Can diode balanced modulator be used to generate AM?

[joymukherji2001]

Hi,

Hi,
I have been experimenting with 7 mhz AM for quite some time now(just for the sake of generating AM at shortwave frequencies).If I modulate final alone I cannot get more than 70-75% modulation depth.So I modulated the oscillator too and now I get 100% but fidelity is not so nice for music and OK for voice.

Can a balanced diode modulator be used to generate AM? I will not match the diodes so there is carrier leakage this way I get pure 100% AM right or there will be some unforseen problem? Anyone ever tried this method on HF?

Thanks in advance for reply…

JOY

[radio8z]

Joy,

Yes and no. A balanced modulator produces the upper and lower sidebands but no carrier. If the carrier can be added to this in the right proportion then an AM wave will result.

One way this is done is to “unbalance” the modulator with a DC bias. So it can be done. The old Drake TR 3 and 4 used this method.

I have experience with an IC that you might find interesting. Try a search on the MC1496 or the LM1496. From the datasheet you will see that this chip can produce a supressed carrier signal. If you look at the test circuit you will see a pot which adjusts the balance. This pot can also be adjusted off balance to give a really good AM signal, linear modulation up to 100%. Keep in mind that this is essentially multiplying the carrier by the audio and in so doing it produces lots of harmonics of the carrier frequency so filtering the output is really important.

We used this at a college where I taught electronic communications and it always worked well at BC frequencies. Offhand I don’t know the max frequency but you can find that.

Now the bad news. These chips are no longer produced but if you want to get one to try you can probably find someone who has them from a web search.

Neil

[Author]

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