Electronics tutorial - Building an AM modulator

[@stanmeyer9770]

You can modulate the oscillator directly without the need for the second transistor, either by injecting the audio in series with the voltage supply line via a audio transformer or by adding another coupling winding on the oscillator coil with one terminal connected to ground and the second terminal connected to a germanium diode for audio input thus modulating the oscillator by inductive coupling. For best results wind the tank and audio coupling coil on a ferrite rod ( like the ones found in transistor AM radios) but the longer the better 15 or 20cm, this will produce a very stable modulated frequency. To increase the range of the transmitter add a antenna wire any lengh required and connect one end to the circuits ground terminal and 5 turns around the ferrite rod with the other end hanging out to the back of the work bench or attached to the wall or a nearby tree depending on the range you want your transmission to reach e.g with no antenna connected 3 to 10m with a 10m wire anntena 50 to 100m with a 30m antenna about 1km. This explination is just for experimental information and all rules for regulation in the country you reside in must be complied with. Thanks for your informative video.

[@Analog_isnt_dead]

Really enjoying your RF stuff, thanks for posting

[@visai3634]

awesome video , today i learned how to build an am modulator

[@shubhamnayak9369]

Your videos are awesome.. keep making projects. Also Start some intermediate course on such ckts and analytical methods that you use while developing a circuit, it will be huge help for people like us who wants to build intuition for designing ckts

[@edgeeffect]

This is quite a good effect to use in music recording studios.... you could spend hours with audio plugins to get a good "tuning in a radio" sound.... or.... you could just use a radio.

[@falconhawker]

FesZ In order to understand an AM modulator it is necessary to understand WHY a circuit modulates the carrier. AM modulator requires MULTIPLICATION of the carrier AM= (Vm cos 2 pi fm )( Vc sin 2 pi fc ) where mod freq , fm'

[@JushuaAbraham-sj2xl]

I doubt about that when I see the wave from in the simulation ,it looks like summation not multiplication of the two signals ( the carrier and the modulating signal )as it should be in AM

[@vihangatck]

Great video! You are such a good teacher.

[@mancio92M]

I follow all the videos, congratulations, always high quality content!

[@rogeronslow1498]

You don't need a non-linear element to create an AM modulator. You simply control the voltage on collector of the final RF PA stage so that the RF envelope changes in sympathy with the modulation. All you need is an audio amplifier and a transformer to couple to the RF PA stage.

[@joeybuddy96]

Amazing!

[@SaihoS1]

It's hard for me to write in English, so I don't have time to answer in time :) Let me explain. The AM modulator of your choice uses the nonlinearity of Ic vs Vb (as you know, this is an exponent, but we will consider it in a large signal mode, so for simplicity, we will replace the exponent with a straight line with a bend at 0.6V). In proper operation, both the low frequency signal and the high frequency signal must have an amplitude of 1Vpk-pk or more. It is good if they are equal in amplitude to each other. In this case, at the top of the positive half-wave of the audio signal, the high-frequency signal will pass to the output completely, at the top of the negative half-wave it will not pass at all, and when the audio signal passes through 0V, only the positive half-waves of the high-frequency signal will pass to the output. As you can see, the transistor operates in cutoff mode for a significant part of the time, and therefore the output signal will be very different from the modulated sinusoid. Therefore, instead of a resistor, a resonant circuit tuned to the carrier frequency is usually placed in the collector of the transistor. Without such a resonant circuit, the output signal spectrum will be very messy. Here are links to two figures that explain this: conture.by/wp-content/uploads/2011/12/mris23.jpg , conture.by/wp-content/uploads/2011/12/mris24.jpg Naturally, modulation will also occur at not optimal amplitudes of the input signals. It's just that the modulation depth will be much less. By the way, it is better to choose a smaller initial collector current (for example

[@FesZElectronics]

Hello SaihoS1, I agree, as long as the input AF amplitude is large enough, then this operating mode makes complete sense - reduce the Vbe low enough to get the transistor in cutoff mode - no modulation, increase it, get modulation. I initially thought that is how my implementation would end un working - but the main difference I observed (with my particular choice of components and signal levels) was that the larger the AF value (positive peak) corresponding to the higher collector current, the smaller the output RF voltage amplitude (I'm getting highest output amplitude when the AF signal is at the low point). If my understanding is correct, the operating mode you mentioned should work in the exact opposite way. In the simulations Vbe peak stays between 0.628 and 0.632V when both AF and RF signals are applied, so the transistor is nowhere near the cutoff mode. Another interesting phenomenon I observed in the simulation is negative instantaneous base current (with RF signal applied) while collector current was still ~3mA; the only explanation I could come up for this is signal being pushed back trough the base trough collector-base capacity. You also mentioned collector current

[@SaihoS1]

@@FesZElectronics For you, everything works the other way around, for the reason that the transistor is driven into saturation and starts to amplify only when a negative half-wave comes. Ic = 3mA, Vcc = 5V, Rc = 1k, Vb = 2.5V hence Vc = Vcc-Ic * Rc = 2V

[@SaihoS1]

@@FesZElectronics I have not been able to convince you?

[@FesZElectronics]

On the contrary. I think you are right, the effect most predominant is gain loss caused by the transistor going into saturation but I still think that part of the impedance change is caused by BC capacitance variation. I will have to look into this in a bit more detail.

[@SaihoS1]

@@FesZElectronics I think that the capacitance of the collector junction will not noticeably affect the modulation process. Especially in the saturation mode. I read the theory on this mixer and I can say that it can work both in a large signal mode (with cutoff) and in a small signal mode (on a near-quadratic part of I - V characteristic). But only in the first case can the maximum modulation coefficient be achieved. Unfortunately, the book is in Russian and you will probably find it difficult to read even a few pages. But if you wish, I'm ready to provide a link to pdf files. I will patiently await the result of your research. Perhaps you can use this scheme in an unusual way?

[@shaknit]

could you make the rf osc variable freq.

[@shubhamnayak9369]

Make a video on costas loop demodulators

[@jotnarymir1393]

Awesome. Any chance to get an fm version?

[@valentingomez4546]

is the inductor connected to the output to the modulator and ground?

[@MD-qz6gk]

But can you make a radio amplifier for this transmitter circuit?

[@sunilachary123]

SIR Why RF INPUT IN COMMON EMMITTER AND AF INPUT IN COMMON BASE MODE ONLY. ?

[@schwinn434]

I sorry, but I didn't understand the part about the antenna, at the end - needless to say, I don't know much about antenna design; however, aren't all antenna's just emitting electric and magnetic waves. Isn't the coil you built to electro/magnetically couple the output signal to the radio just an antenna?

[@FesZElectronics]

You are right, an antenna is supposed to emit both magnetic and electric fields, and these combine to form electromagnetic waves after a certain distance from the antenna (2 wavelengths). But in the AM band, the wavelength is very long (300m for 1 MHz) so on short distances the 2 types of fields stay separate. With the particular radio I was using, other than an external antenna input, it also had coils on a ferrite rod which works like an antenna only for magnetic fields. So I was trying to take advantage of this principle - generate only magnetic fields.

[@schwinn434]

@@FesZElectronics Thank you for the detail response; again, I'm a novice about antenna design; Yet, I must say, I'm really surprised to learn that at short distances the electric and magnetic fields are separated - I thought I basically, understood that these two fields were always together. I really thought that one require the other. I guess I'm misunderstanding Maxwell's equations. I'll try to study the Maxwell equations more closely, in the future. Again, Thanks for this very interesting info. BTW, your doing excellent work with these videos. Take Care & May God bless you

[@FesZElectronics]

Regarding separation of magnetic and electric fields, I may have expressed myself poorly. Up to a certain distance the 2 fields have different impedance and can be treated separately, only after 2*wavelength do they form a uniform filed with the same impedance. In the case of AM, just to make a decent antenna you need ~30m of cable, so there was no point in making 2 such antennas to transmit my signal over ~1-2m. Therefore I decided it was easier to just focus on magnetic waves and perform signal transmission like that - this allowed for a smaller "antenna" in the form of the inductor.

[@schwinn434]

@@FesZElectronics Thanks, once again, for this detailed and thought provoking response. I had no idea that it took two wave lengths for the magnetic and electric field impedance's to match in an EM wave. IMHO, this is why science (particularly, the field of electronics) is so amazing: even if you "think" you might understand something, it's an understanding of these details which often proves to be the factor in truly understanding a physical phenomena. (And often, these details are critical to understand, in many applications.) I do have a B.S. in EE, so I do have enough formal education to realize you don't truly understand a subject, until you master the details of a subject; for me it became obvious whether I truly understood a subject or not, when I tried to answer (correctly) home work problems - usually, this is when I realize I didn't truly understand a subject (e.g, electromagnetism, etc.) since my answers often weren't the correct ones.

[@FesZElectronics]

I guess this is valid for any field of science - the more you study and learn, the more you realize how much you don't actually know. I guess what made electronics so interesting for me was just how easy it is to test your knowhow, not by taking a test, but rather by building and then improving basic circuits. When it comes to electro-magnetism its a bit more difficult cu try and test things out, but even there, you have the subject of EMC, where all the same rules are applied as with general RF transmissions, only rather than trying to make a good transmitter, you are trying your best to make a bad one.

[@tomyeast3438]

Hello! I'm always watching your videos very beneficially ! But can you implement a PID controller in LTspice? I would like to receive the output from the boost converter as feedback and construct a circuit that controls the PWM, but I can't find any reference materials suitable at Google.

[@SaihoS1]

I cannot recommend you a good material about the boost converter, but I can about the flyback converter. I think that this will be enough so that later you can return to the boost converter yourself. Search google "AN2122 Flyback SMPS Using a Microcontroller as Control Unit"

[@mattmoreira210]

“Building an amplitude modulator modulator” lol

[@buscoparan1772]

amplitude modulation modulator