Discussion in 'General Electronics Chat' started by Zara Engineer, Apr 6, 2015.

1. ### Zara Engineer Thread Starter Member

Feb 6, 2015
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Hello

I found this circuit in an old book , I've drawn the circuit .

With :
- F : clock signal 100 Hz
- M is a DC signal from -5 to +5

The author said that the result is M*F.

Zara

2. ### Zara Engineer Thread Starter Member

Feb 6, 2015
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sorry this is the image

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3. ### DickCappels Moderator

Aug 21, 2008
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Please use a more description title next time.

It is a balanced modulator provided that you switch the non-inverting input to ground with your carrier F. This causes the circuit to switch between inverting and not inverting the signal as a function of the state of F.

Among other things, the swich can be a transistor if the base drive swings far enough below ground so that the collector-base junction does not become forward biased, with a MOSFET provided the capacitance is not to high, it can be a transmission gate (such 74HC4066) or a microcontroller out pin switched between being high impedance and ground if the signal is only a few hundred millivolts.

R2 and R3 should be equal.

4. ### MikeML AAC Fanatic!

Oct 2, 2009
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I have used that circuit as a synchronous detector (also known as a synchronous demodulator). Typically, you set the gain of the opamp to alternate between +1 and -1 as a function of the "carrier", which is a square wave with a 50% duty cycle.

5. ### Zara Engineer Thread Starter Member

Feb 6, 2015
53
0
Hi guys
thanks for help
and sorry for mysterious title
but until now, i didn't understand by equation how can we have an output equal to F*M ? or F*Vin

I tried to put per example M = 0 ;
F = V+ = V-
that's mean that (0-F)/R2 = (F-Vout) /R3;

R2 = R3 = R;
=> Vout = 2 * F; ??
and about you it's F*M=F*0 = 0 ;

Please let me know what's the equation of output ! and an example please .
Zara

6. ### MikeML AAC Fanatic!

Oct 2, 2009
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F is equal to + or -C, where C is a constant, like 1 or 1.345. It is not a true multiplier where C can be varied dynamically. (C cannot be a function of time)

7. ### Zara Engineer Thread Starter Member

Feb 6, 2015
53
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Hello Mike

Ok let's suppose the F clock is between -1 and +1 .

how to conclude the equation of the output in function of M and F.

Oct 2, 2009
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9. ### joeyd999 AAC Fanatic!

Jun 6, 2011
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And I use the analogue in the digital realm for digital synchronous detection.

10. ### MikeML AAC Fanatic!

Oct 2, 2009
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Here is how to implement that function using an opamp. The switch shown is usually replaced with a FET. Note that the switch is closed when V(F)>0V; and is open when V(F)<=0V.

Can you see how the switch reconfigures the opamp so that its gain goes from +1 to -1?

11. ### Zara Engineer Thread Starter Member

Feb 6, 2015
53
0
Hello Mike

thank you very much for your patience and doing the simulation for me .