Hi Good Day! It is TGIF, Sept. 24, 2018

I would to do a write up on the concept of SSB today.
In the old days of tubes, it was explained from the pint of view of Amplitude Modulation (AM), removing the carrier wave, then one side band wave.

Well, it started that wave. Speech was introduced to carrier wave by changing  the strength of the signal in accordance with the speech. The frequency remained the same. This was in fact accomplished in the final stage of the signal before leaving the antenna. an audio amp with a microphone drives a transformer when a secondary controlled the flow of current to the final stage. Without any speech, the carrier wave is stills sent and appears as a hissing sound in the receiving radio. When speech is introduced, the carrier wave as shown in an oscilloscope will appear to be changing in strength and was called an envelope.

     In the receiver such as a simple Crystal receiver, a coil picks up the AC signal. a diode converts it to DC and since the original AC was changing in strength, the DSC also fluctuated in accordance to the speech. The speech is recovered.

     To appreciate the modern explanation, we must first appreciate DIRECT CONVERSION receivers. In a direct conversion receiver, an incoming signal or broadcast is mixed with another frequency created by a local oscillator such that the frequency resulting falls within human hearing range. Then we can hear the speech. There is no need to convert AC to DC to derived the speech from the fluctuating DC. When we lower ther frequency of a signal to about 1.5 Khz, we can hear the speech!

     To create sensitivity, we amplify the signal. To create selectivity, we use tuned rf amplifiers which allows only a certain frequency range to be amplified. We see this in our commercial AM BCB radios. In those radios, the original freq. of the signal received  is mixed with a local oscillator which create a new signal at an "intermediate frequency" (455 Khz). Small transformers are used t tune in and peak at 455 Khz. The signal is amplified. In the end, it is rectified to DC with a diode. We DC follows the strength of the speech, thus we hear it.

     In FM radios. the amplitude is kept fixed and the speech influences the frequency within a certain bandwidth. Same thing is done in the receiver, however, a discrimiator detects the frequency shift and converts it to moving current/voltage which is sound to us.

     Now take note: in AM, the modulation is in the end stage of the TX. In FM, it is in the initial stage.

     These are important backgrounds to understand SSB.

Even in tube days, a circuit was developed wherein 2 frequencies F1 and F2 were mixed. F1 would be the speech at 1.5 Khz. F2 is a selected frequency no fifferenct from the "intermediate frequency" in AM radios.
The intermediate frequency can be anything suitable for the purpose. 455 Khz was used. 12.5 Mhz is used, 10 Mhz is used. 4.43 Mhz is used. Let us take the IF to be 4.43 Mhz. The LO therefore has to be 4.43 Mhz plus 1.5 Khz. Why? so that when mixed with the speech freq. of 1.5 Khz, it cancels the 12.5 Khz leaving the freq. at 4.43 Mhz. The 4.43 Mhz signal is amplified selectively. It is then fed into another mixer, this time to raise it into a frequency at which we want to transmit. Another local oscillator is used. It is called the VFO and in this case  the VFO has range where the output of the mixer is say, 7.1 Mhz or the 40 meter band.(Subtract the 4.43 from 7.1=the VFO frequency needed).

To receive at 7.1 Mhz, the signal is mixed with the VFO to net 4.43 Mhz. It is again amplifed and fed into a second mixer with a local oscillator called the BFO so that the output is 1.5 Khz which is within our hearing. The audio is amplified to drive a speaker.

Selectivity is achieved by using a filter for teh 4.43 Mhz which are 4 xtals at that frequency in series. IF transformers are also used.

WE CAN NOW REVIEW VU3MPX SSB TRANSCEIVER CIRCUIT .