RADIO TRANSCEIVERS

July 24, 2018

Well, we discussed the crude arc transmitters used by Marconi. The modern transmitter revolved around the vacuum tube which made oscillators possible. And crystals made it possible to maintain a certain frequency.
The oscillator 's signal was amplified and the radio waves transmitted into the environment.

That is the hardware side. We have to go back on how information is sent using this oscillator.

MAXWELL SAID THAT IF WE CAN PROPAGATE RADIO WAVES WHICH IS A SINE WAVE WHICH CAN BE PICKED UP AND AMPLIFIED BY A RECEIVER, PERHAPS BY MAKING THE STRENGTH OF THE SIGNAL WEAK AND STRONG, WE CAN DETECT THIS AT THE RECEIVER'S END. WEAK AND STRONG OF COURSE REFERS TO AMPLITUDE. THE WORD USED TO DESCRIBE THIS WAS MODULATION.

AMPLITUDE MODULATION WAS BORN. THE MOST POPULAR FOR A LONG TIME COMMERCIAL   RADIO WAS AM FOR AMPLITUDE MODULATION. THE TOY WALKIE TALKIE IS AM. PILOTS TALKING TO THE AIRPORT TOWER IS ON AM. FM CAME LATER.

WE WILL FOLLOW THE CHRONOLOGY OF RADIO WAVE TRANSMISSION.

IN AM, THERE IS A CARRIER WAVE WHICH IS ALWAYS ON. THE FREQUENCY SET. IF YOU TURN ON YOUR FAVE AM STATION, SAY 630 KHZ, WHEN NO ONE IS TALKING, YOU CAN HEAR THE HISSING SOUND UNTIL THE RADIO TECHNICIAN SAYS GOODNIGHT AND TURNS THE CARRIER OFF.

WHEN THE ANNOUNCER TALKS, HIS SPEECH WHICH IS ON THE AUDIO RANGE INFLUENCES THE POWER GIVEN TO THE TRANSMITTER AND THE AMPLITUDE IS MOVED ACCORDING LY.

IN THE RECEIVER, THE RADIO WAVE IS PICKED UP, AMPLIFIED, AND CONVERTED TO DC. THE DC CURRENT WILL FLUCTUATE IN ACCORDANCE WITH THE SIGNAL STRENGTH RECEIVED WHICH IS THE MODULATION. THE SPEECH OF THE ANNOUNCER IS THEN HEARD.

CONCLUSIONS

1. AM TRANSMISSION SENDS A CONTINUOUS CARRIER WAVE WHICH COSTS ENERGY
    TO SEND. IT IS TRANSMITTED EVEN IF NO ONE IS TALKING.(WASTED ENERGY).

2. AM TRANSMISSION SENDS BOTH THE UPPER BAND, THE POSITIVE SIDE OF THE SINE
    WAY AND THE LOWER BAND. SINCE BOTH ARE COMMONLY MODULATED, THERE IS
     DUPLICATION.(ANOTHER WASTE).

3. IN THE RECEIVER, ONLY ONE BAND IS DETECTED BECAUSE A DIODE WILL IGNORE
    ONE PHASE.. THE OTHER BAND UNNECESSARY.


CONCEPT AND WISH

     IF WE CAN SEND ONLY ONE SIDE BAND  WHICH HAS THE VOICE ANYWAY AND DO
     AWAY WITH THE CARRIER WAVE AND THE OTHER SIDEDBAND, IT WILL TAKE LESS
     ENERGY TO TRANSMIT JUST ONE SIDE BAND..

     CONVERSELY, THE SAME ENERGY AVAILABLE CAN BE DEDICATED TO ONE
     SIDE BAND.  THE ACCEPTED RATIO IS 3:1. A 10 W AM TRANSMITTER WILL LOOK
     LIKE A 30W TRANSMITTER IF ONLY ONE SIDE BAND IS SENT.

      I THINK THE RADIO AMATEURS AFTER THE WAR CONCEPTUALIZED THIS IDEA.
      LATER ON, THE   FCC IN THE US ALLOWED CB RADIOS TO HAVE SSB AND IT CAN
     COVER LONGER  DISTANCES.

      OF COURSE AMATEURS ALL OVER THE WORLD USED SSB IN THEIR HF
      TRANSCEIVERS. AFTER THE WAR, THIS TECHNOLOGY WAS AVAILABLE IN THE
       PHILIPPINES AND SSB HF TRANSCEIVER MAKERS CROPPED UP IN MANILA. ONE
       OF THEM WAS A COMPANY CALLED "MEDALIST". TAIWANESE TECHNICIANS CAME
       TO MANILA TO LEARN HOW TO MAKE TUBE TYPES SSB TRANSCEIVERS WHICH
        CAN REACH GREAT DISTANCES.

        TODAY , I HAVE TWO OF THESE TUBE TYPE SSB RADIOS AND THEY TRANSIT 100
        WATTS.AM NOT OPERATING THEM THO.


        HOW IS A SINGLE SIDE BAND SEPARATED AND TRANSMITTED SINGLY. TO
        UNDERSTAND IT, WE MUST FIRST UNDERSTAND THE BEHAVIOR OF RADIO
        FREQUENCIES WHEN MIXED.


            WHEN TWO FREQUENCIES ARE MIXED, THE RESULT IS THE SUM AND
            DIFFERENCE OF THOSE FREQUENCIES AND THE ORIGINAL FREQUENCIES
            THEMSELVES AVAILABLE AT THE OUTPUT.

             TAKE THE CASE OF THE SUPER HETERODYNE AM RADIO. AN ANTENNA PICKS
             UP A STATION SAY 630 MHZ. IT IS FED INTO THE BASE OF A TRANSISTOR VIA A
             CAP SO THAT THE BASE WILL NOT BE SHORTED TO GROUND.

             THE SAME TRANSISTOR IS SET UP AS AN OSCILLATOR WITH A COIL
              TRANSFORMER TICKLED BY THE COLLECTOR AND A SECONDARY
              FEED BACKING THE SIGNAL TO THE EMITTER OF THE TRANSISTOR. THE
              OSCILLATOR IS VARIABLE BY ONE GANG OF A  VARICAP. THE TRANSISTOR ACTS
              AS A MIXER.

               THE OUTPUT FROM THE TRANSISTOR ARE FOUR FREQUENCIES: ONE FROM THE
               ANTENNA, ONE FROM THE LOCAL OSCILLATOR AND TWO COMBINATIONS,
               THE SUM AND DIFFERENCE OF THE ORIGINAL FREQUENCIES. TO SELECT THE
               FREQUENCY WE WANT, TUNED TRANSFORMERS ARE USED CALLED
               INTERMEDIATE FREQUENCY TRANSFORMERS, IFT. IN AM RADIOS, THIS IS
              AT 455 KHZ.

               THE DIFFERENCE IN FREQUENCIES BETWEEN THE RECEIVED SIGNAL AND
               THE LOCAL OSCILLATOR MUST BE 455 KHZ. THE LOCAL OSC. IS SET TO BE
               455 KHZ HIGHER THAN THE STATION'S FREQUENCY.

                THE 45 KHZ SIGNAL IS AMPLIFIED AND THEN A DIODE CONVERTS IT TO DC.
                 WE CAN HEAR THE INFORMATION.


               CLARIFICATION

                   We say that in AM, the AC is converted to DC and the DC fluctuates with the voice. But
                  actually even though the amplitude is said to have been modulated, there are frequencies
                  involved which follows the same rule in frequencies mixing.

                    Let us say instead of voice, a 1 Khz tone is modulating the transmitter. The envelopes created
                    are still the carrier frequency, in our example, 630 Khz, the upper band, 630+1 Mhz and the
                    lower band 630-1 Khz. It is 631 Khz and 629 Khz.

                    This is where a BALANCED MODULATOR comes in. It cancels the carrier frequency,
                    leaving only the two side bands. How do we then remove one side band. We use a crystal filter
                     which will allow only one side band and not the other. The crystal filter is actually like the
                     intermediate frequency. it could be any frequency.

                     For the example above, we can use a 631 Khz crystals which will pass only the upper side
                     band. But we don't have to to use 631 Khz crystals. We can mix the 631 Mhz with another
                     frequency and get a difference of say 455 Khz..In fact, the tube type SSBs I have both use the
                     455 Khz filter, altho not crystals but mechanical filters. Where is the 629 Mhz. Well it will be
                     2 Khz below the 631 Khz, right? so, it is on 453 Mhz in the IF. The 455 Khz filter will reject
                     it

                      SO, THERE WE GO, WE NOW HAVE ONLY ONE SIDE BAND AND IT AT
                     455 KHZ. HOW DO WE GET THE AUDIO OUT, THE 1 KHZ TONE? WE INSERT
                      TO A SECOND BALANCE MIXER WHERE THE DIFFERENTIAL FREQUENCY
                      IS 1 KHZ! THAT FREQUENCY SHOULD BE 454 KHZ. THEN WE HEAR THE
                      1 KHZ TONE!

                       REMEMBER WE CAN CONVERT INTO ANOTHER FREQUENCY BUT RETAIN
                       THE MODULATION. ISN'T THAT HOW ITS DONE IN THE REGULAR AM RADIO?
                       THE STATIONS FREQ. IS MIXED WITH A LOCAL OSC. WITH A FREQ, HIGHER
                        BY 455 HZ. THE RESULTING FREQ. IS 455 KHZ WHICH IS RECOGNIZED BY
                       THE SUBSEQUENT TUNED AMPLIFIERS.