Converting non quadrature signal to quadrature
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From: Bob on 25 Mar 2010 12:32 Hi Group I have a non quadrature signal at 3 MHz. I need to convert it to quadrature (I and Q) at 1.5 MHz to that I can reduce the sampleing rate at the next stage. What do you experts think would be the best method in terms of resources (for an fpga) or is there much difference? 1 Hilbert Transformer 2 Multiply by a 1.5 Mhz sine and cosine and then filter to get rid of the 4.5 Mhz sum signal. Alternatively, are there any smart tricks that could do this more efficiently? Many Thanks Bob
From: Tim Wescott on 25 Mar 2010 13:23 Bob wrote: > Hi Group > > I have a non quadrature signal at 3 MHz. I need to convert it to > quadrature (I and Q) at 1.5 MHz to that I can reduce the sampleing > rate at the next stage. What do you experts think would be the best > method in terms of resources (for an fpga) or is there much > difference? > > 1 Hilbert Transformer > 2 Multiply by a 1.5 Mhz sine and cosine and then filter to get rid of > the 4.5 Mhz sum signal. > > Alternatively, are there any smart tricks that could do this more > efficiently? Either will work, and it's probably a toss up whether 1 or 2 is best in terms of circuit area and speed requirements (both have been done in analog circuits). But this is a little block in a bigger system, and you're doing something that -- on the face of it -- sounds a bit odd. What are you really trying to do? Normally you want I and Q signals so you can do your processing at baseband. Why don't you demodulate it straight down to DC by multiplying it by 3MHz sine and cosine waves, and filtering as appropriate? -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
From: Jerry Avins on 25 Mar 2010 13:31 Bob wrote: > Hi Group > > I have a non quadrature signal at 3 MHz. I need to convert it to > quadrature (I and Q) at 1.5 MHz to that I can reduce the sampleing > rate at the next stage. What do you experts think would be the best > method in terms of resources (for an fpga) or is there much > difference? > > 1 Hilbert Transformer > 2 Multiply by a 1.5 Mhz sine and cosine and then filter to get rid of > the 4.5 Mhz sum signal. > > Alternatively, are there any smart tricks that could do this more > efficiently? You will have to process the same number of samples either way. 1.5M I plus 1.5M Q is still 3 M samples either way. Whatever, for a 1.i5 MHz bandwidth, you need 3 M samples/sec. They can be all regular samples, half I and half Q, half I and half dI/dt, or any other set. Jerry -- Discovery consists of seeing what everybody has seen, and thinking what nobody has thought. .. Albert Szent-Gyorgi �����������������������������������������������������������������������
From: glen herrmannsfeldt on 25 Mar 2010 15:33 Jerry Avins <jya(a)ieee.org> wrote: >> I have a non quadrature signal at 3 MHz. I need to convert it to >> quadrature (I and Q) at 1.5 MHz to that I can reduce the sampleing >> rate at the next stage. What do you experts think would be the best >> method in terms of resources (for an fpga) or is there much >> difference? (snip) > You will have to process the same number of samples either way. 1.5M I > plus 1.5M Q is still 3 M samples either way. Whatever, for a 1.i5 MHz > bandwidth, you need 3 M samples/sec. They can be all regular samples, > half I and half Q, half I and half dI/dt, or any other set. There might be some cases where quadrature sampling is better, though I am not convinced that there are many. 3MHz isn't fast, so speed probably isn't the reason here. In the case where speed is, you can put two ADCs outside the FPGA and separately clock the two. (Ignoring problems due to non-linearity in the ADCs.) Now, is it better to do the IQ conversion in the analog domain and then send it into the FPGA (More analog circuitry, less FPGA resources) or just sample and ADC at 3MHz, and do the IQ conversion in digital logic? It seems to me that to do it right, you need carefully matched analog filters, where it is easy to do in the digital domain without worry about matching of filters. -- glen
From: Tim Wescott on 25 Mar 2010 16:00
glen herrmannsfeldt wrote: > Jerry Avins <jya(a)ieee.org> wrote: > >>> I have a non quadrature signal at 3 MHz. I need to convert it to >>> quadrature (I and Q) at 1.5 MHz to that I can reduce the sampleing >>> rate at the next stage. What do you experts think would be the best >>> method in terms of resources (for an fpga) or is there much >>> difference? > (snip) > >> You will have to process the same number of samples either way. 1.5M I >> plus 1.5M Q is still 3 M samples either way. Whatever, for a 1.i5 MHz >> bandwidth, you need 3 M samples/sec. They can be all regular samples, >> half I and half Q, half I and half dI/dt, or any other set. > > There might be some cases where quadrature sampling is better, > though I am not convinced that there are many. > > 3MHz isn't fast, so speed probably isn't the reason here. > > In the case where speed is, you can put two ADCs outside the > FPGA and separately clock the two. (Ignoring problems due to > non-linearity in the ADCs.) > > Now, is it better to do the IQ conversion in the analog domain > and then send it into the FPGA (More analog circuitry, less > FPGA resources) or just sample and ADC at 3MHz, and do the > IQ conversion in digital logic? > > It seems to me that to do it right, you need carefully matched > analog filters, where it is easy to do in the digital domain > without worry about matching of filters. > > -- glen Potayto, potahto. Mathematically it's the same to do it in analog or digital. Practically you have all sorts of channel matching issues if you do it in analog, so if you can get away with it, you'd rather just sample fast and downconvert digitally. 3MHz is very slow these days. I'd go as far as to say that if you _did_ have a carrier that was too fast for all-digital conversion, and unless bandwidth considerations ruled it out, you'd be better off to do a traditional superhet stage to a lower IF, filter, and downconvert to baseband from there. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com |