Hello, I have a DDC system. The input signal is sampled at 100 MHz. The carrier is 70 MHz. Signal bandwidth is 30 MHz.(from 55 ~ 85 MHz) The output of the DDC is I and Q of 30 MHz. Can I represent 30 MHz bandwidth simultaneously? How can I do it? How to explain it using Nyquist theory? Thank you. Billy _____________________________________ Do you know a company who employs DSP engineers? Is it already listed at http://dsprelated.com/employers.php ?
Bandwidth of complex IQ signals
Started by ●May 14, 2007
Reply by ●May 14, 20072007-05-14
On May 14, 8:23 pm, "billykao" <billy....@gmail.com> wrote:> Hello, > > I have a DDC system. The input signal is sampled at 100 MHz. The carrier > is 70 MHz. Signal bandwidth is 30 MHz.(from 55 ~ 85 MHz) > The output of the DDC is I and Q of 30 MHz. Can I represent 30 MHz > bandwidth simultaneously? How can I do it? How to explain it using Nyquist > theory? Thank you. > > Billy > > _____________________________________ > Do you know a company who employs DSP engineers? > Is it already listed athttp://dsprelated.com/employers.php?You should be fine, assuming that the signal you sampled is appropriately bandlimited (i.e. you should bandpass filter the 55-85 MHz band before sampling). Your sampling frequency is 100 MHz, so the signal of interest will "alias" down to a center frequency of 30 MHz after sampling. Your signal will then occupy the 15-45 MHz band in the sampled signal, which is all below the Nyquist rate. No problems there. Jason
Reply by ●May 14, 20072007-05-14
On May 14, 8:42 pm, cincy...@gmail.com wrote:> You should be fine, assuming that the signal you sampled is > appropriately bandlimited (i.e. you should bandpass filter the 55-85 > MHz band before sampling). Your sampling frequency is 100 MHz, so the > signal of interest will "alias" down to a center frequency of 30 MHz > after sampling. Your signal will then occupy the 15-45 MHz band in the > sampled signal, which is all below the Nyquist rate. No problems > there. > > JasonTo the original poster: the keyword is "bandpass sampling".
Reply by ●May 15, 20072007-05-15
>On May 14, 8:42 pm, cincy...@gmail.com wrote: > >> You should be fine, assuming that the signal you sampled is >> appropriately bandlimited (i.e. you should bandpass filter the 55-85 >> MHz band before sampling). Your sampling frequency is 100 MHz, so the >> signal of interest will "alias" down to a center frequency of 30 MHz >> after sampling. Your signal will then occupy the 15-45 MHz band in the >> sampled signal, which is all below the Nyquist rate. No problems >> there. >> >> Jason > >To the original poster: the keyword is "bandpass sampling". > >For 30 MHz I signal, the maximum available bandwidth is 15 MHz. So is Q signal. If using I and Q, can I make it 30 MHz bandwidth? Thank you. Billy _____________________________________ Do you know a company who employs DSP engineers? Is it already listed at http://dsprelated.com/employers.php ?
Reply by ●May 15, 20072007-05-15
"billykao" <billy.kao@gmail.com> writes:>>On May 14, 8:42 pm, cincy...@gmail.com wrote: >> >>> You should be fine, assuming that the signal you sampled is >>> appropriately bandlimited (i.e. you should bandpass filter the 55-85 >>> MHz band before sampling). Your sampling frequency is 100 MHz, so the >>> signal of interest will "alias" down to a center frequency of 30 MHz >>> after sampling. Your signal will then occupy the 15-45 MHz band in the >>> sampled signal, which is all below the Nyquist rate. No problems >>> there. >>> >>> Jason >> >>To the original poster: the keyword is "bandpass sampling". >> >> > > For 30 MHz I signal, the maximum available bandwidth is 15 MHz. So is Q > signal. If using I and Q, can I make it 30 MHz bandwidth? Thank you.Yes. You essentially utilize the bandwidth in -Fs/2 to +Fs/2 with an I/Q signal, whereas with a real signal the usable bandwidth is from 0 to +Fs/2. -- % Randy Yates % "And all that I can do %% Fuquay-Varina, NC % is say I'm sorry, %%% 919-577-9882 % that's the way it goes..." %%%% <yates@ieee.org> % Getting To The Point', *Balance of Power*, ELO http://home.earthlink.net/~yatescr
Reply by ●May 15, 20072007-05-15
>"billykao" <billy.kao@gmail.com> writes: > >>>On May 14, 8:42 pm, cincy...@gmail.com wrote: >>> >>>> You should be fine, assuming that the signal you sampled is >>>> appropriately bandlimited (i.e. you should bandpass filter the 55-85 >>>> MHz band before sampling). Your sampling frequency is 100 MHz, sothe>>>> signal of interest will "alias" down to a center frequency of 30 MHz >>>> after sampling. Your signal will then occupy the 15-45 MHz band inthe>>>> sampled signal, which is all below the Nyquist rate. No problems >>>> there. >>>> >>>> Jason >>> >>>To the original poster: the keyword is "bandpass sampling". >>> >>> >> >> For 30 MHz I signal, the maximum available bandwidth is 15 MHz. So isQ>> signal. If using I and Q, can I make it 30 MHz bandwidth? Thank you. > >Yes. You essentially utilize the bandwidth in -Fs/2 to +Fs/2 with an >I/Q signal, whereas with a real signal the usable bandwidth is from 0 >to +Fs/2. >-- >% Randy Yates % "And all that I can do >%% Fuquay-Varina, NC % is say I'm sorry, >%%% 919-577-9882 % that's the way it goes..." >%%%% <yates@ieee.org> % Getting To The Point', *Balance ofPower*, ELO>http://home.earthlink.net/~yatescr >I appreciate your answer. Could you explain why Q signal shows bandwidth from -Fs/2 to 0? Is there any article or book talking about this? Thank you very much. _____________________________________ Do you know a company who employs DSP engineers? Is it already listed at http://dsprelated.com/employers.php ?
Reply by ●May 15, 20072007-05-15
>>"billykao" <billy.kao@gmail.com> writes: >> >>>>On May 14, 8:42 pm, cincy...@gmail.com wrote: >>>> >>>>> You should be fine, assuming that the signal you sampled is >>>>> appropriately bandlimited (i.e. you should bandpass filter the55-85>>>>> MHz band before sampling). Your sampling frequency is 100 MHz, so >the >>>>> signal of interest will "alias" down to a center frequency of 30MHz>>>>> after sampling. Your signal will then occupy the 15-45 MHz band in >the >>>>> sampled signal, which is all below the Nyquist rate. No problems >>>>> there. >>>>> >>>>> Jason >>>> >>>>To the original poster: the keyword is "bandpass sampling". >>>> >>>> >>> >>> For 30 MHz I signal, the maximum available bandwidth is 15 MHz. Sois>Q >>> signal. If using I and Q, can I make it 30 MHz bandwidth? Thankyou.>> >>Yes. You essentially utilize the bandwidth in -Fs/2 to +Fs/2 with an >>I/Q signal, whereas with a real signal the usable bandwidth is from 0 >>to +Fs/2. >>-- >>% Randy Yates % "And all that I can do >>%% Fuquay-Varina, NC % is say I'm sorry, >>%%% 919-577-9882 % that's the way it goes..." >>%%%% <yates@ieee.org> % Getting To The Point', *Balance of >Power*, ELO >>http://home.earthlink.net/~yatescr >> > >I appreciate your answer. Could you explain why Q signal shows bandwidth >from -Fs/2 to 0? Is there any article or book talking about this?Thank>you very much. > > > >_____________________________________ >Do you know a company who employs DSP engineers? >Is it already listed at http://dsprelated.com/employers.php ? >It is the interrelation between the I and Q channels that is able to distinguish between positive and negative frequencies. Search for "negative frequency" and you can find some lengthy discussions of it on this group. -Joe _____________________________________ Do you know a company who employs DSP engineers? Is it already listed at http://dsprelated.com/employers.php ?
Reply by ●May 15, 20072007-05-15
>>>"billykao" <billy.kao@gmail.com> writes: >>> >>>>>On May 14, 8:42 pm, cincy...@gmail.com wrote: >>>>> >>>>>> You should be fine, assuming that the signal you sampled is >>>>>> appropriately bandlimited (i.e. you should bandpass filter the >55-85 >>>>>> MHz band before sampling). Your sampling frequency is 100 MHz, so >>the >>>>>> signal of interest will "alias" down to a center frequency of 30 >MHz >>>>>> after sampling. Your signal will then occupy the 15-45 MHz band in >>the >>>>>> sampled signal, which is all below the Nyquist rate. No problems >>>>>> there. >>>>>> >>>>>> Jason >>>>> >>>>>To the original poster: the keyword is "bandpass sampling". >>>>> >>>>> >>>> >>>> For 30 MHz I signal, the maximum available bandwidth is 15 MHz. So >is >>Q >>>> signal. If using I and Q, can I make it 30 MHz bandwidth? Thank >you. >>> >>>Yes. You essentially utilize the bandwidth in -Fs/2 to +Fs/2 with an >>>I/Q signal, whereas with a real signal the usable bandwidth is from 0 >>>to +Fs/2. >>>-- >>>% Randy Yates % "And all that I can do >>>%% Fuquay-Varina, NC % is say I'm sorry, >>>%%% % that's the way it goes..." >>>%%%% <yates@ieee.org> % Getting To The Point', *Balance of >>Power*, ELO >>>http://home.earthlink.net/~yatescr >>> >> >>I appreciate your answer. Could you explain why Q signal showsbandwidth>>from -Fs/2 to 0? Is there any article or book talking about this? >Thank >>you very much. >> >> >> >>_____________________________________ >>Do you know a company who employs DSP engineers? >>Is it already listed at http://dsprelated.com/employers.php ? >> > > >It is the interrelation between the I and Q channels that is able to >distinguish between positive and negative frequencies. Search for >"negative frequency" and you can find some lengthy discussions of it on >this group. > >-Joe > >_____________________________________ >Do you know a company who employs DSP engineers? >Is it already listed at http://dsprelated.com/employers.php ? >In some commercial RF digitizers, like e.g. Aeroflex PXI-3035 and National Instruments PXI-5661, the real-valued IF signal is digitized with a sampling frequency of 100 MHz, followed by IQ-separation and digital downconversion (DDC) to the baseband. I remember the maximum signal bandwidth, that can be acquired, in both cases is 20 MHz. Still wondering if it is a theoretical limit of implementation constraint. LBB _____________________________________ Do you know a company who employs DSP engineers? Is it already listed at http://dsprelated.com/employers.php ?
Reply by ●May 15, 20072007-05-15
>>>>"billykao" <billy.kao@gmail.com> writes: >>>> >>>>>>On May 14, 8:42 pm, cincy...@gmail.com wrote: >>>>>> >>>>>>> You should be fine, assuming that the signal you sampled is >>>>>>> appropriately bandlimited (i.e. you should bandpass filter the >>55-85 >>>>>>> MHz band before sampling). Your sampling frequency is 100 MHz, so >>>the >>>>>>> signal of interest will "alias" down to a center frequency of 30 >>MHz >>>>>>> after sampling. Your signal will then occupy the 15-45 MHz bandin>>>the >>>>>>> sampled signal, which is all below the Nyquist rate. No problems >>>>>>> there. >>>>>>> >>>>>>> Jason >>>>>> >>>>>>To the original poster: the keyword is "bandpass sampling". >>>>>> >>>>>> >>>>> >>>>> For 30 MHz I signal, the maximum available bandwidth is 15 MHz. So >>is >>>Q >>>>> signal. If using I and Q, can I make it 30 MHz bandwidth? Thank >>you. >>>> >>>>Yes. You essentially utilize the bandwidth in -Fs/2 to +Fs/2 with an >>>>I/Q signal, whereas with a real signal the usable bandwidth is from 0 >>>>to +Fs/2. >>>>-- >>>>% Randy Yates % "And all that I can do >>>>%% Fuquay-Varina, NC % is say I'm sorry, >>>>%%% % that's the way it goes..." >>>>%%%% <yates@ieee.org> % Getting To The Point', *Balance of >>>Power*, ELO >>>>http://home.earthlink.net/~yatescr >>>> >>> >>>I appreciate your answer. Could you explain why Q signal shows >bandwidth >>>from -Fs/2 to 0? Is there any article or book talking about this? >>Thank >>>you very much. >>> >>> >>> >>>_____________________________________ >>>Do you know a company who employs DSP engineers? >>>Is it already listed at http://dsprelated.com/employers.php ? >>> >> >> >>It is the interrelation between the I and Q channels that is able to >>distinguish between positive and negative frequencies. Search for >>"negative frequency" and you can find some lengthy discussions of it on >>this group. >> >>-Joe >> >>_____________________________________ >>Do you know a company who employs DSP engineers? >>Is it already listed at http://dsprelated.com/employers.php ? >> > >In some commercial RF digitizers, like e.g. Aeroflex PXI-3035 andNational>Instruments PXI-5661, the real-valued IF signal is digitized with a >sampling frequency of 100 MHz, followed by IQ-separation and digital >downconversion (DDC) to the baseband. I remember the maximum signal >bandwidth, that can be acquired, in both cases is 20 MHz. Stillwondering>if it is a theoretical limit of implementation constraint. > >LBB > > >_____________________________________ >Do you know a company who employs DSP engineers? >Is it already listed at http://dsprelated.com/employers.php ? >"theoretical limit or implementation constraint" I mean. _____________________________________ Do you know a company who employs DSP engineers? Is it already listed at http://dsprelated.com/employers.php ?
Reply by ●May 15, 20072007-05-15
"billykao" <billy.kao@gmail.com> writes:> [...] > I appreciate your answer. Could you explain why Q signal shows bandwidth > from -Fs/2 to 0?Sure, but it may take some studying on your part to understand it. It's not extremely difficult, though. It's not quite that Q supplies the info "from -Fs/2 to 0". In reality, ALL sampled signals have bandwidth (in the digital domain) from -Fs/2 to +Fs/2. However, if you read up on the properties of the discrete-time Fourier transform, you will see that if x[n] is a real signal, then X(F) is "Hermitian symmetric." Basically that means that the information in the frequency domain from -Fs/2 to 0 is a copy of that from 0 to +Fs/2. However, when x[n] is complex, then the resulting spectrum X(F) is no longer Hermitian symmetric, and the spectrum from -Fs/2 to 0 carries more information than the spectrum from 0 to +Fs/2 alone.> Is there any article or book talking about this? Thank > you very much.Any textbook on linear systems will have this. My favorite (though now somewhat dated) is [signalsandsystems]. --Randy @BOOK{signalsandsystems, title = "{Signals and Systems}", author = "{Alan~V.~Oppenheim, Alan~S.~Willsky, with Ian~T.~Young}", publisher = "Prentice Hall", year = "1983"} -- % Randy Yates % "She tells me that she likes me very much, %% Fuquay-Varina, NC % but when I try to touch, she makes it %%% 919-577-9882 % all too clear." %%%% <yates@ieee.org> % 'Yours Truly, 2095', *Time*, ELO http://home.earthlink.net/~yatescr
