Newbie attempts amplitude modulation - Page 1
Forum section: electronics / rf-microwave

author: scdreger
author_rank: Contributor
timestamp: August 24, 2017, 08:06:05 pm
content: Hello, I’m not sure if I should have put this in the beginner section, but I chose this subforum since it’s RF related even though it’s a newbie type question. I hope that’s OK.I’m relatively new to electronics and being a ham, my curiosity soon turned to RF related circuits. I started with a WBR regen receiver for 40 meters and that came out well.Anyway, this week just for fun I put together a Colpitts crystal oscillator based on a drawing from a W2AEW video (a great channel btw). Photo attached. Anyway that’s working as expected but naturally I started wondering how you would turn that into an AM transmitter. So I began playing around with a jfet to see if I could make it work (rough schematic of what I’ve tried so far attached). Technically it works, but I feel like there’s probably a better way to do this. Any ideas are welcome, or any good resources you could/would recommend specifically for modulation/amplification?Also, I assume that after modulating, you would then amplify again with another bjt, fet or mosfet? As it is now I can receive this clearly with an (all band, all mode receiving) HT a couple inches away without a TX antenna.Of course it also occurs to me that I also have no idea how to make sure the output is 50ohms should I ever want to make a real (legal) transmitter to use with an antenna. I have so much to learn.

author: BigBoss
author_rank: Regular Contributor
timestamp: August 25, 2017, 08:21:04 pm
content: In order to obtain a AM Transmitter, you should absolutely use a "AM Modulator" You circuit will somehow work but it's not a Modulator, it's a mixer.When you combine two signal then you pass this combined signal through a "nonlinear" component as you have done, you somehow obtain sums and differences due to nonlinear transfer characteristic of the active element because second order nonlinear term will give you sums and differences.But your case shouldn't be that..For a newbie I don't suggest to use complicated oscillator+modulator circuits because their tuning is might really be cumbersome.instead, you may use a simplified integrated solution with SA612.It's a Double Balanced Modulator and it will work easily @ 7MHz with a proper crystal.

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: August 25, 2017, 10:11:01 pm
content: AM is simple to generate but can sometimes be a little challenging.You need three stages to generate it.* RF oscillator (to generate the HF signal)* RF power amplifier (to amplifier the signal)* Modulator (to amplify the audio signal and impress this on the RF signal)In the old days you could do it all in one stage (with a carbon microphone in the supply lead) but it's best to use separate stages.The output of the modulator is connected to the supply rail of the RF power amplifier. Rather than being a constant voltage the supply to the transmitter power amplifier varies. This causes the RF signal to be modulated at audio frequencies. You can do this via a transformer but these can be heavy or hard to obtain.For low power you can just have a direct connection between the modulator and the RF power amplifier supply rail.The Poppet transmitter athttp://www.delboyonline.co.uk/m0dad/construcion/poppet_top_band_am_transmitter.htmorhttp://www.geocities.ws/raiu_harrison/mwa/tech/circuits/poppet.htmlis an example of an LM386 being used as a modulator. Q4 is the RF power amplifer while Q2 & Q3 are the VFO (which can be made crystal control) and driver amplifier.One thing you need to be careful of with circuits like this is blowing the LM386 by drawing too much current from Pin 5. You might want to put a low value resistor (say 4 - 8 ohms) betweenthis and the PA stage to offer some protection.I used this design in a 500mW AM transmitter on 1.8 MHz. It was heard more than 40km away across salt water. The video below is a demonstration.The SA612 mentioned above contains an oscillator and mixer (used as a balanced modulator in this case). This is a simple approach if you want to generate double sideband suppressed carrier (which is a more efficient communications mode than AM as there's no carrier). But you can still get AM if you unbalance the balanced modulator. The main thing with the SA612 is its RF output is low and you will need at least two stages of linear RF amplification to get even a few hundred milliwatts. So I'd go with the LM386 / discrete transistor approach.

author: medical-nerd
author_rank: Regular Contributor
timestamp: August 25, 2017, 11:13:10 pm
content: HiyaThat pcb and holder look interesting, where did you get them?Cheers

author: scdreger
author_rank: Contributor
timestamp: August 26, 2017, 03:39:43 pm
content: Quote from: vk3yedotcom on August 25, 2017, 10:11:01 pmThank you very much (and Big Boss too). This gives me a lot to go on. The poppet seems like an easy build and I like how the schematic is grouped into sections with a label for their function.

author: scdreger
author_rank: Contributor
timestamp: August 26, 2017, 03:47:23 pm
content: Quote from: medical-nerd on August 25, 2017, 11:13:10 pmThe PCB holder is this:https://www.amazon.com/Aven-17010-Adjustable-Circuit-Holder/dp/B00Q2TTQEE/ref=sr_1_3?ie=UTF8&qid=1503762033&sr=8-3&keywords=pcb+holderIt has worked well for me and allows you to flip the pcb over while mounted.The PCB is made by busboard prototype systems. They come in a few different grid (and overall) sizes. I tried it for the first time as an alternative to the island cutter method on solid copper clad. I liked it a lot. It's easy to make isolated nodes on the different areas of grid on the front and the back is solid ground plane, so just send things through the closest "via" and solder to ground. The one in the picture is this:http://www.busboard.com/SP2-100x100-G

author: 4CX35000
author_rank: Regular Contributor
timestamp: August 26, 2017, 09:44:31 pm
content: Might be a good idea to stick with NPN transistors and get familiar with a modulating a carrier, DC characteristics of the transistor and looking at the signals using a oscilloscope. Anyway the website below shows a simple AM transmitter with a oscillator and modulator which to me looks like a reasonable starting point.http://www.electroschematics.com/6821/basic-low-power-am-transmitter/Don't worry over the impedance matching to 50 ohms, at these low powers it does not matter too much. It is best to get familiar with looking at the signals using a oscilloscope and reading the voltages using a multimeter. Calculate the DC bias voltages using the components in the circuit diagram and then carryout some measurements and see if you get similar results. Apply a 1 KHz tone signal to the audio input and see the signal at various levels and see what happens when you exceed the maximum level and listen to the output of a radio.You might even be able to copy he modulator stage of this transmitter and add it to your present oscillator circuit and see what the results are.For Amateur Radio then the modulation tend to be done at low power level and then boosted to high power using a selection of amplifiers, this is called low level modulation. This is ideal for Amateur radio use, but broadcasters, the traditional method is called high level modulation were the oscillator and audio are amplified to a high power level and then modulated.

author: bd139
author_rank: Super Contributor
timestamp: August 27, 2017, 10:13:44 am
content: Grab a copy of Experimental Methods in RF Design if you can. This carries lots of information on practical ways to do AM modulation and why. Why is arguably as important as how. This book is just amazing. It will cover all your impedance matching theory as well.What you're really doing is mixing the carrier and the audio. This is done by multiplying the carrier by the audio. The word multiply is important because this leads to an important trigonometric identity which I won't go into here in detail. Fundamentally when you mix the carrier with a 1KHz tone you end up with two signals (carrier + 1KHz) and (carrier - 1KHz). At this point even AM starts looking complicated! Effectively you have two side bands then and sometimes we want both, sometimes just one (SSB). Hence the book above that covers the theory first.Now there are a few ways to do that but the JFET as described while it works isn't necessarily ideal.1. Use a balanced mixer. This is usually two transformers and four diodes. This used the logarithmic V-I curve of the diodes to multiply. These are dirt cheap and very good.2. Use a Gilbert cell mixer. The SA602 IC recommended above is a pretty good one. It has an oscillator built in too. This uses transistors to do the multiplication.3. Use a dual gate FET such as the 40673 (unobtanium now). I haven't built one of these so I can't characterise how they work.You can use a single gate JFET. The usual approach is to use a simple JFET amplifier and inject the carrier into the gate and the modulation into the source. These work but I couldn't get one that didn't have some serious distortion.The first thing I built when I was playing around with RF was an SSB modulator and filter and then a detector. Nothing was ever transmitted.I don't have a license yet so transmitting is off the cardsEdit: not sure I even want to talk to anyone, just play with RF things