Inverting a modulated signal (OOK).

[Alex_Bam]

Hello,
I'm modulating the message signal with OOK (on-off keying) modulation. Multisim software is used to model the schematic [attached]. At the resistor R1, the modulated signal is obtained. I wanted to invert the modulated single, for that I used a simple NOT gate (U3), however at the NOT's gate output, I got the same signal as input means without inverting [attached]. So my question is, why isn't the NOT gate inverting? Am I doing something wrong?

As hit and trail, I added two more NOT gates (U4 and U5) but this time I got an inverted signal, although further improvement is needed to achieve a 180-degree phase shift. So my second question is: Why is it an inverter this time? and how to get exactly a 180-degree phase shift between two signals?

Thanks for your time.

 

[bertus]

Hello,

Each gate will have a delay.
Wich type of gate are you using?
There are several types with different delay times.

Bertus

 

[AnalogKid]

You don't give a part number for the NAND gate, so we don't know if it has hysteretic inputs. And, usually there is a resistor in the feedback path for more control and stability of the oscillating frequency.

Beyond all of that, the oscillator probably is producing a symmetrical output, which looks the same on a scope inverted or not because of how a scope trigger works. The extra inverters are merely a delay line, giving a time-shifted (not inverted) image.

What is the purpose of R1?

ak

 

[bertus]

Hello,

as @AnalogKid said, there is usualy a resistor in the feedback.
Have a look at this TI note:
https://e2e.ti.com/support/logic-gr...4093-schmitt-trigger-gated-oscillator-trouble

Bertus

 

[Alec_t]

I replied over at AAC.
The post #1 sim works ok as is with the default 'A' devices in LTspice (providing the AND gate is given a finite td parameter), so there could be a problem with the Multisim models or implementation.