How to modulate a 50KHz square, usign a 1kHz triangle pulse

[pakachunka]

How can I modulate the amplitude of a 50 kHz square wave form, using a 1 kHz triangle pulse? Can I use a 741 for that?

 

[Nigel Goodwin]

Re: How to modulate a 50KHz square, usign a 1kHz triangle pu

How can I modulate the amplitude of a 50 kHz square wave form, using a 1 kHz triangle pulse? Can I use a 741 for that?

You need some kind of VCA ( Voltage Controlled Amplifier ), one way would be to use an FET to make a controlled attenuator - it's often used in AGC and compressor circuits (try a web search). You used to be able to buy specific attenuator I/C's - for volume control and such like, but I think they've been discontinued now.

 

[Ali Sajjad]

How about using the circuit below?? its works quite easily.

-Ali Sajjad

 

[Nigel Goodwin]

How about using the circuit below?? its works quite easily.

-Ali Sajjad

That's delightfully simple - and should work quite well for a squarewave input - bearing in mind the limitations with the control voltage, because of the 0.7V Vbe drop.

 

[Ali Sajjad]

Nigel, i am quite impressed with your experience in the field of electronics.. u r very experienced and skilled ( perhaps "skilled" does not suit here)

Would you please explain the reason why this circuit would not be suitable for waves other than sqaure wave? For the nolinear behaviour of a BJT or the distortion encountered at region near the 'C-E' cuttof?

I hope you will clarify..

- Ali Sajjad

 

[Nigel Goodwin]

Would you please explain the reason why this circuit would not be suitable for waves other than sqaure wave? For the nolinear behaviour of a BJT or the distortion encountered at region near the 'C-E' cuttof?

To be honest, I'd have to try it to be sure, but with a sinewave (or other slow rising shape) I suspect it would probably distort it at the bottom end, due to operating in a non-linear region. Used with a square wave, essentially as a switched HT supply, it's not going to matter.

One thing I had just forgotten - you wouldn't get a sinewave out at all (I don't think) - the actual input to the circuit is to the base, the collector is just providing a switched HT supply. So a sinewave wouldn't be attenuated through the transistor, it will just provide a varying HT supply, which will be clipped at the voltage on the base - probably :roll:

It's easy enough to test, feed the base from a potentiometer across the supply (no need to bother with a triangle wave for testing), and input a 5V p-p sinewave to the collector - obviously, the sinewave must be positive going only - referenced to 2.5V rather than ground, otherwise it would be rectified.

An interesting little experiment!.

 

[Roff]

If you want to build a true amplitude modulator (analog multiplier), see Fig. 5 in National Semiconductor's **broken link removed**.

 

[pakachunka]

Thanks

I was thinking on a 2 transistor solution, but Al solution sounds brillant.

Let's see if the transistors agree with him!

I will try, and tell you what I got later.

 

[pakachunka]

Success! And a lot of noise!

Dear Friends,

I tried something similar to what Ali suggested, but I little bit different, if you understand what I mean!

Check it below!

 

[pakachunka]

Complement

By the way,

I forgot to say that the circuit above works, but generates a lot of noise...

But the AM is completely triangle, except for a few glitches on the scope screen, which I don't know if are there because of noise, or because of the trigger of the scope.

Now I will use this to drive a flyback, and try to get a nice 0-3kV ramp... hopefully.

 

[pakachunka]

It Works!

I put the modulated output of the circuit showed in the primary of a Flyback, and it works!

Thanks for every body!