I've built many Crystal radios in the past and was amazed at how much volume you could get into high impedance earphones, I even managed to drive a speaker using transformer matching to fairly good volume, the transmitter was high power 500 kw only 30 miles away!
I totally agree! I’ve recently acquired an old Philips PM3215. Though it can’t match my digital oscilloscope in functionality, I freaking love it. It does what I need 80% of the time.
I think your description of AM demodulation is wrong, especially the part about sidebands. With a simple diode detector you are always going to have both sidebands present since you have no filtering but you can't see them on a scope unless you use FFT or a spectrum analyzer. Simply reversing the detector diode does nothing but invert the audio signal which doesn't make any difference since it's just audio.
I think even saying the word sideband in this demo is probably a mistake. Just calling it the upper half and lower half would have been enough. Sidebands are a deep topic, and I've never seen a discussion about them that didn't start a fight.
I'm a total noob at AM decoding but i was expecting the rectified signal to contain the carrier: is the diode also doing a low pass filter (is it bandwidth limited) ?
@@chillidogkev Yes, the carrier IS in the signal after the diode. One of the problems using a digital scope for these demonstrations. On an analog scope it would have been easier to see. The process is identical to using diodes for power supply rectification, you get the half of the signal you you want to keep, then use caps to fill in between the peaks. Difference is for a power supply you use big caps to maintain a steady voltage, with a demodulator you use a pretty small cap because you want to follow the audio signal.
The process of AM modulation is the same as heterodyning. In fact, you can create AM modulation with a mixer. If you are discussing conventional AM Modulation, you do not vary the amplitude of the carrier. In fact, the FCC rules and regulations specify that the carrier level shall not vary by more than 4% under any condition of modulation. There are some methods of AM modulation that vary the carrier level, controlled carrier modulation, but this is not generally used in domestic AM broadcast stations. The output of the transmitter is a mixer product, the original carrier frequency, the sum of the modulating audio and carrier frequency, and the difference between the carrier and the modulation frequency. The original audio is dissipated as heat in the process. All the information is contained in the sum and difference signals. When you look at an AM signal on an oscilloscope, digital or analog, what you are seeing is the modulation envelope, not the carrier and sidebands. This is the resultant of the sidebands and the carrier caused by the fact the sidebands are out of phase with each other and the carrier. To actually see the amplitude of the sidebands and the carrier it is best to look at them in the frequency domain on either a spectrum analyzer or FFT. When you do that, you can clearly see the amplitude of the carrier does not vary when modulation is applied. To detect the signal, you feed it through a reverse of the modulation process and pass the two sidebands and the carrier through a nonlinear device where they are heterodyned with each other, producing a number of products, including the difference between one or the other sidebands and the carrier, which is the original modulation frequency. All the other products are simply thrown away by filtering. I know there are is probably a shelf full of high school physics books and popular magazine articles, which talk about varying the carrier with modulation, but this simply not what happens and study of more advanced books and articles will support that as well as looking at AM signals in the proper domain. When you look at the envelope on a scope, you are seeing the effects of what is commonly called upward and inward modulation. The waveform above the resting carrier level is upward, or positive modulation, and the modulation below that level is the inward or negative modulation. The modulation is identical on the two sidebands, but that is not what you are looking at on an oscilloscope. If you look at the upward and downward modulation on a commercial AM station, you will see that it is not symmetrical about the carrier peak level. Stations usually apply 135% positive or upward modulation and 100% inward or negative modulation.
Like I mentioned in your last Item about that Rare radio could a sample of the IF be limited before being fed to a PLL which outputs an signal at 4x455khz of which a small sample is divided by four and compared with the 455khz IF which has been limited to remove the AM signal but the x4 signal from the PLL is used to drive a balanced Tayloe mixer direct conversion stage (Quadrature Sampling Detector) that will allow you to listen to AM without other signals interfering with the station you want to listen to.
The device on the left of the scope should show the l_sideband-carrier-u_sideband view if i am not mistaken. or fft on the scope. Just my humble opinion. Great video as always.
It is not correct to describe the upper and lower halves of an AM signal as "side bands". AM modulation works the same way as the mixer stage in a superheterodyne receiver. When you mix an AF signal with an RF carrier you get 3 different frequencies: the original carrier frequency and frequencies that are the sum and difference between the carrier and AF frequency. It is these sum and difference frequencies that are called "side bands".
The most technical thing I have ever heard about AM. Your comment is most underrated and should in text books. 100% people dosent know this. I was having this doubt for over a decade and now got a confirmation. Thanks alot
@@psionl0 may be. But most of the people have an understanding that these are side bands. I enquired to many to get the same response. Figured it out on my own and left it. Now when I saw your comment I got excited. You can also ask people to get the answer as side bands, I am sure
@@ramrajpanthukalathil1601 Are you saying that some text books call the upper and lower halves of an AM signal "sidebands"? I have never seen that before.
The fact Tony doesn't seem to understand the limitations of DSO's in these sort of measurements and takes no action to mitigate them (chief among which is using the wrong kind of DSO to begin with), like turning off interpollation or putting a low pass filter on the trigger, is no justification for sticking to analog scopes. Don't get me wrong, I'm hanging on to my analog scope for sentimental reasons... it was a graduation gift from my parents after all, but I don't think I've used it in the past 12 years. Even though I, like Tony, don't own a digital phosphor oscilloscope (which mitigates the fact a digital oscilloscope displays a datapoint at a standard brightness, regardless of whether it occurs infrequently or not), but, nowadays, even just entry level, digitiser+laptop DSO's are far beyond analog scopes' capabilities. Compared to early, low end, digital scopes, analog scopes are inherently better, at their pricepoint, than digital scopes in exactly two areas: displaying AM modulated signals and videorecorder head alignment. And both of those are thoroughly obsolete technologies. I mean, I listened to AM radio as a novelty, as a teen, in the 80's. Meanwhile, digital scopes have come a long way, to where, if one knows what one is doing, there is no real downside except being not as intuitive. Get over it.
Jan Verschueren I don't see why some people think that we should only have one kind of scope, or which one is the best. Who cares?? Why can't we have many different kinds of scopes?? I love my old Tek 2465A DV, but that's not gonna stop me from getting a digital scope. At some point I'll probably get a PC based scope as well. If I had the money I would have one of each kind of scope that's ever been made. I don't understand why anyone would want to limit themselves to having only one kind of scope. I say get many kinds of scopes and enjoy the benefits of each!
All kinds of tools for different work. As much as I love my Fluke handhelds, there will always be a place on my bench for a VTVM and my Simpson 260. Same with scopes.
The RF envelope is simply the part of an RF signal that is produced by the modulation. PEP, or peak envelope power would be the amount of power of an RF signal with 100% modulation. This, of course, would be higher than the RF carrier by itself.
You can use FFT function to do the same and turn on interpollation and would have stable trigger ad getting into sideband is a whole another topic that goes much deeper, but using the FFT feature to better explain the upper and lower sidebands would have been better.
