Hi Everyone! I have to fast forward a bit on AM radio due to an inquiry from "Catzilla" at Elab about IC AM radios. Chips for AM or for AM/FM radios use an entirely different in the IF section. AS we have been discussing, the original AM radios used inductors/traffos to narrow the band and amplify only the 455 khz intermediate signal. There is a way to do this eliminating all those IFT's! This was used even before the advent of IC's. THE CERAMIC FILTER The ceramic filter is that yellow square part with 2 or 3 legs and it usually labelled on its side is 455 Khz. It is a ceramic resonator. It resonates strongly at 455 Khz and will only allow that signal to pass through. One small part replaces the 3 traffos! The local oscillator coil and the antenna are still needed. Some ICS have built in audio amp and others without. THE TA2003 Toshiba introduced this IC in the recent past which has caught on in the commercially manufactured AM/FM radios. Most of the newer portable radios use this simple IC. The mixer-oscillator tranny is built in. The detector diode is also built in. It has a sensitive built in intermediate frequency amplifier. This IC is also commonly available in Raon.

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Hello Everyone! Tapos na ang Undas, tulog na ulet mga patay. Gisingin natin itong topic! A little bit more about tuned coils. We all played with magnets,right? In fact, it will be the first display in my SCIENCE FAIR for school children. MAGNETISM IS AWESOME! REALLY IT IS. ALL OF US GOT EXCITED AT THE INVISIBLE ENERGY IN A MAGNET. WOWOWEEEE! We also learned polarities. North, South we called it. North and North repels and so with South and South. But N and S attracts. First law! Well, look at batteries, you cannot connect the minus to the minus nor the plus to the plus. It's always plus to the minus to chain a series of batteries. So, with magnets, you can connect several into a link but it has to be N to S, N to S, etc.... MAGNETISM IS LOCKED ELECTRICITY! WOW! WHATDYAMEAN? Let us get a coil and connect the terminals to a tester or even an LED. Get a magnet and swing it in front of the coil. VOILA, the tester jumped or the LED lighted. This is my second demo in the Science Fair. The magnet INDUCED current into the coil. MAGNETISM BECAME ELECTRICITY! The current was not produced when the coil sensed the magnet. It was when the magnet was removed! The magnet produced a magnetic field around the coil and when the field collapsed, energy was absorbed by the coil. WAIT A MINUTE, WE ARE SUPPOSED TO DISCUSS COILS. EHEM...her we go.. The above was important cuz we will discussed magnetic fields. Ah, we go no further, let us talk about the AM antenna, which is a tuned coil! The antenna coil is like the coil discussed above. We do not need a magnet cuz electric fields abound in the atmosphere. All transmitters put electromagnetic waves. and while we had to swing the magnet, we do not have to swing these waves simply because they are ever changing poles. They are AC and AC can induce current to another coil! If the coil is by itself an electric generator, how do we collect the energy? Well, the capacitor is actually a battery with a very short storage life. We put a capacitor across the coil and might as well make the capacitor variable. When the coil discharges into the capacitor, the capacitor , being a battery, will in turn re energize the coil. The result is a SYMBIOTIC relationship. The pair will alternately charge each other. and not only that, changing the value of the capacitor will change the frequency of the exchange. A pair like this became known as a "tank". YOU MEAN THERE IS ALWAYS CURRENT THERE? Yes, and we call it signal because it is minute. If we connect a long wire antenna and ground the other end of the coil, we can obtain max energy and we DIRECTLY convert the signal to DC with a diode and hear the station! It is called direct conversion and uses no battery. They were called Foxhole radios during World War II. SO, THE FIRST TRANNY IS TO AMPLIFY THIS TINY SIGNAL! YEP...

THE OSCILLATOR COIL So, the red can, the oscillator traffo is actually oscillating? Yes. Bring a working AM radio near it and tune it around. You will will pick the oscillation. It will sqeal.You can tune either the 1st or 2nd radio or both. Do you want it to be a pure oscillator? Well, see the plus going to the C coming via the yellow IFT. The red and yellow traffos are in series, right? Well, remove the yellow traffo and connect the plus to the primary of the oscillator. It will continue to oscillate without the yellow traffo. . Next, you don't want the incoming station, right? Well remove the antenna. What is left is an oscillator and it is tunable with the tuner capacitor. You can make an AM transmitter! But not now. THE INTERMEDIATE FREQUENCY, 455 KHZ. Let us start with the yellow traffo. The C of the first tranny drives this traffo, right? In the schematic, you will see that there is a capacitor across the 3 pins. It is a tuned coil! So the coil peaks at 455 Khz and rejects frequencies below and after 455 khz. There is a measure on how well it is doing this. It is called the "Q" of the traffo. The second tranny does the same thing to the white traffo as a driver. See where the 2nd tranny gets its signal. From the yellow traffo! Another tranny to drive the black traffo. With 3 tuned traffos, only 455 Khz gets through. It has been AC all the while. Now, look at the black traffo as like the traffo in your power supply project. Its secondary has 2 pins, right? Let's make a DC power supply but with very little current. Ground one pin and run a diode on the 2nd pin. Now it is DC! Let's filter it, so add a 1K Res with caps before and after. Remember in your P.S. project, you had eliminated the hum, the AC hum. And heard nothing. But the house electricity is steady. In the AM radio, the AC signal is fluctuating in strength. That's because the announcer is talking!

November 6, 2017 Hi, its Monday! Soon December for all you know it. Let's learn AM radio before they invade North Korea. All Hell will break loose. See headline today. Ok, so far I explained the AM radio in general. Time to explain certain sections and componenets in details. Let us start with the front-end, the antenna. It's coils of wires. Well, there are two coils in there, the long one which is tuned by the variable cap. Then the short coil which connects to the B of the first tranny. Why 2? Well, first it has something to do with IMPEDANCE. This has been a difficult topic to explain really. It is also measured in ohms but your tester-ohmeter cannot test it. Well, lets talk of the speaker where many of us first encountered IMPEDANCE. Most speakers have 8 ohms. The world started with tubes and the electrons flowing through a tube is moving at a very high rate (that is the Voltage). Hi-Fi's has over 500V DC. It can operated at 90 V. Energy is in watts and to get it, it is voltage x amperes. You will note the tubes operated at high V and Low ampere in milli-amps in fact. It is very similar to the step down traffo you have for your power supply project. Primary is 220V and secondary is 12V. The wattage on either side is the same. Let us start with the seconday, 12V at 1 amp. that will be 12 watts. So, what will be the amps in the primary. It will be 12W/220V, very tiny. It is called inversely proportional. Back to the tube amp and the speaker. if one were not to use a final traffo, one has to use a very high impedance earphone or speaker. The final traffo is really just a step down traffo. Something like 550V to less than 20 V to the speaker. BUT THE CURRENT IS INVERSELY PROPO. SO, HIGH CURRENT DRIVES THE 8 OHMS SPEAKER. Now, the epeaker should have less turns but with heavier wire to carry more current. Impedance is close to heaving a 12 V bulb connected to a 12V source. Back to the antenna. The tuned signal from the long coil is transformed to a lower impedance in the short coil. The signal in the short coil actually had more current. It is fed to the B of the first tranny. One added advantage is that the tuner is isolated from the input load at the B of the tranny. OSCILLATORS How do they work? Look at the oscillator coil, the red traffo. The primary has e pins, the secondary has 3. It is the secondary that is tuned. See the capacitors across it. The primary has no cap and is picking up whatever go through the collector of the first tranny. A COIL AND A CAP IN PARALLEL IS CALLED A "TANK" CIRCUIT. If energy is introduced, the cap discharges to the coil and the coil will be magnetized but when the field collapses, it recharges the cap! It will oscillate at a frequency depending on the coil and the cap. The tuning cap is able to change the oscillation. The red ferrite slug affects the inductance of the coil and can also change the frequency. The local oscillator is designed to oscillate at about 1 khz to 2khz.

I dabbled a little into tubes but let us go back to the transistor radio. The first lesson I would offer the nubie is about the transistor. THE TRANSISTOR (TRANNY FOR SHORT) Ok, the tranny is a semiconductor device. It is natural material, silicon. It has 3 layers, the outer layers the Emitter and Collector and the middle layer is the Base. Tickle the based and electrons flow between the E and C. Let us talk of NPN trannies. Electrons flow from the E to the C if B is made a bit positive. Let us use an analogy, the faucet. The E is where water comes in, the C is where it comes out. The B is the rotary valve. We can shut off the faucet, open it slightly or open it full. We can make the outlflow of water dance by playing with the knob. Back to the tranny. We ground the E to the minus rail. If we connect the C to the plus and put some plus to the B.ZAP! we can blow up the tranny. Too much current flowing and we shorted it! We should limit the current even if the tranny is fully ON. How, we put a resistor in series as a ballast. When we turn in on fully, the resistor heats up and the tranny might survived. Or even better, we series an LED from the plus to the C. Get a 1k R and tap the plus and the Base. VOILA! the LED lights up and the LED itself limits the current flowing between the E and C. Now get a 5k volume control, ground one end and put the other end to Plus. The mid pin goes to the base. We have a voltage divider. Turn the POT and control the brightness of the LED. Okay, Okay.... 1. When a plus is put on the Base, it is called a FORWARD BIAS. 2. When it is minus, it is called REVERSED BIAS. Take a look at the first tranny in the AM radio. It is about 820k connected to the Base and the plus line. So, the tranny is forward biased a bit. The faucet is just slightly open. That is the region where the tranny is very sensitve to the signal to its Base from the antenna coil and the signal from the oscillator coil to its E. The signal from the OSC coil is NOT shorted cuz the E is not grounded. There is a resistor there to ground. So, in respect to ground, the E is a plus! What are the parameters of the tranny? Well, first it has to be able to handle a certain frequency. The 9011 is an RF tranny and can handle probably up to 100 Mhz. The next parameter is the current handling capacity. For the RF tranny, it does not have to be able to handle high currents. For the audio ampli trannies, the 9012 (PNP) and 9013 (NPN), these are high current trannies but does not have to be high frequency trannies. The third parameter is the BETA, the amplification ratio or factor. EXERCISE: Google the trannies and note their parameters. THE INTERMEDIATE FREQUENCY TRANSFORMERS These are no different from the traffo in your power supply project. They have a primary and a secondary windings. They also have ratios. In your p.s. traffo, the primary and secondary ratio is 220:12 if your secondary is to give 12 volts. The IFTs is designed to be able to operate MAX at a certain frequency, 455 khz. To fine tune, the ferrite slug is moved up or down. Now, take note. From the yellow IFT to the next tranny, there is no wired connection! The signal is magnetically transferred to the secondary and on to the next tranny. This is called "isolation". BE CAREFUL YOU DO NOT SHORT THE PRIMARY OR SECONDARY OF THE IFTs, YOU CAN BURN THEM. AND THAT IS WHY BEFORE YOU INSTALL THE IFTs, CONDUCT CONTINUITY TEST TO THE BOTH PRIMARY AND SECONDARY. SET TESTER TO RX1 AND TEST. THE OSCILLATOR COIL The OSC coil is also a traffo cuz it also has a P and S. DO THE SAME TEST AS THE IFT. The OST(lets call it that) gets fed a signal from the C of the first tranny. The signal is magnetically transferred to the other side of the OST which has 3 pins. The outer pins are paralled the second gang tuning capacitor. It tunes the OST. TAKE NOTE OF THE CAPACITOR TO THE MIDDLE PIN. IT GOES TO THE E OF THE FIRST TRANNY. SO, WHEN A SIGNAL IS FED THROUGH ITS PRIMARY, IT IS FED BACK TO THE TRANNY VIA ITS SECONDARY. AND SO AN OSCILLATION TAKES PLACE. THE DIODE This section is called the detector cuz it detects the audio. But what it does, it simply converts the AC to DC just like your power supply project. Note the caps before and after the resistor after the diode. That is the filter! THE AGC This is something you will never learn if you jut assemble an AM radio from a kit. AGC stands for Automatic Gain Control. Some stations are very strong and some are weak. We need something like a governor. In steam engines, they have a mechanical governor. Two balls is rotated by the engine. The balls fly out further the faster the speed. Two levers are set on the center rod which compresses the higher the speed. It pushes a valve off to slow down the steam. SAME PRINCIPLE IN THE AM RADIO. Take note, the DC from the diode is brought back to an earlier tranny via a resistor in series. It connects to the Base. Take a look at the polarity of the diode. It is a minus out A strong signal will make the minus greater and it is fed back to the base of the tranny to slow it down. When a signal is weak, that tranny is more sensitive. Got it?
To be continued.........

Oops, I found the edit pencil. Pls disregard my comment. A bit on the "American 5 tube radio". Well, as I said previously, 3 tubes were needed to process the RF.  The third tube actually has a diode to it     and so the audio is pre-amplified by the 3rd tube. The fourth tube is the final audio amp. Then one more tube to act as the rectifier to convert AC to DC voltage to operated the tubes.

When I see a tube radio with a transformer, it means the tubes' heaters are not in series. I eliminate the heater tube and just use a silicon diode.

WELCOME EVERYBODY!

     This is created especially for young students of electronics, specifically for those putting together for the first time, an AM RADIO.

     BK, yours truly , is a major contributor at Electronics Lab Forum or Elab and that is where many youngsters sought my advice on constructing their first AM radio. They usually buy a local kit which do not include any explanation on how the radio works. A leaflet included is purely the schematic diagram and parts list.

IT IS THE INTENTION OF THE AUTHOR TO HELP THE NEOPHYTE BUILDERS NOT ONLY TO BUILD BUT TO UNDERSTAND HOW AN AM RADIO WORKS. A KIT WILL BE MADE AVAILABLE IN THE FUTURE THAT IS SIMPLE WITH THE LEAST PARTS. AND THIS BLOG WILL BE THE LITERATURE!


HOW RADIO WORKS


    Let us go straight to the point. An electronic oscillator can produce a signal which is AC and in fact, can be propagated in the atmosphere. This signal is called electromagnetic waves. The energy keeps changing polarities, plus-minus, thus AC for alternating current.

    This signal can be picked up by a coil of wire tuned to it. A condenser is paralleled to the coil to tune it to the signal. This is what you see in an AM radio, a coil wrapped around a ferrite bar. Yes, the signal is there but human ears cannot hear anything.  To start with,the human ear can only here below 30 kilohertz and an AM radio transmitter in the medium wave range called the BROADCAST BAND is somewhere between .540 Mhz annd 1.6 Mhz! And even if we could, we cannot hear the AC signal!

    A way was developed to mixed human speech to the AC signal. AM was born. It means Amplitude Modulation and the way it works, the frequency is kept steady but the signal STRENGTH is varied in accordance with the speech. SO, THE AC SIGNAL FLUCTUATES IN STRENGTH OR POWER.
Ok fine the AC is varying in strength but how can we hear the speech when the signal is still in the high radio frequencies. For the sake of familiarity, let us say the broadcast is at 1,000 Khz or 1 Mhz. The signal has the upper positive cycle and the lower negative cycle. If we imagine seeing it in an oscilloscope, the AC signal is steady in frequency but the height of the signal keeps changing. We know the fluctuation is the speech pattern of the announcer. How do we extract it? Well, suppose we convert the AC to DC by the use of a simple diode. THE FREQUENCY BECOMES IRRELEVANT CUZ IT WILL BE NO LONGER AN AC SIGNAL! It will be a DC SIGNAL! BUT WHERE IS THE VOICE? HA, THE DC CURRENT WILL FLUCTUATE IN ACCORDANCE TO THE AMPLITUDE WHICH WAS CREATED BY THE HUMAN VOICE! VOILA, THE SOUND........AUDIO...The diode can be placed in either direction cuz the upper and the lower cycle has the same amplitude movements. And when it is AUDIO, we just amplify it to drive a speaker.

CAN WE SIMPLY PUT DIODE TO THE COIL/CAPACITOR AND DIRECTLY HEAR THE AUDIO? IN FACT WE CAN. For a long time people heard the announcer with this set up and long antennas collected the signal and no amplification was needed, NO BATTERY! The AC was directly converted to moving DC straight from the coil! a high impedance headphone was used. What is a high impedance headphone. Well, the speaker has an impedance of 4-8 ohms and the headphone 2,000 ohms! The wire in the coil in the headphone is so tiny but very very long. As such, it does not short out the tiny signal and converts it to magnetic pulses in a diaphram. And we could hear it.

The advent of amplifiers introduce the use of batteries and the first amplifier was the vacuum tubes. The high frequency radio signals was amplified before being converted to DC. Since several stages of amplification was needed, there were many ganged capacitors used until it was strong enough then converted to DC.

Then someone conceived the idea of converting any incoming frequency to a fixed intermediate frequency. 455 Khz was chosen. Then the inventor thought that there were no need for many tuning capacitors. The intermediate frequency can be handled by an amplifier that handled a selective frequency only., in fact, the 455 Khz. Small transformers are were used which are tuned to 455 Khz. These transformers can be fine tuned by moving a ferrite slug in and out of its coil. HOW IS THE FREQUENCY CONVERTED THEN? There is one factual reality. When 2 frequencies are mixed, the sum and difference of those frequencies are generated. So, a local oscillator produced one frequency and the incoming signal is the other frequency. They are mixed. So a local oscillator was designed to generate a frequency that when subtracted from the incoming frequency results to an intermediate frequency of 455 Khz. So, for the 1,000 Khz station, the local oscillator has to produce 545 Khz. The tuner of the antenna coil has a ganged capacitor to also tune the oscillator coil. So, when the antenna's frequency is tuned to a frequency 100 Khz higher, the local oscillator is tuned up by another 100 Khz too. So, the intermediate frequency is maintained at 455 Khz. Haha! only a two ganged tuning capacitor is needed, the tuning capacitor in the AM radio! The oscillator coil is the one with the red ferrite slug. What makes up the intermediate frequency amplifier? They are the 3 IF transformers colored yellow, white, and black. They are attached to the collectors of driver-amplifier transistors. The black coil has a driver tranny, so with the white. But the tranny driving the yellow IFT is given multi-tasking (modern language). It not drives the yellow IFT but it also oscillates at a frequency set by the oscillator coil. So, the tranny is called a mixer-oscillator tranny, the first one. Actually, it is a mixer-oscillator-ampli tranny. In the original tube AM radios, only two IFTs were used, not three. So, there was only one driver tube to the last IFT. The first tube was the mixer-oscillator-ampli. A third tube was used as a diode. A transistor radio will also work with 2 IFTs and there is no need for a 3rd tranny since a simple diode can be used. SO, ONLY 2 TRANNIES WILL BE NEEDED! With 2 trannies, the RF portion of the radio is complete and there is audio after the diode! A single tranny can amplify the audio and drive an earphone! The usual audio amp needed to drive a speaker uses 4 trannies. But today, an small IC is the entire amplifier. So, an AM radio can made with 2 trannies and one IC!