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What is the mix of a received frequency 27 MHz and a oscillator frequency of 27.001 MHz?

I expect the mix of them would be 1 KHz.

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    $\begingroup$ Beware of the language: summing frequencies is not the same as summing signals of different frequencies, which is also different from mixing two signals. Summing signals is a linear process, so you don't get new frequency components. Mixing is a non-linear process, so you get intermodulation components as explained by @pete-nu9w $\endgroup$ – Juancho Aug 10 '15 at 19:09
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If you simply add two sine waves together, it's a linear operation and you do not get mixing. So you would see only the 27.000 and 27.001 MHz components.

If you want to get the sum or difference frequencies, you have to "mix" them in a non-linear device, like a diode or a switching transistor. Then you will get components at .001 MHz (the difference), 54.001 (the sum), as well as some left over power at the original 27.000 and 27.001 frequencies.

Actually, you will get even more frequencies than these as "higher order" mixing products, like 2x27.000 - 27.001 = 26.999 MHz and many other combinations, as well as the harmonics (double, triple, etc.) of 27.000 and 27.010. (But the high order mixing products are probably going to be weaker.)

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When you mix two sine waves with different frequencies you get four frequencies in the result: the two that are coming in, the sum of the two, and their difference. So with 27 MHz and 27.001 MHz you'd see 27MHz, 27.001 MHz, 54.001 MHz (27 + 27.001), and .001 MHz (27.001 - 27). For a radio receiver you'd typically run that mixed-up output throw a low-pass filter so that all you see is the difference frequency, 1 kHz.

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  • $\begingroup$ I mixed two sine waves on audacity, and looking at the plot spectrum I only see 880 and 1000hz waves, the ones I generated. I don't see the 120hz or the 1880 on the spectrum. Can mixing be shown with audio? $\endgroup$ – Skyler 440 Aug 10 '15 at 19:38
  • $\begingroup$ @Skyler440 - it works for all waves. As Juanco points out in a comment, this comes from mixing the signals, not just adding them together. $\endgroup$ – Pete NU9W Aug 10 '15 at 19:43
  • $\begingroup$ How do you mix two signals without adding? $\endgroup$ – Skyler 440 Aug 11 '15 at 15:44
  • $\begingroup$ @Skyler440 - you multiply them. <g> $\endgroup$ – Pete NU9W Aug 11 '15 at 16:54

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