# Upconverting an already modulated signal using mixers or multipliers

#### samihawasli

Joined Oct 19, 2020
10
Hello all,
Most of my experience in frequency conversion lies within mixers, so I had a few questions concerning frequency multipliers. I can understand how a mixer can upconvert an already modulated signal (both analog and digital) while still preserving the modulation. The mixer will actually multiply the two signals together, and thanks to a non-linear relationship of your mixer, higher order terms are created. Those higher order terms preserve the amplitude and phase information. (please correct me if I'm wrong)
A frequency multiplier is much different, obviously. From what I've read so far, it seems a frequency multiplier can't upconvert an already modulated signal? Clearly something would be upconverted, but would you lose most of your modulation info because a multiplier also multiples the phase? Someone at my work sort of confirmed this, but then told me a multiplier would still work for digital modulation? Perhaps I fundamentally don't understand what digital modulation is, but can someone with more experience shed some light on this? (if you could use some equations to help show why, bonus points!)
I found a few papers, behind pay walls, that use multipliers to upconvert different bits of QAM. Does the multiplier work in this situation because you are only upconverting a single vector? Wont the phase of that vector get altered the same as an analog modulated signal would?
I will keep researching myself, but any help would be much appreciated and save me a lot of time,

#### Ramussons

Joined May 3, 2013
1,059
AFAIK, you can have Frequency Multiplication using a Non Linear Device or a PLL. In a Frequency Multiplier for a FM signal, the nth Harmonic with have 'n' times the original frequency deviation. Can that be described as "retaining the Modulation"?
That is, a 1 MHz FM with a 50 KHz deviation will be, on the 5th Harmonic, 5 MHz with a 250 KHz deviation.

You will loose all the modulation from an AM / QAM signal on multiplication since the Amplitude Information is stripped on a frequency multiplication if you use any of the 2 methods mentioned above.

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