Making a first AM transmitter with modular parts - Page 1
Forum section: electronics / beginners

author: Beamin
author_rank: Super Contributor
timestamp: October 23, 2017, 04:47:37 am
content: So the crystal thread gave me an idea of how to make a simple low power fox hunt radio.Build it in moduals so it's good to learn on and each stage can work off the last without too much trouble shooting.I have already created the crystal oscillator. Next will be a AM modulator that puts in a ~500Hz tone. I want to make the output of this variable so the gain can be adjusted to the amplifier.Next will be the amplifier. I prefer to make it from scratch but if I find there are cheap ebay amps I would go with one of those. It will put out between 100mW and 1 W.The power supply is going to be a 12 volt probably (depends on what the amp requires) Then fed into adjustable ebay buck boost converters. The adjustable converter should allow me to vary the gain of the signal that goes into the amp and ideally it will be as low power as possible with the antenna doing all the work.Do you think this is a good way to learn? The thought of making all this in one circuit seems like it would be too much to trouble shoot if it doesn't work. The scematic are going to be ones I find on the internet because I don't know enough to make this from scratch.Attached is a rough idea with the first stage already to be built.

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 23, 2017, 05:21:42 am
content: The licensed users of the 36 MHz spectrum would be annoyed by your illegal transmissions. To the point where they may even complain to the government authorities who might send out investigators to shut you down or worse. Since you have not identified what country you are in we are left to surmise that you are likely not authorized to transmit anywhere near that frequency.Suggest studying some amateur radio ("ham") websites and see what is legal and how to get a licence (pretty easy these days). And if you can't find at least two or three dozen circuits for fox-hunt transmitters, you aren't trying very hard. People have been doing this likely since before you were born and there are some clever circuits out there that would be a very valuable learning experience to study.

author: Zero999
author_rank: Super Contributor
timestamp: October 23, 2017, 08:24:55 am
content: Have you tried modulating the power supply voltage to the oscillator?

author: Beamin
author_rank: Super Contributor
timestamp: October 23, 2017, 09:33:19 am
content: I haven't played with the crystal oscillator until I get my copper clad board in the mail and make a proper ground plane using the "island" technique. Then a bunch of different crystals are going to go into it 10m 11m 49mhz etc etc.Quote from: Richard Crowley on October 23, 2017, 05:21:42 amAlready have a license (but thank you for reminding me what a "ham" was I almost forgot). Reread the thread and let me elaborate: The 36MHz is for low power testing. Also using a very high gain directional RX antenna in this area I have found that this is an unused part of the spectrum by monitoring all around the frequency for 24 hours several days a week. Once the radio is working I will switch to a 49 MHz band or 27 or perhaps a ham band I'm licensed for. I'm trying to build this myself from a "modular design" a rube Goldberg device where I can mess around with each part. It's not for fox hunting but for testing out antennas that I build. I appreciate you looking out for me when you insinuated I was "Young(or younger then you), naïve, and will be operating malicious radio equipment with reckless abandon and blatant disregard for legitimate users of the allocated spectrum in my country or territory". Feel free to report me to the FCC or local judicial body of the appropriate government(s) for trying to learn something fun. But remember cindy no one likes a tattle tale. Furthermore if I am caught I will not implicate you in anyway should any legal action ensure; in fact I will state the opposite that you told me not to but I did it anyways. If you feel more comfortable you can send me a legal disclaimer by certified mail barring you from any harm; legal, emotional, or physical that you suffered during one of my transmissions. But seriously I do realize that one touch of the TX button outside of the legal bands will send you down a one way street to jail followed by a life of crime and banishment from society as a convicted felon. Worse then pulling the trigger of a gun if you ask me.Now that we have the obligatory lecture out of the way we can get back to fun stuff.

author: Damianos
author_rank: Frequent Contributor
timestamp: October 23, 2017, 04:39:30 pm
content: Quote from: Hero999 on October 23, 2017, 08:24:55 amBy doing so, the signal will be multi-modulated. In addition to the amplitude, the shape and the phase / frequency will also be modulated.A scheme, that I am thinking on:Oscillator --> Buffer amplifier (preferably tuned) --> modulated amplifier --> final power amplifier (linear, class A or B-PP) --> output couplingThe buffer amplifier protects the oscillator from disturbances from loading. If it is also tuned, it will make-up the shape of the signal.The separated power amplifier protects the operation of the modulator from the load variations.The output coupling adapts the load impedance and is the final filter of unwanted products.

author: dmills
author_rank: Super Contributor
timestamp: October 23, 2017, 05:05:46 pm
content: I would advocate getting a copy of 'experimental methods in RF design', it really is very good.Have you actually tried modulating the oscillator supply? With a crystal in play the PM is typically not overly problematic and it does make for a gloriously simple QRP set (For all that you are limited to something well short of 100% negative mod).Here is how I would skin this.Drive the final amplifier with a square wave and modulate the supply to the final amp, then clean up with a LPF. That way the final is in class C and your power dissipation moves to the modulation amplifier. The hardcore go class E here, but that is tricky for a beginner.You do not want to have to build a linear final amp if it can be reasonably avoided, they are never very efficient.A crystal osc, buffer (Can be code for a gate driver chip!), and a smallish mosfet (Cdg is the figure to watch here, miller capacitance will kill you if you are not careful), with a Vds rating at least three of four times the maximum supply voltage makes a reasonable RF power source.Output network is probably some kind of L network followed by a LPF.Modulator can be single ended or push pull to taste.73 Dan.

author: Damianos
author_rank: Frequent Contributor
timestamp: October 23, 2017, 06:11:11 pm
content: Of course, you can make a transmitter with a single stage (a modulated oscillator), but the OP says he wants to learn ...

author: Beamin
author_rank: Super Contributor
timestamp: October 25, 2017, 04:32:44 am
content: Quote from: Damianos on October 23, 2017, 04:39:30 pmDo you have schematics?

author: ebclr
author_rank: Super Contributor
timestamp: October 25, 2017, 04:43:40 am
content: https://www.onsemi.com/pub/Collateral/AN531-D.PDF

author: Damianos
author_rank: Frequent Contributor
timestamp: October 26, 2017, 07:43:23 pm
content: Quote from: Beamin on October 25, 2017, 04:32:44 amNothing special. I had played with the simple circuits found in magazines, radio amateur handbooks (such as ARRL) and related websites.