[SOLVED] AM modulator with MC1496 project problem

[tzitzikas]

Hello, i have done this project. It is an experimental oscillator - modulator for mw frequencies. The schematics are here:
oscillator:
https://tzitzikas.webs.com/oscillator-1.JPG
modulator:
https://tzitzikas.webs.com/modulator-1.jpg

The problem is that the designer told me to use an 1kohm variable series resistance between oscillator and modulator (after

c9=22n capacitor of oscillator) to adjust the carrier level. I saw that the 1kohm variable series resistance is not enought

because with 1kohm series resistance the audio was't very good. I think that the carrier level was high. So i change this

resistance with another 4.7kohm. Now i am using about 2kohm of this series resistance. The modulation is very good But i

think that the audio signal in the radio is not at the center of the frequency but a little above.
(The operation frequency is 1000khz but i listen the audio with quality and strong voice at 1003khz.)
which is the problem? how i can fix it, to listen the audio very well at center frequency and not at sideband?

 

[FvM]

The circuit seems to generate a supressed carier AM modulation. It can't be received correctly with a regular AM receiver.
Check the output signal with an oscilloscope and take care, that the modulation index is kept below 100%. I guess you have
to add a DC offset between pin 1 and 4 of the modulator to achieve this.

See https://en.wikipedia.org/wiki/Amplitude_modulation for explanation of the basic properties of AM.

 

[tzitzikas]

.... I guess you have
to add a DC offset between pin 1 and 4 of the modulator to achieve this....

thank you. how i can add DC offset between pin 1 and 4? Give me please an idea, i don't understand it.

 

[vfone]

By definition MC1496 is a balanced modulator and the main application is DSB modulation (Double Side Band – suppressed carrier). If is used as an AM modulator to don’t get carrier suppression (and getting instead, a DSB) you need a shift from suppressed carrier, which your circuit already have it (R5, R6, R10, R11, R9, R12).

You can add a potentiometer as in fig.27 below:
https://www.onsemi.com/pub_link/Collateral/MC1496-D.PDF