Implementing demodulation in a different way

[atferrari]

I revisited the concept of demodulation of an AM signal.

Isn't the output of the 455 KHz IF filter, similar to a sampled signal as I could get from a DAC process?

If so (in the hope of not being wrong assuming the above) could I obtain the baseband just by using an active filter implemented with opamps of suitable GBW, surely very expensive/hard to find?

What are the flaws of my reasoning?

 

[Nigel Goodwin]

You need to rectify it as well, then filter it.

 

[atferrari]

What op amps?

Ah, yes. Rectifying first.

Actual implementation as I propose would it be hard to do?

What opamps should I try?

 

[Nigel Goodwin]

Ah, yes. Rectifying first.

Actual implementation as I propose would it be hard to do?

It would be fairly pointless, the whole idea of AM modulation is that the carrier frequency is considerably higher than the data, so a simple RC filter after the rectifier is all that's needed.

 

[RadioRon]

The output of a 455KHz IF filter when receiving an AM broadcast is not the same as a sampled signal. An active filter can be one of many configurations so you should specify one of the basics, like high pass, low pass, bandpass for example. In any case, if you operate the op-amps in their linear range then you will not recover the modulation. A non-linear process of some sort is needed.

 

[atferrari]

Ron,

It seems that I failed to convey the idea of a filter. What else if not an LPF?

Pity that I cannot try that right now.

 

[Nigel Goodwin]

It seems that I failed to convey the idea of a filter. What else if not an LPF?

As I've told you repeatedly, you MUST rectify it before filtering, obviously a low-pass filter is what's required, but an opamp active one isn't needed.

 

[RadioRon]

If you look at the spectrum of a 450Khz IF signal from an AM receiver, you will see a carrier at 450Khz surrounded by sidebands extending perhaps 5KHz either side of the carrier. Those sidebands contain the information that you want. The problem is that you need to move those sidebands from 450KHz down to 0 Hz. This is essentially what AM demodulation does. Passing the signal through a low pass filter will not move the sidebands down to 0 Hz. In order to do that you need to perform a mathematical multiplication of the incoming signal with either a carrier from a local oscillator (the old BFO function found on shortwave receivers), or more simply by multiplying the incoming signal by itself. Here is the math: Multiplying a sin(2*pi*f1*t) by a cos(2*pi*f2*t) results in 1/2*(sin(2*pi*(f1+f2)*t) + sin(2*pi*(f1-f2)*t)). so, you end up with the sum (in frequency) of the two incoming signals and the difference (in frequency) of the two. Now, if f1=f2 then you end up with the signal at 2*f2 plus another copy at 0Hz. If one of these signals has sidebands, those also end up around 2*f2 and 0Hz. If you mix an incoming AM modulated carrier with itself, the sidebands retain their relationship to the carrier and so you still get a clean copy of the sidebands on either side of 0Hz. A low pass filter can then pass these sidebands that got moved down to around 0Hz (baseband). This multiplication process can only be done with a non-linear function. You can use a mixer, for example. The simplest type of mixer is a single diode, which is the classic simple AM demodulator.

 

[atferrari]

Clear now. Thanks to you both.

 

[WTP Pepper]

Use a synchronous demod circuit. You amplify the AM signal to give you a square wave at the carrier frequency. A phase locked loop would also help when high levels of modulation are present. You then use this to "sample" the un amplified AM signal to extract the modulation (known as the envelope) and filter. 1 x high gain amp at the carrier freq + 1 x CMOS 4066 to act as a sampler + a filter.

Any four quadrant multiplier circuit can do this although it will also respond to FM signals.