I want to up-convert a baseband sinusoidal signal of frequency $4000 \, \rm{Hz}$ using a carrier frequency of $20\, \rm{KHz}$. I read that one can up-convert a baseband signal to a passband signal using the following.

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But after convolution with a carrier cosine, I get a $16\, \rm{KHz}$ signal. What could be the problem?

I used following Python code to do convolution with carrier frequency: samples * cosineSignal.

  • 1
    $\begingroup$ Multiplying a 4kHz tone with a 20kHz carrier will produce 2 tones, at 16 and 24kHz. What else were you expecting? What is your sample rate? $\endgroup$
    – Juancho
    Oct 18 '17 at 1:01
  • $\begingroup$ I want to up-convert the signal to 20 kHz passband signal to send. I'm using 48 kHz samp rate on baseband signal. $\endgroup$
    – JayHeo
    Oct 18 '17 at 1:34
  • 2
    $\begingroup$ You should not convolve with a carrier signal. You should multiply! $\endgroup$ Oct 18 '17 at 13:04

You are right in that: multiplying your signals in the time domain corresponds to a convolution in the frequency domain. A pure complex exponential carrier is basically a peak (a Dirac delta) in the frequency domain. Convolving with that corresponds to a "shift" - which is how you modulate. So far so good.

However, you forgot that your signals are real-valued sinusoidals, which have a (Hermitian) symmetric spectrum. Your baseband signal in the frequency domain then consists of two peaks, one at $4\,\mathrm{kHz}$ and one at $-4\,\mathrm{kHz}$! (The same goes for your carrier actually, but this is not relevant in this case.) Both peaks are then shifted by $20\,\mathrm{kHz}$, so that you end up with $16\,\mathrm{kHz}$ as well as $24\,\mathrm{kHz}$.

Since you seem to be simulating this, and use a sample rate of $48\,\mathrm{kHz}$ across the board, you might run into sampling issues as explained in @hotpaw2 's answer, which is why you might only see the $16\,\mathrm{kHz}$ peak.


A 48k sample rate is too low to reliably reproduce your upper side of 20k+4k = 24kHz. Thus you end up with a LSB (lower sideband SSB) signal, not an AM signal.


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