why i/q downconverter or upconverter mixer is used

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

ads iq mixer

i/q modulation means double the information on the same BW, hence more spectrum effficiency

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

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.

baseband i and q downconverter

safwatonline said:

i/q modulation means double the information on the same BW, hence more spectrum effficiency

Click to expand...

The signal content transmitted by I and Q from Baseband DAC are identical
or not identical? Or, the I path and Q path have a 90° phase difference?

Added after 1 minutes:

khouly said:

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

Click to expand...

Why I need I and Q in Direct Conversion? The I Q path required by low IF
is due the image rejection, but how come Zero IF need I and Q also?

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.

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?

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: 1-sin(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

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

biff44 said:

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

Click to expand...

Does baseband processor use adding and substracting to get real and imag
part signal?

biff44 said:

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

Click to expand...

Does the different signals of I and Q have any relation, such as phase delay but
identical content? Or, I and Q signal from baseband are totally different in QPSK
mode.

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.