1
\$\begingroup\$

In am receivers, we detect the modulating signal by using envelope detector. So, the diode rectifies the signal, and the detected signal has doubled frequency than the original. Does this mean we will hear distorted sounds, if we connect audio amplifier. Is it possible to decrease the frequency by two?

\$\endgroup\$
4
  • \$\begingroup\$ Why do you say the signal is doubled in frequency? \$\endgroup\$
    – Majenko
    Dec 28, 2014 at 17:52
  • \$\begingroup\$ Because the diode will behave as a full wave rectifier. \$\endgroup\$ Dec 28, 2014 at 17:55
  • \$\begingroup\$ You can't make a single diode be a full wave rectifier. You need 4 for that. \$\endgroup\$
    – Majenko
    Dec 28, 2014 at 17:57
  • 1
    \$\begingroup\$ Even with a full wave rectifier (uses at least 2 diodes) the carrier frequency may be doubled but the modulating frequency is not. (Unless you're using a suppressed-carrier modulation technique like DSBSC) \$\endgroup\$ Dec 28, 2014 at 18:02

1 Answer 1

3
\$\begingroup\$

In AM you have a waveform typically formed like this:

enter image description here

Rectification chops off the negative portion of that signal, thus:

enter image description here

Filtering then removes the high frequency component:

enter image description here

A capacitor then removes the DC offset:

enter image description here

Nowhere in that does the frequency change.

Even if you were to use full-wave rectification, only the frequency of the carrier would change - the modulated signal frequency will be just the same as it was.

In the image you provided in your comments:

enter image description here

The modulated signal is modulated twice - once on the positive axis, and once on the negative axis. This is the same as the top image above. However, the modulating signal has an amplitude that crosses the zero axis, so you actually end up with two signals crossing over each other, like this:

enter image description here

When you then rectify and filter that waveform you get just the positive portions of both waves:

enter image description here

With pure audio modulation (modulating two audio signals together) this can be desirable as it produces very noticeable affects and artefacts (you would never typically demodulate this signal, it would be the finished audio product in its own right). In RF modulation though it's not wanted, so the incoming signal should have an amplitude of no more than 50% of the carrier frequency, and be off-set to half-way up (and down) the carrier wave so the two sides of the wave don't cross over.

\$\endgroup\$
7
  • \$\begingroup\$ I might be doing something wrong, but my simulations give decreased frequency. By the way, how can I draw and post graphs like you? \$\endgroup\$ Dec 28, 2014 at 18:06
  • \$\begingroup\$ For that one I found a waveform on google images, then tinkered with it in Gimp. Are you doing your rectification after the filtering by any chance? \$\endgroup\$
    – Majenko
    Dec 28, 2014 at 18:12
  • \$\begingroup\$ Yeah, after the filtering. Isn't it like that: sfu.ca/sonic-studio/handbook/Graphics/Amplitude_Modulation2.gif You can see that when modulating signal reaches one period, the am signal has two periods \$\endgroup\$ Dec 28, 2014 at 18:14
  • \$\begingroup\$ Ah, you're working with pure audio AM, not RF AM. In audio AM you don't demodulate, the modulated signal is the finished product. Also you're not offsetting the incoming signal to keep it just to one side of the 0 axis - so it's crossing over in the middle. \$\endgroup\$
    – Majenko
    Dec 28, 2014 at 18:16
  • \$\begingroup\$ Sorry for not providing enough information. So, what would you suggest to do? \$\endgroup\$ Dec 28, 2014 at 18:18

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.