Quadrature low pass filter question
in [DSP]
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From: suren on 12 May 2010 10:58 On May 10, 9:24 pm, "jacobfenton" <jacob.fenton(a)n_o_s_p_a_m.gmail.com> wrote: > 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 Hi, If you are talking about implementation on a FPGA/ASIC, you CAN implement using one LPF but with multiplexed I and Q data. That is you need to convert IQ parallel streams into a single serial stream with I and Q alternating. However this will require you to run the logic at double the original rate and of course the number of states in the filter would need to be doubled too. Regards suren
From: Zhi.Shen on 12 May 2010 23:36 Yes, you're right, I realize my mistake. Because of the filter's memorial, the bypast buffers must be kept. I used to increase the system clock to meet one filter's tap-increasing, so I gave a wrong method without too much thinking. It could complete the 2 filters' process. "Jerry Avins" <jya(a)ieee.org> ??????:_uyGn.17826$_84.6238(a)newsfe18.iad... > On 5/12/2010 12:39 AM, Zhi.Shen wrote: >> "Jerry Avins"<jya(a)ieee.org> ??????:GIfGn.1275$gv4.1122(a)newsfe09.iad... >>> 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"? >>> >> >> I'm not very sure what the "transients" are. >> In my opinion, for example, if the I,Q interpolation need 10-tap >> filtering >> at sampling rate 10MHz each, I can use two 100MHz 'Serial' LPFs or just >> one >> time division multiplex 200MHz 'Serial' LPF to complete it. > > A filter has history. In the case of an FIR with taps, is is the contents > of the circular buffer. Each output is computed from all of the buffer > stages. If you alternate inputs between channels, the outputs will be > garbage. There are ways to combine resources, but beyond reusing the tap > coefficients, they save very little memory. > > Jerry > -- > "I view the progress of science as ... the slow erosion of the tendency > to dichotomize." --Barbara Smuts, U. Mich. > �����������������������������������������������������������������������
From: Randy Yates on 13 May 2010 22:35 Jerry Avins <jya(a)ieee.org> writes: > On 5/12/2010 6:22 AM, Randy Yates wrote: >> Jerry Avins<jya(a)ieee.org> writes: >>> [...] >>> As long as we both know what we mean and have the same operations in >>> mind, straightening out the semantics os secondary to me. >> >> I didn't think this was simply a matter of semantics. I know a company >> that was getting 130 MHz bandwidth in the late 80's - no mean feat even >> for today - by utilizing complex sampling (of course that meant they had >> to convert to quadrature in analog). >> >> My main point is that there are practical, as well as theoretical, >> differences between the two techniques and they are not "the same >> thing." >> >> Now before you tell me how you can build a "2x"-Hz A/D from x-Hz A/D >> components, let me say that this is an exercise in A/D converter design >> and not directly relevent to the discussion. For once you've got a >> "2x"-Hz A/D designed, you can then get 2x-Hz bandwidth through it by >> utilizing two such converters on a complex I/Q signal. > > Same thing or different, one can get (nearly) X bandwidth with 2X > samples/sec. Those samples can be sequential, I&Q, X&dX/dt, and other > possible pairs. Do we agree on that? I believe I've already agreed to that Jerry. The problem with your point is that it is presuming the metric is "samples per second," but there are other metrics that could be more important in certain cases. -- Randy Yates % "So now it's getting late, Digital Signal Labs % and those who hesitate mailto://yates(a)ieee.org % got no one..." http://www.digitalsignallabs.com % 'Waterfall', *Face The Music*, ELO
From: Jerry Avins on 13 May 2010 23:27 On 5/13/2010 10:35 PM, Randy Yates wrote: > Jerry Avins<jya(a)ieee.org> writes: > >> On 5/12/2010 6:22 AM, Randy Yates wrote: >>> Jerry Avins<jya(a)ieee.org> writes: >>>> [...] >>>> As long as we both know what we mean and have the same operations in >>>> mind, straightening out the semantics os secondary to me. >>> >>> I didn't think this was simply a matter of semantics. I know a company >>> that was getting 130 MHz bandwidth in the late 80's - no mean feat even >>> for today - by utilizing complex sampling (of course that meant they had >>> to convert to quadrature in analog). >>> >>> My main point is that there are practical, as well as theoretical, >>> differences between the two techniques and they are not "the same >>> thing." >>> >>> Now before you tell me how you can build a "2x"-Hz A/D from x-Hz A/D >>> components, let me say that this is an exercise in A/D converter design >>> and not directly relevent to the discussion. For once you've got a >>> "2x"-Hz A/D designed, you can then get 2x-Hz bandwidth through it by >>> utilizing two such converters on a complex I/Q signal. >> >> Same thing or different, one can get (nearly) X bandwidth with 2X >> samples/sec. Those samples can be sequential, I&Q, X&dX/dt, and other >> possible pairs. Do we agree on that? > > I believe I've already agreed to that Jerry. The problem with your point > is that it is presuming the metric is "samples per second," but there > are other metrics that could be more important in certain cases. OK. I'll have to think about that. I thought that the sampling theorem related bandwidth to sample rate, but maybe there's more. Jerry -- "I view the progress of science as ... the slow erosion of the tendency to dichotomize." --Barbara Smuts, U. Mich. �����������������������������������������������������������������������
From: Randy Yates on 14 May 2010 09:41
On May 13, 11:27 pm, Jerry Avins <j...(a)ieee.org> wrote: > On 5/13/2010 10:35 PM, Randy Yates wrote: > > > > > Jerry Avins<j...(a)ieee.org> writes: > > >> On 5/12/2010 6:22 AM, Randy Yates wrote: > >>> Jerry Avins<j...(a)ieee.org> writes: > >>>> [...] > >>>> As long as we both know what we mean and have the same operations in > >>>> mind, straightening out the semantics os secondary to me. > > >>> I didn't think this was simply a matter of semantics. I know a company > >>> that was getting 130 MHz bandwidth in the late 80's - no mean feat even > >>> for today - by utilizing complex sampling (of course that meant they had > >>> to convert to quadrature in analog). > > >>> My main point is that there are practical, as well as theoretical, > >>> differences between the two techniques and they are not "the same > >>> thing." > > >>> Now before you tell me how you can build a "2x"-Hz A/D from x-Hz A/D > >>> components, let me say that this is an exercise in A/D converter design > >>> and not directly relevent to the discussion. For once you've got a > >>> "2x"-Hz A/D designed, you can then get 2x-Hz bandwidth through it by > >>> utilizing two such converters on a complex I/Q signal. > > >> Same thing or different, one can get (nearly) X bandwidth with 2X > >> samples/sec. Those samples can be sequential, I&Q, X&dX/dt, and other > >> possible pairs. Do we agree on that? > > > I believe I've already agreed to that Jerry. The problem with your point > > is that it is presuming the metric is "samples per second," but there > > are other metrics that could be more important in certain cases. > > OK. I'll have to think about that. I thought that the sampling theorem > related bandwidth to sample rate, but maybe there's more. > > Jerry > -- > "I view the progress of science as ... the slow erosion of the tendency > to dichotomize." --Barbara Smuts, U. Mich. > ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ That's like saying, "I thought power was I*V. Maybe there's more to it than that" without thinking about the almost infinite number of ways to design a system that yields that power (e.g., high voltage/low current, low voltage/high current) and the practical pro's and con's of each! --Randy |