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What are the main advantages from a theorical point of view of a Heterodyne transmitter versus a Homodyne Transmitter?

I've seen the main difference is the VCO or PLL connected to the Mixer after the Modulation/Demodulation phase to take the modulated signal to RF frecuencies for the antenna.

If would be possible to take the modulation signal directly in RF frecuencies taking a high Carrier Frecuency, wouldn't be a easier choice?

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    \$\begingroup\$ Good luck doing that at 78GHz \$\endgroup\$ – PlasmaHH Dec 1 '17 at 13:44
  • \$\begingroup\$ No, but maybe at 400 Mhz would be possible \$\endgroup\$ – EduardoG Dec 1 '17 at 13:52
  • \$\begingroup\$ but with what effort? And who is using 400MHzish signals these days anyways? Had a look at G5 recently? \$\endgroup\$ – PlasmaHH Dec 1 '17 at 13:54
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    \$\begingroup\$ 400 Mhz is into UHF band, are u going to say UHF is useless nowadays? \$\endgroup\$ – EduardoG Dec 1 '17 at 13:56
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    \$\begingroup\$ For different applications different receivers are used. If you do dsp direct conversion is probably the thing to go because you can easier deal with LO leakage and frequency images, which is rather hard to do over a wide range with analogue stuff. There is no one size that fits all \$\endgroup\$ – PlasmaHH Dec 1 '17 at 14:09
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The advantages of a hetrodyne TX are basically that the modulator does not need to run at the output frequency, and it is MUCH easier to build a modulator having good linearity at 70MHz or so then at 24GHz. Because the modulator output frequency is fixed, you can also add useful filtering here to restrict the occupied bandwidth at the output of the modulator, much harder to produce a 2MHz stopband at 24GHz then at 70MHz.

Now you do see homodyne designs, usually where modulator linearity is unimportant or at lowish frequencies, most of the FM broadcast sets are homodyne, as are the 433MHz SRDs and most of the AM broadcast stuff (Which actually often looks more like a modern switch mode supply then a classical transmitter), the common thread is that none of these things are particularly fast by modern standards.

One interesting development has been the use of digital upconversion directly from a complex baseband signal, you can think of this as homodyne or even zero IF hetrodyne depending on your view, this is usually seen at VHF or below with some parts that interpolate and work in a higher Nyquest zone getting up into the UHF, however you can view the DAC in such things as being a mixer, so these are hetrodyne if you squint just right.

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