IQ modulator or mixer？

[justtry114]

Hi all,
I have been always confusing that,why we always choose to design a transimitter or receiver architecture with IQ signal, why not use real signal with mixer? Use transmtter for example, if we use IQ modulator, than we need a dual channel DAC and a modulator, if we use real signal, then a single channel DAC and a mixer is ok. That is if we don't use IQ signal, we can save one channel of DAC and also the PCB size. The only advantage using IQ I know so far is image rejection.
Can anyone heple me to make it clear? Thanks in advance!

 

[Terminator3]

Maybe search for SSB and DSB modulation will help you clear this out. All i can remember now is about some about power efficiency.

 

[biff44]

to achieve a complex digital modulation, you need a much higher clock rate (and more bits) on the DAC to generate the real signal as opposed to using two DACs and an I/Q generation method. Even today, high effective bits and clock rate are not easily achieved.

 

[BigBoss]

I-Q Modulation is used for Bandwidth and Spectrum Efficiency because I-Q Modulated signals occupy less bandwidth (like SSB).

 

[justtry114]

yeah, thanks!
Some kind of complex modulation like QPSK,QAM need IQ modulation. But this complex modulation can be implemented in baseband in FPGA by using NCO and digital modulator, then this modulated signal can be digitally upconversion to IF, then the pysical analog frontedn only need a single channel DAC and a mixer. And I think the cost of a DAC channel is less than the additional cost of FPGA to achieve digital modulation and up conversion. Ofcoz the FPGA has to run at a higher speed at this case.

 

[FvM]

I fear the question is too unspecific. Which transmitters or receivers are you exactly referring to?

I presume you know that IQ-topology can't be avoided for complex modulation schemes, e.g. QAM, both with TX and RX. So I assume you are talking about simple modulation schemes like ASK or FSK.

Transmitters for the said "simple" modulation schemes usually don't utilize IQ modulators, receivers however do. And yes, image rejection is a sufficient reason to do so.

P.S.:

But this complex modulation can be implemented in baseband in FPGA by using NCO and digital modulator, then this modulated signal can be digitally upconversion to IF, then the pysical analog frontedn only need a single channel DAC and a mixer. And I think the cost of a DAC channel is less than the additional cost of FPGA to achieve digital modulation and up conversion.

How do you implement the "digital modulation"? The standard implementations I'm aware of, even in FPGA based SDR are using IQ-modulation. And transceivers usually don't involve FPGA rather than simple RF mixed signal ASICs.

 

[justtry114]

I fear the question is too unspecific. Which transmitters or receivers are you exactly referring to?

I presume you know that IQ-topology can't be avoided for complex modulation schemes, e.g. QAM, both with TX and RX. So I assume you are talking about simple modulation schemes like ASK or FSK.

Transmitters for the said "simple" modulation schemes usually don't utilize IQ modulators, receivers however do. And yes, image rejection is a sufficient reason to do so.

P.S.:

How do you implement the "digital modulation"? The standard implementations I'm aware of, even in FPGA based SDR are using IQ-modulation. And transceivers usually don't involve FPGA rather than simple RF mixed signal ASICs.

I am sure there is digital demodulatio for FPGA, for IF receiver, IF signal is downconversion to IF and sample directly, then digtal downconversion to baseband and digital demodulated. And I think it's possible to implement the transmitter with same inverse process. Because I need to design a OFDM RF frontend with baseband signal processing in FPGA, and the size of PCB is the most important thing!

 

[Terminator3]

I am sure there is digital demodulatio for FPGA, for IF receiver, IF signal is downconversion to IF and sample directly, then digtal downconversion to baseband and digital demodulated. And I think it's possible to implement the transmitter with same inverse process. Because I need to design a OFDM RF frontend with baseband signal processing in FPGA, and the size of PCB is the most important thing!

i think problem would be here: suppose we use some one-channel IF signal to modulate LO with simple one-diode upconverter, then we get LO+IF and LO-IF. DSB modulator, regardless of all digital parts.

 

[FvM]

I am sure there is digital demodulatio for FPGA, for IF receiver, IF signal is downconversion to IF and sample directly, then digtal downconversion to baseband and digital demodulated. And I think it's possible to implement the transmitter with same inverse process. Because I need to design a OFDM RF frontend with baseband signal processing in FPGA, and the size of PCB is the most important thing!

It's not possible with baseband processing only. Using a single DAC for the transmitter is possible if the DAC runs at least at a subharmonic of the carrier and the carrier band can be selected with a band pass filter. Respectively, a receiver with oversampling ADC must be able to supress all image frequencies in front of the ADC, as shown in your block diagram.

 

[Fovakis]

I think the anwer said by FvM "" ""I fear the question is too unspecific. Which transmitters or receivers are you exactly referring to?

I presume you know that IQ-topology can't be avoided for complex modulation schemes, e.g. QAM, both with TX and RX. So I assume you are talking about simple modulation schemes like ASK or FSK."""""""'

You cannot avoided for the QAM modulation that maybe you will use cause this is how it works. If you were speaking about ASK/FSK or AM modulation shemes we have to speak about the waste of energy from the image range that is useless for voice transmission for example.

 

[FvM]

The signal to the antenna is real, in so far it might be generated directly, if you have a sufficient fast DAC and can supply the data stream. "Can't avoid IQ for complex modulation schemes" is more a practical consideration for VHF up to microwave frequencies. But direct RF generation is surely an option for SW transmitters.