hi all, is it possible to get real data by doing non complex downconversion? assuming we receive real signal at some IF can you do conversion to baseband by using just a sine or cosine? --------------------------------------- Posted through http://www.DSPRelated.com
How to reconstruct original waveform from IQ
Started by ●November 12, 2008
Reply by ●October 18, 20152015-10-18
Reply by ●October 18, 20152015-10-18
On Sun, 18 Oct 2015 12:07:04 -0500, "b2508" <108118@DSPRelated> wrote:>hi all, >is it possible to get real data by doing non complex downconversion? >assuming we receive real signal at some IF can you do conversion to >baseband by using just a sine or cosine? >--------------------------------------- >Posted through http://www.DSPRelated.comIf you use one of each, yes. Eric Jacobsen Anchor Hill Communications http://www.anchorhill.com
Reply by ●October 18, 20152015-10-18
On Sun, 18 Oct 2015 12:07:04 -0500, b2508 wrote:> hi all, > is it possible to get real data by doing non complex downconversion? > assuming we receive real signal at some IF can you do conversion to > baseband by using just a sine or cosine?This is more of a word definition issue than a mathematical one. If you define "baseband" as "meaningful frequencies extend all the way to DC then yes, sorta-kinda. I don't like that definition of "baseband" and I suspect a lot of other people agree with me. If I were teaching a class I would only mention it in passing, and then only to rip it apart. If you define "baseband" as "carrier frequency is really close to zero" then no -- mixing down by a single sine wave mangles the signal. That's because in the full frequency domain, a sinusoidal carrier has components at +fc and -fc; when you mix by a single sine wave, you get a "baseband" signal that has the original signal spectra twice, superimposed on itself. Basically, you've halved the total signal bandwidth and in the process you've irrevocably destroyed information. When you do quadrature mixing each channel has the +fc and the -fc components superimposed upon one another, so each in itself lacks information for the reasons given above. However, with quadrature mixing the inphase channel* is the sum of the +fc and the -fc components, while the quadrature channel is their difference. Because the superposition is a linear operation, all of the information can be recovered. * barring arguments about what the starting phase is. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Reply by ●October 18, 20152015-10-18
On Sun, 18 Oct 2015 12:07:04 -0500, b2508 wrote:> hi all, > is it possible to get real data by doing non complex downconversion? > assuming we receive real signal at some IF can you do conversion to > baseband by using just a sine or cosine?It depends on your signal design and what sort of information you need to recover from your signal. If you want all the information in a RF signal within some passband centered around that RF center frequency, you cannot mix down to "zero IF" (that is, where the local oscillator is at the RF center frequency) without losing some information, unless you use a quadrature mixer, which gives you a complex output. If you need only certain information, such as the amplitude envelope, then you're okay without quadrature mixer, and with a real output at (so-called) baseband. This is a standard way of mixing down an AM-modulated signal. If you need all the signal information, but are okay with low-IF instead of zero-IF, then you can also use a non-quadrature mixer. This is a standard way of mixing down an SSB-modulated signal. The result may or may not be describable as "baseband" but as Tim points out that is terminology. The mixed-down signal is in a low band near DC. Steve
Reply by ●October 18, 20152015-10-18
>On Sun, 18 Oct 2015 12:07:04 -0500, b2508 wrote: > >> hi all, >> is it possible to get real data by doing non complex downconversion? >> assuming we receive real signal at some IF can you do conversion to >> baseband by using just a sine or cosine? > >It depends on your signal design and what sort of information you >need to recover from your signal. > >If you want all the information in a RF signal within some passband >centered around that RF center frequency, you cannot mix down to "zeroIF">(that is, where the local oscillator is at the RF center frequency) >without losing some information, unless you use a quadrature mixer, >which gives you a complex output. > >If you need only certain information, such as the amplitude envelope, >then you're okay without quadrature mixer, and with a real output >at (so-called) baseband. This is a standard way of mixing down >an AM-modulated signal. > >If you need all the signal information, but are okay with low-IF >instead of zero-IF, then you can also use a non-quadrature mixer. >This is a standard way of mixing down an SSB-modulated signal. The >result may or may not be describable as "baseband" but as Tim points >out that is terminology. The mixed-down signal is in a low band >near DC. > > >SteveHow can I determine lowest IF to which I can move signal by just multiplying it with sine or cosine? I have a real signal at 60 MHz originally and assuming that it has bandwidth B and I want to preserve all of it, what would be the way to calculate this lower IF? Thank you for responses. --------------------------------------- Posted through http://www.DSPRelated.com
Reply by ●October 18, 20152015-10-18
>On Sun, 18 Oct 2015 12:07:04 -0500, b2508 wrote: > >> hi all, >> is it possible to get real data by doing non complex downconversion? >> assuming we receive real signal at some IF can you do conversion to >> baseband by using just a sine or cosine? > >This is more of a word definition issue than a mathematical one. > >If you define "baseband" as "meaningful frequencies extend all the way to>DC then yes, sorta-kinda. I don't like that definition of "baseband" and>I suspect a lot of other people agree with me. If I were teaching a >class I would only mention it in passing, and then only to rip it apart. > >If you define "baseband" as "carrier frequency is really close to zero" >then no -- mixing down by a single sine wave mangles the signal. That's>because in the full frequency domain, a sinusoidal carrier has components>at +fc and -fc; when you mix by a single sine wave, you get a "baseband">signal that has the original signal spectra twice, superimposed on >itself. Basically, you've halved the total signal bandwidth and in the >process you've irrevocably destroyed information. > >When you do quadrature mixing each channel has the +fc and the -fc >components superimposed upon one another, so each in itself lacks >information for the reasons given above. However, with quadrature mixing>the inphase channel* is the sum of the +fc and the -fc components, while>the quadrature channel is their difference. Because the superposition is>a linear operation, all of the information can be recovered. > >* barring arguments about what the starting phase is. > >-- > >Tim Wescott >Wescott Design Services >http://www.wescottdesign.comThank you as well, this was very helpful for understanding some parts of the problem. --------------------------------------- Posted through http://www.DSPRelated.com
Reply by ●October 18, 20152015-10-18
On Sun, 18 Oct 2015 16:10:56 -0500, b2508 wrote:>>On Sun, 18 Oct 2015 12:07:04 -0500, b2508 wrote: >> >>> hi all, >>> is it possible to get real data by doing non complex downconversion? >>> assuming we receive real signal at some IF can you do conversion to >>> baseband by using just a sine or cosine? >> >>It depends on your signal design and what sort of information you need >>to recover from your signal. >> >>If you want all the information in a RF signal within some passband >>centered around that RF center frequency, you cannot mix down to "zero > IF" >>(that is, where the local oscillator is at the RF center frequency) >>without losing some information, unless you use a quadrature mixer, >>which gives you a complex output. >> >>If you need only certain information, such as the amplitude envelope, >>then you're okay without quadrature mixer, and with a real output at >>(so-called) baseband. This is a standard way of mixing down an >>AM-modulated signal. >> >>If you need all the signal information, but are okay with low-IF instead >>of zero-IF, then you can also use a non-quadrature mixer. This is a >>standard way of mixing down an SSB-modulated signal. The result may or >>may not be describable as "baseband" but as Tim points out that is >>terminology. The mixed-down signal is in a low band near DC. >> >> >>Steve > > How can I determine lowest IF to which I can move signal by just > multiplying it with sine or cosine? > > I have a real signal at 60 MHz originally and assuming that it has > bandwidth B and I want to preserve all of it, what would be the way to > calculate this lower IF?It's complicated, mostly by the fact that the edges of a "bandwidth" are fuzzy, but also practical considerations in radio design. If 'B' is the bandwidth occupied by the signal plus sufficient guard bands such that images won't cause an issue, then the lowest "possible" IF is B/2, assuming you adequately filter the signal at 60MHz. Adequately filtering the signal with bandwidth B centered around 60MHz may be a challenge if it's not already in the digital domain and if B < 6MHz -- this is why superheterodyne radios (and quadrature downconversion) were invented. If you were going to do it "the old way" with LC filters and single-LO (not quadrature) downconversion, you'd go down in IF by a factor of 10 each time until B >= 0.1 * carrier F, then you'd do one more conversion. Or you'd design a crystal filter with bandwidth B (assuming 0 < B < 10kHz or so), then spend a large amount of money producing your filters. For that matter, adequately filtering the signal if it _is_ in the digital domain may be a challenge if B << 60MHz, because your filter length is roughly proportional to carrier/B (it's something like five times as long, probably, for a filter with halfway decent skirts, and longer if you need sharper skirts). So if B, for instance, is 600Hz then you'd need a filter that's something like 50,000 taps long and that iterates at something well in excess of 60MHz. -- www.wescottdesign.com
Reply by ●October 18, 20152015-10-18
b2508 <108118@DSPRelated> wrote:> spp wrote,>>If you need all the signal information, but are okay with low-IF >>instead of zero-IF, then you can also use a non-quadrature mixer. >>This is a standard way of mixing down an SSB-modulated signal. The >>result may or may not be describable as "baseband" but as Tim points >>out that is terminology. The mixed-down signal is in a low band >>near DC.>How can I determine lowest IF to which I can move signal by just >multiplying it with sine or cosine?>I have a real signal at 60 MHz originally and assuming that it has >bandwidth B and I want to preserve all of it, what would be the way to >calculate this lower IF?If your local oscillator is at less than 60 - B/2 MHz, or greater than 60 + B/2 MHz, it will potentially work. Note, your signal will be contaminated by an image, which may or may not require mitigation or otherwise be a problem, depending on details. Steve
Reply by ●October 19, 20152015-10-19
>On Sun, 18 Oct 2015 16:10:56 -0500, b2508 wrote: > >>>On Sun, 18 Oct 2015 12:07:04 -0500, b2508 wrote: >>> >>>> hi all, >>>> is it possible to get real data by doing non complex downconversion? >>>> assuming we receive real signal at some IF can you do conversion to >>>> baseband by using just a sine or cosine? >>> >>>It depends on your signal design and what sort of information you need >>>to recover from your signal. >>> >>>If you want all the information in a RF signal within some passband >>>centered around that RF center frequency, you cannot mix down to "zero >> IF" >>>(that is, where the local oscillator is at the RF center frequency) >>>without losing some information, unless you use a quadrature mixer, >>>which gives you a complex output. >>> >>>If you need only certain information, such as the amplitude envelope, >>>then you're okay without quadrature mixer, and with a real output at >>>(so-called) baseband. This is a standard way of mixing down an >>>AM-modulated signal. >>> >>>If you need all the signal information, but are okay with low-IFinstead>>>of zero-IF, then you can also use a non-quadrature mixer. This is a >>>standard way of mixing down an SSB-modulated signal. The result mayor>>>may not be describable as "baseband" but as Tim points out that is >>>terminology. The mixed-down signal is in a low band near DC. >>> >>> >>>Steve >> >> How can I determine lowest IF to which I can move signal by just >> multiplying it with sine or cosine? >> >> I have a real signal at 60 MHz originally and assuming that it has >> bandwidth B and I want to preserve all of it, what would be the way to >> calculate this lower IF? > >It's complicated, mostly by the fact that the edges of a "bandwidth" are>fuzzy, but also practical considerations in radio design. > >If 'B' is the bandwidth occupied by the signal plus sufficient guard >bands such that images won't cause an issue, then the lowest "possible" >IF is B/2, assuming you adequately filter the signal at 60MHz. > >Adequately filtering the signal with bandwidth B centered around 60MHz >may be a challenge if it's not already in the digital domain and if B < >6MHz -- this is why superheterodyne radios (and quadraturedownconversion)>were invented. If you were going to do it "the old way" with LC filters>and single-LO (not quadrature) downconversion, you'd go down in IF by a >factor of 10 each time until B >= 0.1 * carrier F, then you'd do one more>conversion. Or you'd design a crystal filter with bandwidth B (assuming>0 < B < 10kHz or so), then spend a large amount of money producing your >filters. > >For that matter, adequately filtering the signal if it _is_ in the >digital domain may be a challenge if B << 60MHz, because your filter >length is roughly proportional to carrier/B (it's something like five >times as long, probably, for a filter with halfway decent skirts, and >longer if you need sharper skirts). So if B, for instance, is 600Hz then>you'd need a filter that's something like 50,000 taps long and that >iterates at something well in excess of 60MHz. > >-- >www.wescottdesign.comI am not sure I understood this. My signal is centered at 60 MHz in digital domain and if it is wide, say 40 MHz, I also have an image of it at the negative axes, right? What am I suppose to filter with this filter? From my point of view I could shift it to the left until some extent and then filter out replicas or stuff that will not come into my signal (and its replica on the negative side). When I try to move it to baseband I do get this mixing of spectral components coming from positive and negative side of frequency axes of original signal. --------------------------------------- Posted through http://www.DSPRelated.com
Reply by ●October 19, 20152015-10-19
>On Sun, 18 Oct 2015 16:10:56 -0500, b2508 wrote: > >>>On Sun, 18 Oct 2015 12:07:04 -0500, b2508 wrote:>For that matter, adequately filtering the signal if it _is_ in the >digital domain may be a challenge if B << 60MHz, because your filter >length is roughly proportional to carrier/B (it's something like five >times as long, probably, for a filter with halfway decent skirts, and >longer if you need sharper skirts). So if B, for instance, is 600Hz then>you'd need a filter that's something like 50,000 taps long and that >iterates at something well in excess of 60MHz. > >-- >www.wescottdesign.comI do not have so huge differences between CF and BW, the ratio carrier/B goes from 0.75 to 1.5. All is done digitally but I am not sure if I am suppose to filter out the components on negative axes and leave those around 60 MHz. Does this mean I need bandpass filter? I thought this can be done after multiplication with sine? --------------------------------------- Posted through http://www.DSPRelated.com
