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Author Topic: THE SUPERHETRODYNE INTRODUCTION TO RADIO  (Read 4611 times)
K0YNE
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« on: April 23, 2009, 09:51:21 PM »

IN THE EARLIEST YEARS OF RADIO, RECEIVERS WERE NOT THE MOST SELECTIVE, SO IN CITIES WHERE
SEVERAL RADIO TRANSMITTERS WERE OPERATING, SEPARATING STATIONS BECAME DIFFICULT.
NEWER RADIOS, ADDED MORE TUNED RF CIRCUITS TO NARROW THE BANDWIDTH OF WHAT IT RECEIVED,
BUT STILL STATIONS WOULD GET MIXED TOGETHER. THEN LATER IN THE 20'S OR EARLY 30'S, A NEW
INNOVATIVE CIRCUIT WAS CONTRIVED. THE SUPERHETRODYNE CIRCUIT RADICALLY CHANGED RADIO
RECEIVING FROM THAT POINT ON. THE OLD CONFIGURATION, AMPLIFIED, THEN CASCADED THAT SIGNAL
TO ANOTHER RF STATE FURTHER NARROWING THE BANDWITH, AND SOMETIMES ANOTHER OR 3RD STAGE WAS ADDED THEN DETECTED. THE EARLIEST RADIOS WERE NOT MUCH MORE THAN CRYSTAL RADIOS WITH HIGH IMPEDANCE HEADPHONES TO LISTEN BY. SPEAKERS WERE ADDED, SO NOW EVERYONE IN THE ROOM COULD HEAR THE BROADCASTS. NOW THE SUPERHETRODYNE RADIO RECEIVER
MAKES IT WAY INTO THE MARKET. I SUPPOSE AT THE TIME, IT WAS THE GREATEST THING SINCE '
SLICED BREAD..

ALL MANUFACTURERS MUST HAVE BEEN ABLE TO USE THIS NEW CIRCUIT, SINCE THE SWITCH TO
IT WAS FOUND IN ABOUT ALL PREVIOUS RADIO MAKERS. SOME RADIOS USED AN RF AMPLIFIER TUBE SEPARATE FROM THE MIXER AND SOME USED A RF AMP WITH A SECTION OF THE TUBE FOR A MIXER.

A STATION LIKE KHAS RADIO AT 1230 KHZ , SIGNAL WOULD  BE AMPLIFIED IN THE FIRST SECTION OF THE TUBE. THE SECOND SECTION OF THE TUBE WOULD BE PART OF A VFO CIRCUIT, NAMELY A VARIABLE FREQUENCY OSCILLATOR. NOW WE CAN SELECT THE "IF" FREQUENCY. A VERY COMMON
"IF" FREQUENCY WAS 455 KHZ. NOW WE MUST CONSIDER THE LOWEST FREQUENCY OF THE AM BROADCAST BAND AS BEING 540 KHZ AND IN THOSE DAYS, 1600 KHZ WAS AT THE TOP.. NOW CONSIDER THE LOWEST OSCILLATOR OR VFO FREQUENCY WE CAN USE TO BEAT AGAINST 540 KHZ TO EQUAL A DIFFERENCE OF 455 KHZ THE "IF FREQUENCY.. CONFUSED YET? THIS IS NOT EASY....
SO NOW LETS ADD 455 KHZ AND 540KHZ TOGETHER.  TOTAL IS 995 KILOHERTZ AT THE LOWEST END OF THE BROADCAST BAND.  WHEN THE VFO  OR OSCILLATOR IS GENERATING THE 995 KHZ BEAT FREQUENCY, AND THEN ADDED TO THE 540 KHZ BROADCAST STATIONS SIGNAL, THE DIFFERENCE OF THE TWO FREQUENCIES IS 455 KHZ.  SO NOW SINCE THE DIFFERENCE OF 455 KHZ IS FED INTO A TRANSFORMER "IF" CAN, THE TUNED CIRCUIT COUPLES IT EASILY TO THE NEXT AMPLIFIED STAGE.
WE HAVE JUST NARROWED DOWN THE BANDPASS TO APPROXIMATELY 15 TO 20 KHZ ON EACH SIDE OF THE 540 KHZ STATION. FURTHER STAGES WILL BE REDUCING THE "IF BANDPASS EVEN FURTHER, SHUTTING OUT NEARBY STATIONS TO A GREATER DEGREE.. WHEN MIXING TONES,  ONE IS 300 HZ AND THE SECOND TONE IS 700 HZ. THE DIFFERENCE IS 400 HZ, AND THE SUM IS 1000 HZ AND OF COURSE YOU STILL HAVE THE ORIGINAL 300 AND 700 HZ TONES.
THE SUM AND DIFFERENCE. IN THE SUPERHETRODYNE CIRCUIT , IS THE DIFFERENCE. SO HOW HIGH WILL THE VFO OSCILLATOR HAVE TO GO TO BEAT OUT THE 1600 KHZ STATIONS AUDIO? 
IT WOULD HAVE TO BE 1600 KHZ PLUS 455 KHZ. OR 2055 KHZ. THE VARIABLE CAPACITOR IN THE VFO IS GANGED WITH THE ANTENNA PEAKING CIRCUIT NORMALLY SO AS THE RADIO IS TUNED, UP FREQUENCY, SO THE ANTENNA TUNING CAPACITOR
 IS MOVED TO A LESSER CAPACITANCE AT THE HIGHER FREQUECY..
IN MODERN DAY RADIOS, DIFFERENT "IF FREQUENCIES ARE USED AND SOMETIME TRIPLE CONVERSION
IF'S ARE EMPLOYED. SOME OLD TIMERS THINK SINGLE CONVERSIONS SUCH AS THE OLD 455 IF'S ARE
BETTER AND WON'T INTRODUCE EXTRA NOISE FOUND IN THE ADDITIONAL STAGES. THEY MAY BE RIGHT.
THE VFO'S FOUND ON RADIOS OLD AND NEW CAN VARY GREATLY IN RANGE. THIS ILLUSTRATION MERELY EXPLAINS WHAT SUPERHETRODYNE IS ABOUT AND WHY IT GREATLY REDUCED THE INTERFERENCE FROM STATIONS ACROSS THE BAND.
HOPE NOT TOO MANY MISTAKES, PAUL K0YNE
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kbasq
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« Reply #1 on: April 23, 2009, 10:00:46 PM »

Paul,

Once again a great article and contribution.  Keep up the good work! Wink
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