The QPSK baseband signal is composed of real (I) and imaginary (Q)
components, or complex values. Using a Quadrature modulator, you can
combine these onto one sideband. Like the SSB signal of FreeDV, it is
a complex signal. But creating an analog SSB signal is what makes VHF
SSB so expensive. If you could do all of this digitally, and then
modulate with a complex signal, I would imagine expense and stability
requirements would be reduced. The signal would be half the bandwidth,
and no carrier.

If you look at FM radio stations on your SDR, you see the signal
duplicated on both sides of the carrier, and also a large carrier when
the modulation is low. I guess you could say FM has built-in

Although IQ values have to be calibrated, as any error results in a
large carrier and/or alternate sideband. A radar I used to work on,
balanced the IQ channels every slant. The beam would go to the top,
the IQ channels balanced, the beam would scan down (phased-array),


On Fri, Feb 2, 2018 at 11:43 AM, Adrian Musceac <> wrote:
> If you don't mind me asking, why do you need a complex input? The job
> of the frequency modulator is to transform a real domain signal into
> complex domain. The usual case is you have baseband audio represented
> as floats as an input.

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