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:
http://tzitzikas.webs.com/oscillator-1.JPG
modulator:
http://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?

 

[(*steve*)]

Have you confirmed that your carrier is actually 1MHz? On the other hand, are you sure your receiver is accurate?

Those would be the first things I'd look at.

edit: You're using a VCO. It's amazing you're within 0.3% of the frequency you're after without some form of feedback (PLL?). I would be less than surprised if the frequency doesn't wander around with temperature, humidity, and possibly even phase of the moon.

 

[55pilot]

Considering the problems I pointed out with the other circuit you posted from this source, I suspect the problem is a poor design. I would look at things like not taking component tolerances and other real life issues into account.

To answer your question any further, I would need to read up on the chips in question since I am not familiar with them. Like I suggested in your other thread, I would go back to the website you got the circuit from and ask a few questions there.

---55p

 

[tzitzikas]

55pilot :the circuit is from a Greek website.
(*steve*): i controll the vco , by using a pll with SAA1057 and pic16f628 microcontroller.
Do you have any idea why the modulation is not at the center frequency? Maybe improper armour?

 

[(*steve*)]

Armour? Shielding? I doubt it. That's more likely to introduce noise than a systematic frequency error.

Have you independently measured the carrier frequency?

 

[tzitzikas]

yes i have measured the carrier frequency. It is ok. i work the vco with pll (SAA1057 and pic16f628 microcontroller)
i was meaning good sield, sorry.

 

[davenn]

Do you have any idea why the modulation is not at the center frequency? Maybe improper armour?

yes because you appear to have the MC1496 wired as a DSB (double sideband) suppressed carrier modulator. So therefore you are not going to see a signal on the main carrier freq as that freq is suppressed. Rather you are going to see a signal on the 2 sideband freqs.

Dave
VK2TDN

 

[tzitzikas]

yes because you appear to have the MC1496 wired as a DSB (double sideband) suppressed carrier modulator. So therefore you are not going to see a signal on the main carrier freq as that freq is suppressed. Rather you are going to see a signal on the 2 sideband freqs.

Dave
VK2TDN

do you know what modifications i must do at this circuit http://tzitzikas.webs.com/modulator-1.jpg to work as am modulator? the designer claims that this schematic is for am modulation.

 

[davenn]

have a look at the data sheet
http://www.datasheetcatalog.org/datasheet2/c/0gh5wy71w3dsw8x23f8l8pw8d97y.pdf

on page 10, Fig 27 is an AM modulator note the component values and how they differ
a bit to the ones you are using. There isnt a big difference between its setup for a
DSB,SSB, or AM modulator (demodulator)

see how ya go

Dave