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!