Dsb

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dr.power

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Hello guys,

I am newbie here.

Sorry I got a question regarding to modulation.
Please take a look at the below PDF which is an AM DSB modulator out of 4066!
The Two questions which arrive to me are:
1. Is this circuit actually reliable as a DSB modulator?? For instance, is it able to honestly work instead of an MC1495 AM modulator?

2. It uses a band pass filter (I am not sure why we need it to remove unwanted upper and lower frequencies?), How to calculate the bandwidth for it? Say I have a modulating signal of 4kHz and a carrier signal of say 400kHz, How do I calculate the best order filter for the said modulator?

Thanks a bunch.
 

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Yes, it is a reliable DSB modulator. They're often used by hobbiests to demodulate FM stereo signals.
Sorry, I'm not much help with audio filters, but if you're trying to filter out one of the sidebands, obviously a high Q filter is required.
At RF this task is usually done with crystal filters. But at 400KHz, it could easily be done with a good DSP filter.

If the purpose of the task is use some sort of LCR filter combination, then something with five stages should be more than enough to filter out the unwanted sideband. Parallel and series frequency response formulas is where I'd start.
 

Thanks sir,

Did you ever have used a modulator out of 4066 or have seen any AM circuit out of it somewhere? I would like to know the other AM structures by 4066.

Are you sure about using it to "demodulate FM stereo signals"?!

I do not know what is the usage of the bandpass filter in the above modulator but I am almost sure that it is not there to remove one of sidebands. Is there anybody to says us the main reason of using that bandpass filter?
 
dr.power said:
I do not know what is the usage of the bandpass filter in the above modulator but I am almost sure that it is not there to remove one of sidebands. Is there anybody to says us the main reason of using that bandpass filter?
Click to expand...

As I see it, that circuit isn't really an AM modulator - it's simply chopping the audio signal at a high frequency.

An AM modulator should vary the level of the carrier according to the modulation.
 
Thanks Nigel, but the PDF file tells that it is an actual DSB-SC and if needed a DSB-WC modulator!
I am in doubt about it's ablity yet. But I guess that 4066 Switch, starts switching and hence multpelling 2 signals connected to the left and upper sides of it (I mean the pic at my first post)
 
dr.power said:
Maybe looking at the below pic enlightens you what a 4066 cip is:
Click to expand...

I already know exactly what it is, as does (I would imagine?) everyone on these forums.

It's a simple switch, and it's just going to chop the audio ON and OFF.

I suppose it 'might' give a crude simulation of AM modulation, based on the tuned circuit on the right been tuned to the 'carrier' frequency. But I doubt it's something you would use if you had any concerns about quality.
 
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Well, Is there any expert in the field to kindly simulate the above circuit fro us to see if it is able to perform as a good AM modulator? I think there are several free simulators in the net but unfortunetely I never have worked by them.

thanks beforehand
 
Nigel Goodwin said:
As I see it, that circuit isn't really an AM modulator - it's simply chopping the audio signal at a high frequency.

An AM modulator should vary the level of the carrier according to the modulation.
Click to expand...

That is what it does Just the modulation is a little less precise that you're thinking of.

I'd send you a copy of my stereo FM decoder circuit which uses them but my hard disk crashed and it's away for data recovery.
There's heaps of very similar circuits on the net... like this one....
https://electroschematics.com/325/stereo-encoder-high-separation/

I thought I might have some of my circuits floating around on another forum, but this is about the best I could cough up at short notice.
https://www.austech.info/225786-post27.html

The 4066 just performs the same task as a transistor in your circuit's case. On-Off keying, which is just another form of AM.
For DSB (another form of AM again) four 4066 switches are used as a "double balanced mixer".
 

Have you 'lost the plot'?

The two examples you posted are for stereo encoders, nothing to do with AM modulators.
 
Pardon my ignorance, and my jumping into the middle of this, but if I understand Nigel's objections correctly--that the 4066, far from "modulating" the signal, simply chops it--couldn't it nonetheless operate effectively as a modulator if there was a wave-shaping circuit (e.g., tank) following it?

I'm thinking of a basic class C amplifier, which essentially does the same thing: generate a chopped (square wave) output, which then gets shaped into nice sine waves by a resonant tank. Wouldn't the same principle apply here? or am I missing something really obvious?
 
I believe CZ is correct. If you look at the OP's attached thumbnails in post #1 it is doing what CZ suggests, it has a tank filter after the chopper to filter out the harmonics. That can thus generate a DSB, suppressed carrier signal or a normal AM signal depending upon the DC bias at the input to the chopper.
 
Not really , it's not some much about the modulation as such, it's about the 4066 bilateral switches being used.

A single element used as OOK as a form of AM, and a 4066 used as a double balanced circuit for DSB.
Both are functioning as you might say, "a chopping circuit".
 
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