures the truth that there really /are/ twice as many samples. The second set of samples needn't be Q. d(Real)/dt, for example, would serve as well. Jerry -- "I view the progress of science as ... the slow erosion of the tendency to dichotomize." --Barbara Smuts, U. Mich. �����������������������������������������������������������������������
From: Jerry Avins on 11 May 2010 12:36 On 5/11/2010 5:21 AM, Zhi.Shen wrote: > If you just don't want to implement the LPF twice, you can double your LPF > system clock to complete the I and Q's LP process in only one filter. Oh? How do you deal with perpetual start-up "transients"? > "jacobfenton"<jacob.fenton(a)n_o_s_p_a_m.gmail.com> д����Ϣ����:DrKdnXQKFfXUrnXWnZ2dnUVZ_qCdnZ2d(a)giganews.com... >> I have I and Q data that I want to low pass filter. From what I have read I >> would impliment the same LPF twice, one on I and one on Q. Is that the >> only >> way to do this, or is there some trick out there I have not found out >> about? Also I would assume my LPF must have linear phase? >> >> Thanks. >> >> -JF Jerry -- "I view the progress of science as ... the slow erosion of the tendency to dichotomize." --Barbara Smuts, U. Mich. �����������������������������������������������������������������������
From: Randy Yates on 11 May 2010 12:46 Jerry Avins <jya(a)ieee.org> writes: > [...] > It's the same thing. So, e.g., using complex sampling doesn't allow you to gather more bandwidth using today's technology in ADCs than real sampling does? -- Randy Yates % "Maybe one day I'll feel her cold embrace, Digital Signal Labs % and kiss her interface, mailto://yates(a)ieee.org % til then, I'll leave her alone." http://www.digitalsignallabs.com % 'Yours Truly, 2095', *Time*, ELO
From: Randy Yates on 11 May 2010 12:50
"jacobfenton" <jacob.fenton(a)n_o_s_p_a_m.gmail.com> writes: > I have I and Q data that I want to low pass filter. From what I have read I > would impliment the same LPF twice, one on I and one on Q. That is true if your lowpass filter is a real filter. If it is a complex filter, you'd have more work to do. > Is that the only way to do this, or is there some trick out there I > have not found out about? Mathematically, it's the only way to get from here to there. However, there are infinite choices in how you design your filter and perform the convolutions. > Also I would assume my LPF must have linear phase? Depends on whether or not you need a linear-phase filter. If not, it doesn't have to be linear phase. Let me put it this way: there is nothing inherent in quadrature lowpass filtering that requires a linear-phase filter. -- Randy Yates % "She has an IQ of 1001, she has a jumpsuit Digital Signal Labs % on, and she's also a telephone." mailto://yates(a)ieee.org % http://www.digitalsignallabs.com % 'Yours Truly, 2095', *Time*, ELO |