balanced modulator

Hi there,

Currently, i have build the modulator and demodulator as shown in Fig.5 and Fig. 29.

The frequency for modulating signal input = 1kHz
Carrier frequency = 100kHz. (for both modulator and demodulator)

The modulator output (either V+ or V-) is able to get the modulated AM output. (However, i only use V- to transmit, V+ i left it as no connection). I use two wires to act as antenna for the modulator and demodulator. By means of wireless transmission, I cant obtained the modulated output from the demodulator. Even if the antenna is very very near. What i obtained is only the carrier signal. But, if i let the two antenna contact with each other, i can get the output and it is same as the modulating signal.

So my doubt is:
i. Is it the length of antenna affecting the transmission?
ii. Is it the transmission power is not sufficient?
Or is there any other factors?

Thanks in advance.

Regards,
cwt

Hi CWT!
1. The 1496 is a low Power device; It cannot transmit or radiate any usable power, especially at Low frequencies (< 500KHz).
2. You will need a Buffer-Power amplifier stage for this. A buffer-biased FET with an RF choke at the drain can do, although 'carrier Feedback' will affect the carriur null, if you are not careful with circuit wiring and layout
(RC circuits have very little radiating property. You need to convert into an LC circuit.)

3.
3. The 'Antenna' for 100KHz....:D

1/2 wave dipole(in Feet) = 468/Freq. in MHz

4. The input signal for the product-detector should be > 400mv for proper detection.

You can use 'loop' antennas, but first -please answer:

What do you want the antenna for ?
Have you checked the output with an oscilloscope?

I wish to do wireless communication between the modulator and demodulator. So thats why i need an antenna. But for my case if i use antenna, it'll huge. I cant imagine that. So is there any option where i can transmit wirelessly? My carrier is in the range of 100kHz to 500kHz.

I have used the function generator to provide the carrier frequency and the analog signal.

I have checked the output of modulator using the scope. It is an AM modulated signal. Tuning the carrier null will affect the shape of the modulated signal.
As for the input of modulator i only see the carrier signal at the scope.

Regards,
cwt

cwt!
Perhaps, you want to build this ? http://imagenisp.com/jsm/VA7LF.html :D

Interesting, no doubt: did some experiments in LF and VLF, long ago....

What I suggest is -
1. A transmitter for the Modulator.
An LM386 works great with a tuned loop in place of a speaker. This will give you a range of about 5 meters (15ft).

2. A Receiver using another LM386. The input with another tuned loop antenna. The output coupled to the demodulator with a capacitor (AC coupling)
You can also use a MC34119.

The Lm386 and MC34119s are just power opamps. They will work well upto 400khz.

...or build the PA0SE FSM circuit in the attachment. Thats a great circuit and works very well! Please send him a 'tnx' mail if you do!

Is it all types of modulator and demodulator requires power amplifier to assist the wireless transmission?

I thought they only need a suitable antenna to transmit. :(

So now if i use any power opamp other than those suggested by you, is it wireless transmission is able to achieve?

You said that they work well up to 400kHz. Then what about 500kHz? My transmission requires the carrier freq ranges from 125kHz to 500kHz.

As for the tuned loop antenna suggested by you, can i use a normal antenna (the "stick-like" antenna) to transmit the signal?

Man, i need your help so badly, i am not good in communication aspect.

Thanks for the help so far.

Regards,
cwt