MC1496
Double Balanced Modulator

A mixer is a nonlinear element that combines two signals. A mixer has three ports: F1 receives low level signal; F2 is high level signal (local oscillator) and F3 is the resultant mixer product.

The output of the mixer contains number of different frequencies that obey the relationship   Where m and n are integers 0, 1, 2, 3,...

There are three types of mixers: single-ended, single balanced and double balanced. Double Balanced Mixer (DBM) suppresses F1 and F2 components of the output signal leaving only the sum and difference frequencies. Double Balanced Mixer provides superior suppression of the local oscillator and RF signals in the output leaving only the sum and difference frequencies. This is known as port to port isolation

This is a circuit by VU3MPX. Let us start with this Block Diagram.


The MC1496 is MIXER.We will see the detail later on.  The VFO is tunable from 2.567 to 2.667 Mhz.

 





The BFO or Beat Frequency Oscillator is at 4.23 Mhz +1.5 Khz.





The 22 PF allows the oscillator to go higher by 1.5 Khz.
Look at the RX mode. When a 7.1 Mhz signal is picked up, it mixed with the VFO which will be tuned so that the output is at 4.43 Mhz, exactly what the filter can recognize. It is amplified and goes to a second MIXER where it is mixed with the BFO to remove the 4.43 Mhz and recover the signal at 1.5 Khz, audible to us. Then it gos to an audio amp.


On TX, the speech at 1.5 Khz is mixed with the BFO. Again since the BFO has an offsewt of 1.5 Khz, the result is asginal at 4.43 Mhz, again recognized by the filter and amplified. This time, it is mixed with the VFO to get 7.1 Mhz for transmission. It of course goes through a linear amp and a final stage. The BPF is the Band Pass Filter at 7 Mhz!



So, where is the carrier wave removed? Ans: In the first MIXER. What is the purpose of the crystal filter: Ans: It is a narrow path which can only pass the upper or lower side band but never both. The tunable BFO is adjusted to find either band.

The signal goes to an IF amp. Yes, the IF frequency is now 4.43 Mhz and is amplified. How does the signal (one side band) becomes 40 M again. It is mixed with the VFO. 4.43+VFO=7.1 Mhz.

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Hi, good morning. This site is dedicated to students in communications engineering. Nothing is new here. It is just a discussion forum about SSB.

SSB is for Single Side Band.

When a carrier frequency is modulated by way of changing its strength, it is called AMPLITUDE MODULATION.

It has three components. The carrier wave, the upper side band wave and the lower side band wave. Since a wave is sinusoidal , it has a positive peak and a negative crest. When it is modulated, the vertical size of the wave moves and the upper side and the lower side carry the same information.

In AM radios, detecting this amplitude change is a cinch. Just use a diode at the end of the RF amplification (in superhets) to detect the fluctuating DC which carries the information. Either side will do. So, the diode can be placed in any direction.

The waves leaving the  AM Tx are the carrier wave, the upper side band and the lower side band. First it s a waste of use of power at the TX to send both side bands. One will do. Second, the carrier wave need not be sent since it can be inserted at the RX's side.

SO, ONLY ONE WAVE NEEDS TO BE SENT-ONE SIDE BAND.

The power available in the TX can all be dedicated to send out one side band and therefore, it is more powerful.

We shall proceed on ow this is done by studying a Single Side Bank transceiver.