AM Modulation

--- OK, let's get into it a little deeper.

Let's take the stuff on the right hand side and make it look like this:

E1 | [R1] | +----E2 | [R2] | GND

Where E1 is a 1VPP 1MHz sinusoid, R1 is the sum of the resistance of the generator and the resistance of the fixed resistor, R2 is the resistance of the LDR, and E2 is the voltage developed at the junction of R1 and R2.

Further, let's say that R1 is equal to 1000 ohms and that the brightness of the LED has been adjusted with the DC supply in order to make the resistance of the LDR equal to 1000 ohms.

Under those conditions our circuit will look like:

1V | [1000R] | +----E2 | [1000R] | GND and the voltage at E2 will be equal to:

E1 * R2 1V * 1000R E2 = --------- = --------------- = 0.5VPP R1 + R2 1000R + 1000R

Let's make the LED a little brighter by cranking up the power supply until the LDR's resistance falls to 900 ohms. E2 will now be equal to:

E1 * R2 1V * 900R E2 = --------- = --------------- ~ 0.474VPP R1 + R2 1000R + 900R

Likewise, if we make the LED a little less bright so the LDR's resistance climbs to 1100 ohms, E2 will be equal to:

E1 * R2 1V * 1100R E2 = --------- = --------------- ~ 0.524VPP R1 + R2 1000R + 1100R

Since we've changed the amplitude of the signal at E2 (let's call it the 'carrier') by changing the resistance of R2, we have _modulated_ the amplitude of the carrier, and we have an AM signal at E2.

Now, let's adjust the DC supply so that E2 is again 0.5VPP, and now let's crank up the output of GEN1. What do you think is going to happen?

OK, I'll tell you. When the AC signal on the secondary of the transformer _adds_ to the power supply voltage the LED will get brighter, and when it _subtracts_ from the power supply voltage it will make the LED a little dimmer.

Remember from before what happened when we made the LED brighter and dimmer? Right! We amplitude modulated the carrier. The same thing will happen now, except that the modulating frequency will be 1000 Hz.

Don't believe me?

Build it or simulate it.

-- John Fields Professional Circuit Designer

You're playing with words. Amplitude modulation is multiplication, and whether you consider multiplication to be "linear" is up to you.

John

--
Well, words are really all I have to play with, here, but since:


     y = mx + b


and since:


     dy
    ----
     dx 

is constant everywhere on that line, multiplication is _inherently_
linear.

I can simulate it well enough in my head. Your circuit is not a multiplier so it does not create the sidebands required of an Amplitude Modulation process.

Your circuit is a mixer in the sense of an audio mixer. Yes you are "modulating) a summation point in the circuit, but that is very different from "AM,"

Let's make a couple minimal changes to obtain an Amplitude Modulation circuit, the output of which will contain the products and also the unfiltered inputs:

5VPP Carrier (f1) input (referenced to gnd) + | [1000R] | +-------------+----------------+ | | [1000R] [Diode] AM signal output | | | +----------------+ | | | GND +

Variable Modulation signal (f2) input (referenced to gnd)

Linear Modulation is merely a term to denote that, in the case of AM, the modulation product(s) contain one or more sidebands, each of which has the same bandwidth and shape as the modulating signal (voice, tone, whatever).

The process of doing that is necessarily, non-linear.

Don

We keep going over and over this ground, about whether you need nonlinearity to get sidebands. It tends to be, as in this case, people who half-remember stuff from lectures vs other people, some of whom know nothing and some of whom can actually still do algebra. (You know who you are.)

We talked about this in September, and actually generated some insight:

Cheers,

Phil Hobbs

--
Not well enough to work it out all the way through, I\'m afraid.

It's an herb, kind of like parsley or oregano, but it tastes kinda like laundry soap.

Cheers! Rich

You guys just have no taste... Cilantro is good stuff.

But then, I'm the kind of guy who like fennel ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
     It\'s what you learn, after you know it all, that counts.

--
Gringos call Cilantro Coriander.

It's great stuff, when fresh, IMHO. My recipe for Jinga (shrimp) Dhania (cilantro) Masala has a cup of fresh coriander/cilantro in it, along with cumin, fennel and green chilies. The sauce is bright green.

Or sometimes "Chinese Parsley".

[Oblamehumor] What's the difference between parsley and pussy? Did you hear about the parsley farmer who got behind in his child support payments? They had to garnish his wages.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

Nobody eats parsley.

John

--
I have just posted something to abse that I think you\'ll be very
interested in.  It\'s under the same heading as this thread.

Enjoy! :-)

The LDR divider does change the amplitide of the carrier GEN2, in response to another signal, GEN1. So it's a modulator, and it does create sidebands. Heck, it generates *more* sidebands than any self-respecting AM modulator should.

John

Saw an interesting study about a year back which showed that the average length of a marriage these days is a gnat's whisker over 20 years, the same as it was several decades ago, and several hundred years before that. The changing part of the equation is lifespan.

Cheers.

Ken

Like I said - cucumber!! :-)

Cheers.

Ken

Aha. That grows in small bottles in supermarkets doesn't it?

Cheers.

Ken

[snip]

Divorce is so easy these days, and no one seems to take the trouble to make sure they've chosen a partner correctly.

Ours is one of those teenage marriages that was supposed to fail... in fact my mother-in-law was fond of saying, "Wait one more day, until April Fool's Day" ;-)

March 31 will mark our 46th year!

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
     It\'s what you learn, after you know it all, that counts.

--- We like it (cilantro) in beef fajita tacos, sometimes. Beef fajitas are flank steak that's been marinated and grilled after the membrane's been removed. To make a beef fajita taco, you get some beef fajitas that've been sitting on the grill for a while and are ready, and you roll a flour tortilla around them. Some folks like to add guacamole, or maybe pico de gallo, or maybe chile con queso to the fajitas, once they're on the tortilla, and then roll it up and eat it.

I like to add chile con queso, and to have a nice, cold Negra Modelo on the side to wash it down with.

BTW, "fajita" is pronounced like 'fa' in do,re,mi,fa, 'he' like the opposite of 'she', and 'ta' like the 'ta' in 'tardy'

--- Larkin beat me to it...

---

--- Now my day is complete!

Reminds me of...

Did you hear about the woman who ate a doorknob? It turned her stomach.

-- John Fields Professional Circuit Designer