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26th October 2006, 16:50 #1
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i/q mixer
new question:
than in the transimiter, quadrature structure is still used,why is it used , is the i/q mixer be modulated with the same signal or different two signals ???
and I have this question
for qam or qpsk etc........
when a I/Q(different) modulated signal s1*sin(w*f1)+s2*cos(w*f1) generated by upconverter,when it travel trought I/q downconverter,the original two different signal will be converted to two different complex signal s1*sin(w*f2)+s1*cos(w*f2) and s2*sin(w*f2)+s2*cos(w*f2),f2 can be equal to f1 if it is a direct convertion receiver, these two if signal have carrier overlap not orthogonal~~~~~,how can it be rendered or demodulated?
the original question:
why the quadrature mixer structure is used especially in some low if receiver, just for the convience of adding a polyphase filter to reject image?
got it! thanks to 2,3,4

26th October 2006, 17:11 #2
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ads iq mixer
i/q modulation means double the information on the same BW, hence more spectrum effficiency

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26th October 2006, 17:15 #3
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iq mixer upconverter
IQ mean that ur dealing with complex signals , so u can make complex filters , and so on , it is essential for direct conversion , and low IF
khouly

26th October 2006, 17:24 #4
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q upconverter
You kind of get twice the information with an I/Q output than with a single output. For many demodulation or other applications, that allows you to run your A/D converters and 1/2 the clock rate. So there is a clear tradeoff between microwave complexity and DSP complexity.

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27th October 2006, 03:50 #5
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baseband i and q downconverter
Originally Posted by safwatonline
or not identical? Or, the I path and Q path have a 90° phase difference?
Added after 1 minutes:
Originally Posted by khouly
is due the image rejection, but how come Zero IF need I and Q also?

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27th October 2006, 18:00 #6
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Re: why i/q downconverter or upconverter mixer is used
think of i and q as x and y cartesian plane.
with x and y, you get two dimensions space. if your data is represented by x and y plane, you get more data points. i.e., (3,4), (3,5).
with just i or q, you get x or y only, a one dimenion space.
i and q can be different data. they are modulated by two orthogonal signals (cos and sin) which are 90 degree phase apart. it doesn't mean the data is 90 degree apart.
i and q channel can also be used as synchronization detection.

28th October 2006, 05:35 #7
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why i/q downconverter or upconverter mixer is used
to szekit
I know that is qpsk/qam etc
the problem is that when a I/Q(different) modulated signal s1*sin(w*f1)+s2*cos(w*f1) ,when it travel trought I/q downconverter,the original two different signal will be converted to two different complex signal s1*sin(w*f2)+s1*cos(w*f2) and s2*sin(w*f2)+s2*cos(w*f2),f2 can be equal to f1 if it is a direct convertion receiver, these two if signal have carrier overlap not orthogonal~~~~~,how can it be render?

28th October 2006, 10:17 #8
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why i/q downconverter or upconverter mixer is used
hello,
i don't get ur question clearly but if i understand correctly , the signal u r talking about I and Q signals are s1 and s2 , then to separate them in superheterodyne Rx u first get ur modulated signal to the IF frequ. i.e. signal=S1*sin(IF*t)+S2*cos(IF*t)
then u separate them at the second stage by using two IF signals (sin(IF*t) and cos(IF*t)) which will separated the two signals i.e. sI=s1 and sQ=s2
notes: 1sin(w1*t) is the right expression not w*f1
2 if u use real signals for downconversion then u get ashifted version of the o/p
3 if u use complex signals for downconversion u still get shifted versions (i.e. in both cases u will never get a case like the one u stated as the I and Q signals are 90 out of phase and are treated by the same manner so they will stay out of phase

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29th October 2006, 08:57 #9
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Re: why i/q downconverter or upconverter mixer is used
If you use an I/Q upconverter, and feed the same signal into both I and Q ports, you would have BPSK modulation. If the I and Q signals were different, you would have a more complex type of modulation, such as QPSK, NPSK where N is 3,4,5..., QAM, etc

17th July 2007, 08:16 #10
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Re: why i/q downconverter or upconverter mixer is used
Originally Posted by biff44
part signal?

9th September 2007, 12:40 #11
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Re: why i/q downconverter or upconverter mixer is used
Originally Posted by biff44
identical content? Or, I and Q signal from baseband are totally different in QPSK
mode.

10th September 2007, 16:29 #12
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why i/q downconverter or upconverter mixer is used
I/Q is used to perform complex modulation/demodulation. In low IF， I/Q down mixer
is used to do spectrum shifter and reject image at negnative frequency with polyphase filter.
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