# How to shift signal to baseband using local oscillator frequency?

If the input to a demodulator is a signal having the spectrum shown below, how would I figure out what local oscillator frequency is required to shift the signal to baseband?

Is there a formula for this? I'm getting no where on google.

Thanks.

• Mix with the fundamental to get to baseband. (This can sometimes not work as well as expected... any slight differences in frequency (of the fundamentals) becomes a slow modulation.) Apr 27, 2015 at 19:59
• Could you explain a little better? I'm trying to do this out on paper as study revision Apr 27, 2015 at 20:07
• Remember that multiplication in the time domain corresponds to convolution in the frequency domain. Also remember that a sinusoid in the time domain corresponds to an impulse in the frequency domain (at the sinusoid's frequency, and another at the negative of the sinusoid's frequency). The question then becomes: When convoluted with the signal spectrum shown, which is centered around 100MHz, what impulse will produce a signal spectrum that's centered around 0Hz after convolution?
– Zulu
Apr 28, 2015 at 0:30