Narrow-Bandwidth Television Association

I am currently playing with the MC1496 with a view to using it as a double balanced modulator, ie. with the carrier optionally disappearing when no modulation is coming in. I am currently at the stage of trying to understand how it should be best connected...

Would the attached circuit work? It is derived from one on the data sheet, but redrawn for simplicity. Assuming low impedance signal and modulation sources, I am still worried that the presence of the 100 ohm resistors feeding DC bias to pins 1 and 4 would render the effect of the 47K 'carrier null' pot - null and void! Or am I mistaken?

Thanks for any help.

Steve O

Hi Steve,

I've done a little digging of circuits that have used the 1496 in the past.
From CQTV, my own PAL encoder, and Linsley Hood's synchrodyne AM Rx.

Certainly the 100R looks a little low.
The general consensus seems to be a 1k resistor to a reference potential (rail or mid rail) from each of pins 1 and 4
Then a mid range pot, say 4k7 or 20k across pins 1 and 4, with a 47k resistor from pot wiper to either supply rail, or 0v.

What you are trying to do is introduce a small balancing current so the wiper can go via the 47k to either rail

From what I remember in my PAL coder design the chip is well balanced and only a very modest correction was needed to achieve accurate sub-carrier nulling

Hope this is of use

Cheers,

Graham

Useful tips - many thanks!

I believe what I am basically trying to do is create a Ring Modulator. So I am also going to try doing this with a couple of transformers and some diodes.... could be simpler.

Steve O

Good luck,

Cheers,

Graham

This second method has had good results. I have been amplitude modulating a high-end audio carrier with lower audio frequencies. With this double-balanced modulator the 'AM output' has a suppressed carrier. In other words, the carrier only appears under conditions of modulation.

It strikes me this method could be used for broadcasting NBTV wirelessly across a room, using 40KHz transducers and ultrasonic sound.

Steve O