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70

Neither, there is no modulation involved. The magnetization on the media is directly (and hopefully fairly linearly) related to the waveform amplitude. There is a high frequency bias signal added to the audio signal, to get the resulting signal on the tape to a linear range of the magnetization curve, but the signal at the head is a sum of the two, not a ...


28

It is neither, the signal is recorded as a magnetized pattern without any form of modulation. This can give an impression of how that works: I got that from here. See, no modulation needed, the amount of magnetic flux is proportional to the actual audio signal. Some HiFi Stereo Video recorders do use FM to record the audio though.


26

It uses something called a filter. You can build filters out of all sorts of different things. RC filters made out of resistors and capacitors are probably the simplest to understand. Basically, the capacitor acts as a resistor, but with a different resistance at different frequencies. When you add a resistor, you can build a voltage divider that is ...


20

They need not, narrowband FM occupies the same bandwidth as AM, for the same audio bandwidth. Transmitted AM bandwidth is fixed at twice the audio bandwidth. However, wide deviation FM provides signal to noise ratio improvements above that which is possible for AM. It also provides the so-called capture effect, whereby a weak station near to a stronger ...


13

This is done using a Heterodyne tuning system. For example, let's say you want to tune in a station at 1200kHZ. You set your tuning dial to "1200", which sets a local oscillator to generate a frequency of 745kHz. When you mix them, one of the resulting frequencies is the difference (the freq. you want to tune - 745kHz). The next stage is a narrow band ...


11

Analog tape recording for audio cassette doesnt require modulation. But since magnetization ability of the particles on the tape does not have linear response at low signal levels of the audio to be recorded, poor response will result during playback. To prevent this, technique called biasing is applied. This creates very linear response of magnetization ...


10

Simplest possible AM receiver that outputs to an 8 ohm speaker: simulate this circuit – Schematic created using CircuitLab This radio has: extremely poor selectivity extremely poor sensitivity extremely poor volume extremely poor fidelity It is however, without a doubt, the simplest radio that meets your requirements. It works because the voice ...


9

Your basic strategy to use a piece of wire for the antenna, into a L-C resonant circuit tuned to the frequency of the desired station, amplify that, then connect to a speaker is on the right track but misses at least one important step. That is to detect the AM signal, or to demodulate the amplitude modulated radio signal to get the original audio signal. ...


9

The concept here is that you are adding the signal currents before feeding the sum to the diode; the resistors are there to convert the voltage sources "NF" and "HF" to current sources. You'd get exactly the same effect by adding the signal voltages directly — simply connect the "NF" and "HF" boxes in series, without any resistors. The only downside ...


8

A single transistor used as a multiplier (modulator) is basically imperfect. Yes it will amplitude modulate the carrier but it will also inject the output with the modulation signal and this is what you have. As Brian Drummond says in his comment, there is amplitude modulation occurring AND regular additive mixing. Look at the envelope valleys; the upper ...


8

The main body of this answer was made before the OP added an edit section to his question - that edit section is inapplicable because the type of modulation (although AM) is full double sideband suppressed carrier modulation and diode detectors will be useless if trying to recover decent audio or linear base-band signals. A rather idealized but useful ...


7

Yeah the incoming RF frequency is fixed, but every station has a different fixed frequency. That makes tuning the sort of filter you need for decent reception really tedious! Especially since early radios often had a separate tuning knob for each tuned circuit. Brilliant new innovation : the Osram Music Magnet Four has THREE tuned circuits, with all the ...


7

There are quite a few advantages. Taking a look at a typical superhet (up to the IF): The input signal at the RF input is small (as low as -122dBm in some narrow band voice systems I have worked on - that is about 6.3fW) To amplify a signal at a high RF (say a few GHz) is expensive compared to doing that amplification at a lower frequency. A few dB of RF ...


7

First, I do not understand how the LC filter selects the frequency. I know that it's impedance is 'infinity' at the resonant frequency, but I don't understand how it rejects the other frequencies: no matter the impedance of the LC filter for a given frequency, the voltage is applied at the 'X' node and hence every frequency is applied at the input of the ...


6

The simplest antenna for a crystal radio is a long wire (say 10m), and up as high as you can get it. On the roof is best if you can get up there. The wire should be stretched out, not rolled up. Outside is best, to avoid signal loss in any metal in the building's structure. Make sure the antenna is attached with insulators at both ends. To make it simple, ...


6

Further to my comment. You are quite correct about needing some form of multiplier. Perhaps the following addition could be made to improve the circuit. The FET acts as a voltage controlled resistor altering the gain of the op amp creating an AM signal.


6

The difference has less to do with the modulation, and more with the frequencies used. FM broadcast radio in the United States operates from 87800 to 108000 kHz. The most common AM radio operates from 540 to 1610 kHz. There are other areas of spectrum reserved for AM frequencies globally, but they are all much lower in frequency than those used for FM ...


6

Some of the simplest AM radio receivers were based on the ZN414 - a radio on a single chip in a TO-92 package. (replaced by the MK484). It is poorly suited to drive an 8\$\Omega\$ speaker directly (it won't be very loud) but you could add an LM386 amplifier as shown here. If you really want to work with discrete transistors rather than ICs hiding in ...


6

I don't understand how you could have studied AM demodulation and not understand the purpose of an envelope detector. Something is lacking in your studies. In any case, in AM (the M stands for modulation) all of the information is contained in the amplitude variations of the carrier. That amplitude variation is usually called the envelope of the resultant ...


6

This isn't a direct answer to your question, but too long for a comment. You are going about this completely the wrong way. Your teacher assigned you something so that you will learn about it. You are not only trying to get away with just copying something, but in addition, you clearly don't understand the fundamentals behind what you are trying to do. ...


6

There is no paradox. Periodic functions have no beginning and no end. Signals always have a beginning and an end. Therefore, no signal can be exactly represented by a periodic function. But, the mathematical techniques that we use to analyze periodic functions are both powerful and useful. So useful that we can pretend that a small segment of some ...


6

Note that this answer is skewed toward analog radio reception. The rules are different for software-defined radios, and for digital services. The biggest drawback to direct conversion is sideband suppression. If you use a single mixer, a signal at \$f_c + f_s\$ is indistinguishable from a signal at \$f_c - f_s\$, where \$f_c\$ is the carrier and \$f_s\$ ...


5

My idea is to amplify signals from TMP36 (temperature sensor), give it to AM transmitter, use a ADC (i.e. Arduino) at receiver side to convert output of AM receiver into digital signals It won't work the way you describe. Your proposed method implies you take a slow moving signal and modulate a carrier. This will just be a carrier with some slight ...


5

Adding two signals together doesn't give the right result. What you want with AM is the audio signal moved up in frequency to around the carrier frequency so that many AM channels can co-exist and be individually tuned to by a radio receiver. If you added 1 MHz with an audio source the resultant specturm is plain ordinary 1 MHz and unaltered 20Hz to 20 kHz. ...


5

The coil will have a certain amount of self-capacitance, which will be enough for resonance. The resonant frequency will be in the broadcast band if it has enough turns.


5

This is a very crappy "AM transmitter" circuit which isn't worth its name. It "works" by modulating the supply voltage with an Audio signal. The output of a crystal oscillator switches between negative and positive supply. By changing that supply with a signal, the amplitude of the oscillator's output signal will change with that signal. The "modulation ...


4

To keep it simple, you could feed the signal from the Temp sensor into a VCF (voltage to frequency converter) like a TI 231 or I think there are 555 timer based versions. This signal then can go to the modulator and then the tone detected on the receive side. The receive tone could be simply a comparator and timer on the Arduino (don't know details - but ...


4

Here's an idea: A simple and rough solution by biasing a diode with a small current. It becomes non-linear and rectifies even quite small AC voltages. I've drawn a possible circuit. You could refine all of the values to suit your exact requirements. D_A and D_B are a pair in one SOT23 case. R1 and R3 supply the bias current to the diodes, from a 3.3 V rail. ...


4

That's not a very good crystal radio. Try as long a wire as you can find and the strongest AM station in your area. You might be able to get more than the 300 mV it takes to turn on the diode. By the way, nowadays you use a Schottky diode instead of a germanium one. They have about the same forward drop, but are a lot easier to find. Better crystal ...


4

The detector you show is called an Envelope Detector (Wikipedia). You are missing a couple of important components: A resistor and a capacitor after the diode. When the diode conducts it charges up the capacitor which then discharges relatively slowly through the resistor. The time constant is selected to be long relative to the carrier frequency but ...


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