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Click here to play with a mixer spreadsheet that can generate spectral inversion of S-parameters
New for August 2021. Spectral inversion happens when you mix an RF signal with a high-side local oscillator. "High side" in this context means that the LO signal is higher in frequency than the RF signal. As you know, any signal of interest has a certain amount of bandwidth. If you are operating a radar and looking for a Doppler shift, high-side mixing will invert the Doppler so that objects approaching will be shifted down in frequency (opposite of normal), and objects receding will be shifted up. Of course, you can call on your digital people to just deal with it....
What if your signal was audio? Let's say you are processing an AM band signal with 20 Hz to 5kHz for audio transmissions and you mixed it with high-side LO. Let's further say that the signal was someone playing a piano. 20 Hz is approximately the tone of the lowest "E" on the keyboard, while 5000 Hz is actually above the range of a normal keyboard (oops there goes the analogy), approximately a D sharp, about eight octaves apart. When these tones get reversed, along with all other tones, it is easy to see that anything musical is going to be destroyed with spectral inversion. So is the human voice.
Here's a video featuring spectral inversion of audio... and how to recover the signal using an audio signal generator (the LO) and a double-balanced mixer (DBM).In the video, the mixer is also referred to as a ring oscillator, perhaps that is "Ham" terminology but it is confusing when you think about ring oscillators made from an odd number of inverters. The DBM does a good job of suppressing the LO tone, otherwise the processed signal would hurt your ears. This video was produced by Amateur Radio VK3YE. You can look at other videos on this channel, here.