Simple Envelope Detectors

[EEEStudent]

Hi,

I have to write a report about envelope detectors for AM signals and have a few questions that I just cannot find the answers to and so I'm hoping someone here could help me.

Here are the questions:

1) Why is an envelope detector very susceptible to noise?

2) Why can a DSB-SC signal not be demodulated with an envelope detector?

3) Why does the envelope detector not work if the Carrier signal from a DSB-SC is added back in but is out of phase?

I think 2 and 3 are probably linked and it is just a case of me not fully understanding how it works.

I would greatly appreciate any help.

 

[Analog]

Hi,

I have to write a report about envelope detectors for AM signals and have a few questions that I just cannot find the answers to and so I'm hoping someone here could help me.

Here are the questions:

1) Why is an envelope detector very susceptible to noise?

2) Why can a DSB-SC signal not be demodulated with an envelope detector?

3) Why does the envelope detector not work if the Carrier signal from a DSB-SC is added back in but is out of phase?

I think 2 and 3 are probably linked and it is just a case of me not fully understanding how it works.

I would greatly appreciate any help.

Dear EEEStudent,

Welcome to our forum. You may notice that the questions on this forum that have the most answers are from those people who are working on circuits or problems that they have done the work on, are at an impass, or need advice on which way to go.

Most of us are not here to answer homework problems, many times the response "go do your own work" comes to mind. I happen to be a professor in the area of which you are asking questions... If it were not the end of the semester break, I'd suspect you were one of my students, because those are exactly the type of questions I'd ask. Anyhow, to facilitate you doing your own work, below are three links that will get you answers to the questions, but, alas, you must do the reading yourself...

A better question that may get you more answers would be something like this:

"In my reading of the subject of amplitude modulation, I found that a superheterodyne receiver has excellent selectivity, with respect to the generic diode envelope detector. I noticed that a superhet actually incorporates a diode detector as well - so I am wondering how a superhet can be better than a diode detector, when it actually incorporates one to begin with"?

Do you see the difference in the question vs. yours? It shows that the person has done the background work, and is seeking advice based on that. It is not stated in a way that sounds like "please do my work for me".

Does that help?

**broken link removed**
https://en.wikipedia.org/wiki/Envelope_detector
**broken link removed**

 

[stevez]

EEEstudent - thank you for being honest about your role (student) and the assignment (write a report). As you might guess, we see requests that are as bold as "do my homework for me" without so much as "please." Your request is certainly not that. You/we are lucky to have people like Analog to help us.

Analog - I hope EEEstudent appreciates the time you took to write this - my guess is that he/she does. It would be nice to use this response for the many other requests from student. Many of us feel as you do - we don't mind helping those who try to help themselves.

 

[EEEStudent]

Thanks for the replies.

In my defence I did do research before asking; I just could not find the answers to those questions. I will follow the links you provided and hopefully they will help (although I have already seen the wikipedia entry and it didn't).

Asking a question here was a part of my research for the answers, and I do not really see the difference between asking a question on a forum and asking the question of my lecturer directly (something I would have done had it been possible).

 

[EEEStudent]

I hope you don't mind helping me one more time, just to check that I have understood it correctly.

Am I correct that an envelope detector cannot demodulate a DSB-SC signal because |A + m(t)| > 0 for an envelope detector to work and in a DSB-SC signal A=0 and therefore it doesn't demodulate?

 

[Analog]

I hope you don't mind helping me one more time, just to check that I have understood it correctly.

Am I correct that an envelope detector cannot demodulate a DSB-SC signal because |A + m(t)| > 0 for an envelope detector to work and in a DSB-SC signal A=0 and therefore it doesn't demodulate?

Dear EEET student,

The carrier is required in a diode (envelope) detector because the diode detector simply rectifies the incoming signal and eliminates the DC offset. Perhaps a diagram would help? The figure below shows a standard DSB+C modulation scheme. The top signal is the intelligence signal that we start with. Below that is the modulated signal, double sideband with carrier (DSB+C), i.e. standard AM. The third signal down is what the diode detector will do - it simply rectifies the input signal, chopping off the lower half. The final signal at the bottom is what it would look like filtered and DC offset eliminated. Note that it is our original signal back again.

The other figure is the same intelligence signal, this time modulated using DSB-SC (supressed carrier). Along the same lines, my simulation shows what would happen if you demodulated using a diode detector again. Note that you get a full wave rectified version of the original signal! Hardly a satisfactory way of getting the original back.... Therefore, it should be obvious that the carrier is absolutely necessary to get back the original signal using an envelope detector!