How to measure AM broadcast circuit with oscilloscope - Page 1
Forum section: electronics / beginners

author: jj5
author_rank: Unknown
timestamp: March 11, 2024, 04:48:44 pm
content: I'm trying to test the following circuit which is supposed to be an AM CW transmitter. I have numbered five potential test points. I am wondering can someone tell me where I should expect to find the following signals:a) the carrier waveb) the message signalc) the modulated signal (which I understand is carrier wave + message signal?)I am trying to understand how this circuit works so anything you could explain to me would be very much appreciated.

author: bdunham7
author_rank: Super Contributor
timestamp: March 11, 2024, 05:01:56 pm
content: It is a keyed CW (Continuous Wave) transmitter and really isn't AM in the way you'd typically understand amplitude modulation. You could call it AM in the sense that it has two states of modulation--100% and 0%, but that's not a very useful way of describing things. Your oscilloscope probe loading may significantly affect the operation of the oscillator so keep that in mind if you want any sort of precision in your measurments. If your oscilloscope is grounded, then the best place to see the CW output is probably point 2 and you can see the "message", which is simply whether the key is open or closed, by probing point 3.

author: jj5
author_rank: Unknown
timestamp: March 11, 2024, 05:10:03 pm
content: Quote from: bdunham7 on March 11, 2024, 05:01:56 pmMy scope is a Rigol MSO5074 and I am only just beginning to learn how to use it. My probes are PVP2350 passive probes. Do you know what the "probe loading" of my probes would be? I have never heard that term before. Do you mean like the 10 MΩ the scope puts across the probe contacts?Quote from: bdunham7 on March 11, 2024, 05:01:56 pmI have no idea what this means. My probes have a ground pin, is that what you mean? Or do you mean that the scope itself is plugged into the earth cables with the mains supply?I will probe points 2 and 3 to see what I can see, thanks very much!

author: jj5
author_rank: Unknown
timestamp: March 11, 2024, 05:12:41 pm
content: Quote from: bdunham7 on March 11, 2024, 05:01:56 pmOne thing I don't know is what this "message" should "sound like" when I tune in my AM radio. Should I expect a buzzing sound? Silence? Some sort of hiss?

author: bdunham7
author_rank: Super Contributor
timestamp: March 11, 2024, 05:15:31 pm
content: Quote from: jj5 on March 11, 2024, 05:10:03 pmYour probe, presuming you have it in 10X mode which you should, will have an input load of 10MΩ + 10pF and in the context of this circuit, the 10pF is important. Anywhere you probe you are essentially installing a temporary 10pF capacitor between that point and ground. At point #2 this may not be a huge deal, but at the base of the transistor this might cause a large frequency shift or even stop the oscillator.In this case by "grounded" I mean the ground clip or lead of the probe is attached to the ground point indicated in the schematic, although I'd also assume that the scope was plugged into a socket with proper grounding.

author: Mahagam
author_rank: Regular Contributor
timestamp: March 11, 2024, 05:23:55 pm
content: perhaps this particular scheme will not work at all, since no base bias voltage at all. but maybe this is only the sketch

author: jj5
author_rank: Unknown
timestamp: March 11, 2024, 05:27:03 pm
content: Quote from: bdunham7 on March 11, 2024, 05:15:31 pmOh, my probe is in 1x mode. I will switch it to 10x mode. I recall hearing from Dave Jones that I should always have my scope in 10x mode but I didn't understand the rationale for that.Quote from: bdunham7 on March 11, 2024, 05:15:31 pmYes! That's what happened! lol. That's why I asked my question. I mean, there was only 5 potential test points, so I figured I could just try testing each one, but when I probed at the base of the transistor all my readings just flatlined, I've never seen anything like it and have no explanation for that behaviour.Quote from: bdunham7 on March 11, 2024, 05:15:31 pmYes, the ground clip of the probe is attached to the ground point. Thanks again.One thing I think I have learned today is that each of the four probes on the four channels of my scope have their ground leads connected to each other, they are not separate. Is that right?

author: TimFox
author_rank: Super Contributor
timestamp: March 11, 2024, 05:34:32 pm
content: Quote from: jj5 on March 11, 2024, 05:12:41 pmI don’t know how much hiss you would hear in a normal AM radio: the output of the demodulation would be only a shift in the DC level.Proper demodulation of a keyed CW waveform uses a beat-frequency oscillator (q.v.) in the receiver to give you an audible tone when the key is down.

author: bdunham7
author_rank: Super Contributor
timestamp: March 11, 2024, 05:42:49 pm
content: Quote from: jj5 on March 11, 2024, 05:27:03 pmThat's correct, they're all connected to the same ground. As a beginner, you can avoid at least some catastropes by only using one ground clip--simply leave them off the other three probes. That method has its limitations, but it is a good way to start without any fires.

author: audiotubes
author_rank: Regular Contributor
timestamp: March 11, 2024, 06:56:49 pm
content: Quote from: jj5 on March 11, 2024, 05:12:41 pmIt's called a "carrier" and it is the sound of silence as long as the transmitter is free of spurious oscillations.

author: jj5
author_rank: Unknown
timestamp: March 11, 2024, 07:04:38 pm
content: Quote from: audiotubes on March 11, 2024, 06:56:49 pmExcellent. Thank you! Because I did think I had managed to tune in the circuit, but when I pressed the key the radio went silent. I was expecting some sort of oscillating tone, but I think I was mistaken about that, that's not how this circuit works. Thanks again.

author: radiolistener
author_rank: Super Contributor
timestamp: March 11, 2024, 08:59:20 pm
content: CW and AM are different type of modulation.CW just uses enable/disable carrier with about 500 Hz bandwidth.If you use AM receiver, you will hear silence when CW transmitter is enabled.In order to listen CW tone, you're needs additional beat frequency oscillator (BFO) in the receiver circuit.

author: jj5
author_rank: Unknown
timestamp: March 13, 2024, 05:37:23 pm
content: Thanks to everyone who helped me with this. I put together a video where I applied everything I learned from this forum topic:

author: 807
author_rank: Frequent Contributor
timestamp: March 14, 2024, 12:20:23 am
content: You are using the term "DC" in the wrong context here. The transmitter is not transmitting DC, it is simply transmitting with no modulation. On a similar note, many people use the term AM band. It is often used to describe the Medium wave band, but can cause confusion, as amplitude modulation is used on Long wave (LW), Medium wave (MW), & Short wave (SW). So I never use the term AM to describe a waveband. AM is also used on the Aircraft VHF band (118 - 137MHz).A lot of receivers that are able to pick up morse code (CW) are called Communication receivers. The beat frequency oscillator (BFO) is a circuitwithinthe receiver that, as the name implies, generates a signal that mixes with the incoming signal to create a beat frequency that's in the audio range, so can be heard.You can hear this effect yourself if you tune your receiver to a radio station (if there are any still broadcasting in your area) & then gradually tune the transmitter closer to the radio station. You will hear a high pitched tone, gradually getting lower in frequency as you get closer. When your transmitter is on the same frequency as the radio station, you will probably hear a fluttering sound because a simple transmitter like that won't be able to tune exactly to that of the radio station because it doesn't have very good stability. When a transmitter is on exactly the same frequency as the radio station, it is known as "zero beat", as the frequency difference between them is zero, so produses no difference tone.

author: fourfathom
author_rank: Super Contributor
timestamp: March 14, 2024, 03:39:04 am
content: Quote from: Mahagam on March 11, 2024, 05:23:55 pmThat was my first thought as well. It seems that even if the initial power-on transient through the base capacitor turns on the transistor, the base-emitter rectification is eventually going to bias itself to the off state.

author: jj5
author_rank: Unknown
timestamp: March 14, 2024, 03:41:39 am
content: Quote from: fourfathom on March 14, 2024, 03:39:04 amThat would stack with my experience with this circuit being very unreliable. Sometimes the oscillator starts when power is connected, and sometimes... it doesn't.

author: Zero999
author_rank: Super Contributor
timestamp: March 14, 2024, 01:42:00 pm
content: Quote from: TimFox on March 11, 2024, 05:34:32 pmI expect most receivers will emit a pop, when the transmitter is activated and possible deactivated. It might be still useful for Morse code, but probably not.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 15, 2024, 03:18:40 am
content: Similar experimenter kit with AM broadcast band transmitters. One CW, the other MCW. Maybe you will find the text helpful.