Need a circuit-AM modulation using multiplier IC

A mixer ?

Rene

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Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

You need to tell us the frequency of the carrier you need to modulate.

The Analog Devices AD734 is good to 10MHz, the AD834 and AD835 rather higher.

Page 10 of the AD734 data sheet talks about using it as a low distortion mixer.

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Bill Sloman, Nijmegen

On a sunny day (6 Jan 2007 23:28:21 -0800) it happened "venkatsr" wrote in :

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Demodulate AM with a diode :-)

+5 CMOS switch | R1 1/3 74HC4053 [ ] 1k RF C1 + | A1 Y1 C2 AM modulated [ optional ] audio ----|[]--------------0 0-------||-----------------------[ filter ] - Q 1Vpp 100u | R2 / 100p | [ ] 1k / [ ] R3 | B1 0 | | 100k | | | X | /// /// | /// | | | 0-5V 1MHz square wave carrier

Normally A1 is at 2.5 V. The RF at X causes Y1 to switch between 0V and 2.5V, so 2.5 pp RF. C2 blocks any DC and C2 R3 form a high pass for the RF.

When audio signal goes up to say +1V, then A1 will be at 3.5V, and there is 3.5Vpp RF out. When audio goes low to -1V, A1 will be at 1.5V, and there is

1.5V pp RF out.

Pure AM. A filter is needed to get rid of any harmonics in the output.

100 % modulation will happen with 2.5Vpp audio in, as then the RF will change between 0V and 5V pp. RF AM D1 C2 Q ________|\\|__________________||________ Audio |/| | | || | diode | | 100n | germanium [ ] === [ ] | R1 | C1 | R2 | 100k | 220p | 1M /// /// ///

Jan Panteltje a écrit :

Q

Yes but that is not in line with what this homework is about.

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Thanks,
Fred.

On a sunny day (Sun, 07 Jan 2007 13:46:07 +0100) it happened Fred Bartoli wrote in :

- Q

Well, the switch is a multiplier basically. Actually I should have written 5Vpp audio results in 100 % modulation.

Have a look at the MC1496 datasheet.

Then go and look at the Analog Devices AD834 or AD835 - Jim Thompson's MC1496 was a useful circuit in its time but that was long time ago, and Barry Gilbert has done better since then.

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Bill Sloman, Nijmegen

Thanks for all ur replies.. The freq of carrier may be between 10khz and 1Mhz AD634 is a type of ic l look for. I need clarifications about the modulated signal of this ic is DSBFC or DSBSC or SSB ?. And does envelope detector can demodulate any of the above AM signals? AM mod using multiplier ic and demodulate it using envelope detector is my entire work.Please help

A multiplier implies a balanced circuit, so the results will be double sideband with no carrier, since the balance keeps the carrier from getting to the output.

So you need to unbalance the multiplier in order to get the carrier at the output. That's often done by applying some DC bias, but there were all kinds of methods in the old days, any of which unbalanced the modulator so the carrier would feed through.

The only way you'd get single sideband out of this is if you put the DSB signal through a filter to knock out the unwanted sideband, or use one of the phasing based methods, though that would require a second balanced mixer.

Note that you can get AM (ie DSB with carrier) through all kinds of simpler means, since you aren't trying to get rid of the carrying. All AM modulators are mixers, translating the audio signal to radio frequencies. But since the traditional AM modulation scheme feeds an audio signal to a non-balanced stage that acts as the mixer, the carrier is there at the output without any effort.

Also note that an "envelope detector" is misleading, because the name implies that the output is following the envelope of the signal, but in reality it is acting as a mixer, so the carrier of the incoming signal beats against the sidebands to bring them back down to "baseband".

Without the carrier on the transmitted signal, it will never be able to recover the original modulating signal. ON the other hand, if you insert a "carrier" generated at the receiver at the right place, then the receiving mixer will indeed recover the modulation of the signal, beating the one or two sidebands back down to baseband.

The carrier (or one synthesized at the receiver) is always necessary, it's the key to decode the signal, but the carrier is never needed to get the signal to the receiver (as the name "carrier" suggests).

Michael

Thanks Mr. Michael ur explanation is very useful to me

Thanks Mr. Michael ur explanation is very useful to me Tell me more about Gilbert Cell

Don't pay any attention to his comments about the envelope detector.

The audio is recovered by charging a capacitor by the voltage of the envelope. The voltage across the cap, is the audio.

Don

Google is your friend. There is enough information out there for you to be able to learn enough to sort good from bad, information. This is just one.

For more enter "gilbert cell" (with the "s) into the search criteria.

Don

It implies no such thing. Many multipliers are a single diode, or if you're into tubes, a single tube Class C amp.

He can build his AM modulator test bed with one diode, a small cap, and a couple resistors.

What he uses for the carrier source and modulating signal is his choice.

Why did you change your term from "multiplier" to "mixer?" AM modulation processes require the non-linear process to perform the multiplying. Non' linear mixing will not modulate. A "non-balanced stage" has nothing to do with modulation as you have expressed it, but is is one method one may use to reintroduce a carrier to a balanced multiplier's output.

Yes, the sidebands beat with the carrier, and this is what creates the envelope. Then the instantaneous voltage of the envelope is placed across a small cap and this voltage IS the audio, or whatever, of the transmitted signal. It recovers the audio from the envelope, so it is correctly called an envelope detector.

Don

"Michael Black"

** That is a bit obscure for a novice to follow.

More simply, a multiplier will have zero output whenever one of the two input signals is zero.

This is quite unlike a normal AM where the carrier has a *fixed* amplitude under zero modulation - the modulating signal is able to boost the carrier up to double that amplitude or reduce down it to zero.

** More simply, if a fixed DC voltage is added to the (AC) modulating signal so that the sum never changes polarity, then a multiplier will produce an output that is normal AM. The fixed DC voltage then creates a steady carrier when there is no AC modulation. ** Which is exactly what it does. ** The time domain view is reality.

** The frequency domain view is a ( very useful) mathematical abstraction where time ceases to exist.

Better not confuse it with (and all novices by claiming) it is " reality " .

** The concept of a " carrier " that changes amplitude, wobbles ( or suddenly steps ) in frequency and / or phase to convey information to a distant point is a very useful one. It also inherently involves time.

....... Phil

Yeah, but it would sound like crap. ;-)

Cheers! Rich

I don't agree. It just will not be very efficient. Furthermore, the more the OP experiments with many options the faster he will really understand how AM works.

Don