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.