# How to choose frequency of local oscillator for heterodyne principal

I am trying to implement the heterodyne principle in order to make an AM signal demodulator that can demodulate signals on a set bandwidth(550 - 1720kHz). Is there a formula or some other method of deciding which local oscillator frequency to use? Or have I completely missed the mark on what exactly this does?

Your bandwidth is between 550 kHz - 1720 kHz, but that is centered around a particular frequency. Now it depends if you want to move the signal to baseband (centered at zero frequency) or some other intermediate frequency. You get to decide this as the system designer.

Given a signal centered around the frequency $$f_c$$, that you wish to modulate to a new frequency $$f_0$$, the expression is simple for the required oscillator frequency $$f_{LO}$$

$$f_0 = f_c - f_{LO}$$

$$\rightarrow f_{LO} = f_c - f_0$$

The signs assume that you are shifting down from $$f_c$$ to $$f_0$$, which is usually the case. For example, when getting a signal to baseband, you set $$f_c = f_{LO}$$ so that $$f_0 = 0$$.

Some systems do a combination of upconverting and downconverting, in which the equations have their signs changed to accomodate an increase or decrease in frequency.

• Thank you! What resources did you use to learn about this topic? I want to look into this further. Jun 24, 2021 at 18:11
• @user58046 The internet should be good enough to get the basics. Many signal processing texts will expand further. If you think this answers your question, please accept the answer by clicking the check mark to help the site. Jun 24, 2021 at 18:16