AM transmitter - Page 1
Forum section: electronics / beginners

author: interoth
author_rank: Contributor
timestamp: July 05, 2012, 10:10:59 pm
content: Hey, I am creating a simple AM transmitter consisting of just the carrier wave.I have attached a schematic of the 1MHz oscillator. I have made this design from other designs so I am not sure if it will work.Am I right in thinking that the output is a 1MHz Square wave at +9v?If so how would I attach an antenna?I have seen designs that include a inductor to increase the range/power and I am not sure as to how to how to use an antenna.Thanks for any help

author: RCMR
author_rank: Frequent Contributor
timestamp: July 06, 2012, 12:37:18 am
content: You don't want a square wave -- that will produce a whole heap of harmonics.What's needed to create a decent 1MHz RF carrier is a nice LC oscillator that produces a sine-wave output (low in harmonics).There are plenty of examples out there -- go look up "colpitts oscillator" for some suggestions.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 06, 2012, 01:17:15 am
content: Don't attach an antenna to that thing,unless you want to incur the wrath of your neighbours!Take RCMR's advice,as with your square wave,you will interfere with people's radios,& they will become quite upset,plus you may attract the attention of the licencing authority--& believe me,you don't want that!!Even with a nice 1MHz sine wave,I would be very wary about connecting an antenna,for the same reasons.Many Administrations let you play about with very low power,& minimal antennas on the Broadcast Band--some others forbid it !

author: interoth
author_rank: Contributor
timestamp: July 06, 2012, 06:53:57 am
content: Ha ha, thanks for the advice. It was meant to only be on for up to 5 sec as a sort of remote control.

author: Simon
author_rank: Unknown
timestamp: July 06, 2012, 10:40:37 am
content: you need I believe a colpits oscilator, this will have a "tank circuit" in it that is the Lc part and will produce a sine wave. Use only as much power as you need. I'd suggest one of the frequencies used by AM radio as they are seldom used and just enough power to transmit as far as you need.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 06, 2012, 11:24:16 am
content: Quote from: Simon on July 06, 2012, 10:40:37 amThey may be seldom used in the UK,but the MF Broadcast band is full of Radio stations in Australia!

author: Simon
author_rank: Unknown
timestamp: July 06, 2012, 11:43:16 am
content: well due the the not excellent coverage in the UK most are forced to listen to the radio via the internet. I guess that is why we are having to scale up our internet network - just so we can listen to the godamn radioMy paramount suggestion is keep the power as low as possible, if it is just a carrier it should not upset reception to other devices too much unless they are the same distance as the intended receiver (5 meters ?)

author: vk6zgo
author_rank: Super Contributor
timestamp: July 06, 2012, 12:24:03 pm
content: Remember real radios are likely to be considerably more sensitive than the probable receiver!

author: Simon
author_rank: Unknown
timestamp: July 06, 2012, 12:29:31 pm
content: True or the receiver needs to be very sensitive

author: G7PSK
author_rank: Super Contributor
timestamp: July 06, 2012, 03:08:51 pm
content: I would go for a clapp oscillator as they are a bit more stable than the colpitts but if you only want a 1khz signal a wein bridge circuit would be better as no inductance is required.

author: interoth
author_rank: Contributor
timestamp: July 06, 2012, 06:26:39 pm
content: After the good advice of not connecting an antenna to my 1MHz Square wave, I have made a 50MHz Sine wave Colpitts oscillator.I am hopeful this is correct, if you can see any problems please tell me.I want to know what's the best way to attach an antenna is. Do I need an inductor, capacitor or what?Thanks for your patience, I am quite new to electronics.

author: Simon
author_rank: Unknown
timestamp: July 06, 2012, 07:14:47 pm
content: I am interested, how does one select the capacitor values ? I have need for an oscillator around 100 MHz

author: interoth
author_rank: Contributor
timestamp: July 06, 2012, 07:22:13 pm
content: I used a already existing example, butthis pagegoes into detail on how to calculate the values.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 07, 2012, 04:51:58 am
content: Quote from: interoth on July 06, 2012, 06:26:39 pmNow you will have upset all the Hams ,who do "weak signal communications" at frequencies just above 50MHz!There are reasons why people don't just choose frequencies at random to do stuff,& that is because the Radio Spectrum is already allocated to various services,who don't want your little "drifty" oscillator to interfere with them!OK,if it is just for fun,nobody is going to be very worried,but you seem to want to regularly use this thing as a remote control.They won't care,if it is once or twice,but if it becomes a regular thing,youwillannoy someone!As Auric Goldfinger said: "Once is happenstance,twice is coincidence,but three times............".

author: SeanB
author_rank: Super Contributor
timestamp: July 07, 2012, 05:44:28 am
content: Remember that Hams are pretty good at tracking transmitters, as they do it for fun. Cheese those in the neighbourhood off enough and they will come and find you, then the regulatory authorities will come knocking on your door and they have the authority to confiscate as well as fine you.

author: dda
author_rank: Contributor
timestamp: July 07, 2012, 06:13:30 am
content: Crappy RC toys use 27MHz. I was thinking of using it in a similar way to the OP. For freq where no licence is required can you just blast out RF signals?.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 07, 2012, 06:25:34 am
content: No,you are severely limited in power.And not just anywhere on 27MHz-----AFAIK the toys are crystal controlled,so as not to interfere with CB & Marine channels on & around 27MHz.(In Oz)The receivers are often pretty grotty--superregenerative Rx were common a few years back.It you just steal a toy setup from the toy,nobody will know any difference.

author: dda
author_rank: Contributor
timestamp: July 07, 2012, 10:34:26 am
content: I was basically thinking of cannibilising an rc toy / duplicating the radio circuit and using it to send data over short distances.Is there a guide on what is good / legal RF practice?

author: Simon
author_rank: Unknown
timestamp: July 07, 2012, 10:36:18 am
content: you should be ok with radio control frequencies and keep the power LOW. There are regulations on frequencies AND power. For example in Italy we could use the FM band providing we were not transmitting over a few metres.

author: interoth
author_rank: Contributor
timestamp: July 07, 2012, 02:01:02 pm
content: So what frequency should I use?I am only interested in broadcasting it over a few meters such as across a room.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 07, 2012, 02:17:24 pm
content: I think you would get in the least amount of trouble withdda's idea of using the Transmitter (& probably Receiver),out of a toy!Have you thought about what I suggested in the other thread,using Ultrasonics?

author: interoth
author_rank: Contributor
timestamp: July 07, 2012, 03:16:42 pm
content: I am currently making this to learn electronics, so using one out of a toy would not help me too much.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 07, 2012, 03:49:31 pm
content: You don't have to build every circuit in the world to learn Electronics.You can learn a lot from playing with stuff other people have made,as in adapting the 27MHz RC stuff.By the same token, not everything you do build has to have to have an application,Build your oscillators,find out their characteristics,then pull them apart & build the next one!I learned a lot about astable multivibrators on midnight shift at my then job,using spare twin triode tubes from their store ,caps & resistors from my home "junkbox",& an old dead mains radio chassis to supply power,plus the firm's Telequipment "Serviscope" to look at the waveforms.Much more fun at 2am than reading old magazines,the stuff on the noticeboard,etc.My M/Vs were like Cinderella,though,the 12AT7s had to go back into the store before the shift ended,& the old radio became just an old radio again.

author: interoth
author_rank: Contributor
timestamp: July 07, 2012, 05:10:44 pm
content: I would still rather make my own, I find this subject interesting and want to know how to make one. What I am having issues with is finding a good frequency to use.

author: Simon
author_rank: Unknown
timestamp: July 07, 2012, 05:18:07 pm
content: well for starters what country are you in ?

author: interoth
author_rank: Contributor
timestamp: July 07, 2012, 05:22:32 pm
content: United KingdomI will add it to my profile, strange it did not show up.

author: Simon
author_rank: Unknown
timestamp: July 07, 2012, 05:27:05 pm
content: then I would use the same frequency as radio control toys sold here and use low power or try one of the am frequencies on LOW power not that I know what the regulations are in the UK on non licenced transmission

author: interoth
author_rank: Contributor
timestamp: July 07, 2012, 05:50:39 pm
content: After looking up the UK regulations, it looks like I will have to use 27Mhz. How much voltage should I use for the antenna for only low power? My current design calls for 9V but I think that is a bit much.

author: SeanB
author_rank: Super Contributor
timestamp: July 07, 2012, 06:00:16 pm
content: It is not really a voltage thing, but the power you put in. Try to limit the power to under 10mW and you will not realy have much range on it, and a simple wire antenna will be inefficient in most cases.If you get the matching right and the antenna right then 50mw is enough to communicate around the planet in the right circumstances.

author: interoth
author_rank: Contributor
timestamp: July 07, 2012, 06:03:53 pm
content: Would a 750 Ohm resistor in series with the antenna be enough to limit it to 10mW?

author: G7PSK
author_rank: Super Contributor
timestamp: July 07, 2012, 06:14:41 pm
content: Model control is 26.960 to 27.280 at 1.5 watt max.according to the list that I have, there is also 27.450 which is emergency alarm at no more than 0.5 watt. Putting a resistor in line with the antennae will not limit the power we are talking RF here.You need to limit the power generated by your circuit. I would use a 50 ohm resistance across the out put of the transmitter as the antennae that will in all likely hood do for short range work.

author: interoth
author_rank: Contributor
timestamp: July 07, 2012, 06:37:24 pm
content: So will a Colpitts oscillator be accurate enough or will I need a crystal oscillator? If so do you know any good crystal oscillator designs?

author: G7PSK
author_rank: Super Contributor
timestamp: July 07, 2012, 08:52:08 pm
content: You can crystal control a colpitts.

author: interoth
author_rank: Contributor
timestamp: July 07, 2012, 09:37:13 pm
content: How would you use a crystal oscillator in a Colpitts oscillator? I have never used a crystal oscillator before.

author: G7PSK
author_rank: Super Contributor
timestamp: July 08, 2012, 03:25:14 pm
content: Like this.

author: interoth
author_rank: Contributor
timestamp: July 08, 2012, 07:33:28 pm
content: Thanks, but the diagram you gave me only goes up to 10MHz, how would I get it to 27?

author: G7PSK
author_rank: Super Contributor
timestamp: July 08, 2012, 08:07:35 pm
content: You could use a multiplier circuit or re-calculate the circuit shown and change the crystal but you could well be better of using a chip like the NE602

author: interoth
author_rank: Contributor
timestamp: July 10, 2012, 12:15:40 am
content: After looking around it looks like it will be best to use a pre existing design instead of try to use a half baked idea based on little knowledge of wireless electronics.It looks like I will be using the circuithere.One thing I want to ask is how would I change the circuit to flip a relay instead of reversing a motor?Also what would I have to search on sites like Farnel or RS for those inductors? I can not find them.

author: dda
author_rank: Contributor
timestamp: July 10, 2012, 01:04:29 am
content: The inductors he used are hand wound. Look at the photos. But im sure you can find 33uH most places... Im not sure what the deal with the variable one is, is it referred to in the text somewhere?Also,Quote

author: interoth
author_rank: Contributor
timestamp: July 11, 2012, 06:08:17 pm
content: After looking for the design I can still not find details on the inductors on the design onthis page.Both the transmitter and receiver have inductors that are 'measured' by number of turns. I am looking to use stock parts so I would need to know the inductance.TransmitterReceiverWhat should I use for the unmarked inductors? Would a stock inductor do?

author: RCMR
author_rank: Frequent Contributor
timestamp: July 11, 2012, 07:44:07 pm
content: Some of those inductors will need to be "tuned" in order for the system to work.Just buy some coil formers and wind your own -- cheap, easy and very effective.

author: interoth
author_rank: Contributor
timestamp: July 14, 2012, 12:48:28 am
content: For this design I am looking to use stock items that I can get in large numbers, so I want to know what inductance to use.Is it even possible to use stock inductors for those inductors?Thanks

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: July 14, 2012, 03:23:02 am
content: Quote from: interoth on July 14, 2012, 12:48:28 amI've had success with 1uH RF chokes for the 27 - 40 MHz region in oscillator collector circuits. These would resonate with a trimmer in the few tens of pF. For a secondary wrap 2 or 3 turns of enamelled copper wire around it. Altronics and Jaycar sell them, with the former much cheaper.The inductance values for the pi network on the output will need to be much lower than 1uH. As a rule of thumb, 1uH and about 440pF is good for 7 MHz, so for 27 - 30 MHz then 0.22-0.25 uH would be OK. Something on a T37-6 toriod would be OK but to cut costs it should be possible to make something from enamelled wire wound on a 5 - 10mm former. Here's where a low value inductance meter will be handy. Low value slug tuned coils sometimes come up in the Rockby mailer and may be OK.In relation to RF transistors, a 2N2222A (especially the metal can version), is OK up to 50 MHz and puts out a bit more than the BC548. I think Talking Electronics favour the 2N3563 for small signal VHF stuff over the BC548 - this would also be fine for low power. You see overseas circuits try to push a lot out of a 2N2222A but be safe and run them at maybe 20 - 30mW or so only.I've found that a garden variety LM386 audio amp works well as an AM modulator for transmitters up to about a half watt. Use the standard circuit but without the coupling capacitor between Pin 5 and the speaker. Instead put in (say) a 10 ohm resistor (this protects the LM386 as a near short will blow it up).You'll need a crystal oscillator and one or two RF amp stages to get the RF up to a few hundred milliwatts (if that's your aim). Instead of connecting the last stage to the positive rail (probably via an RF choke), connect it to the free end of the 10 ohm resistor from the LM386. Then maybe a 0.1uF from there to shunt RF to ground. You'll then have an AM transmitter.As to its legality, rules vary between countries. Here in Australia we have a 'Low Interference Potential Device Class Licence' (look it up on acma.gov.au ). It basically says that you can use low power on a generous selection of frequencies without needing to take out a licence. Conditions are that you mustn't interfere with anyone and must accept interference from other users.Permitted power outputs vary with frequencies. They're very often 100 mW (plenty of power to have lots of fun with a 1 - 5km range) but there's some (including around 27 MHz) where you can run 1 watt. I like around 36.864 MHz as you can double 18.432 MHz crystals and there's less interference than 27 MHz. Other countries may have 'short range device' provisions or similar.The cheapest way to start experimenting is to buy a set of toy walkie talkies for $6-7. These are often AM with super-regenerative receivers. Maybe get 2 sets, so you have one good receiver, one good transmitter (to use as a reference) and then there's two left over to pull apart or modify.Peter

author: G7OVK
author_rank: Newbie
timestamp: July 26, 2012, 05:13:15 pm
content: Designing your own rf circuits with next to no experience of rf is not the way to proceed. As mentioned, the best suggestion is to play around with designs already published that are known to work. Build up some exerience that way. RF can be good fun, but it can be hard-core analogue and not really something to be starting out with. When building the circuits to experiment with, dont use breadboard. Breadboard is terrible for most rf work with stray capacitances and inductances everywhere. Best way is to build on copper clad board - you can use a large piece of copper clad board for the ground and glue small square copper clad board pieces for component connections. Keep all leads as short as possible.I don't think there will be any complaints from anyone (authorites or neighbours) from interference. As long as the circuits are well built, suitable frequencies are chosen and only a few milliwatts are output, I doubt anything bad will happen. In most cases, the signal will be weak and shouldn't attract anyone's attention of more than a few metres at the most. I also wouldn't touch anything LC based above around 10 MHz. Just stick with crystals - at least you won't have any drift worries.

author: G7OVK
author_rank: Newbie
timestamp: July 26, 2012, 05:18:46 pm
content: Forgot to mention also, don't use anything in the MW broadcast band. I'm not sure who said the AM broadcast bands are seldom used in the UK? They are still very much in use, although I agree most people now listen on FM or via DAB. Biggest problem is the interference you will suffer, especially at night when further afield stations will come in, some of them running hundreds of thousands of watts. You don't exactly want your garage door to suddenly open itself due to DX coming in triggering the receiver!!

author: T4P
author_rank: Super Contributor
timestamp: July 26, 2012, 05:49:49 pm
content: Quote from: G7OVK on July 26, 2012, 05:18:46 pmDX?

author: G7OVK
author_rank: Newbie
timestamp: July 26, 2012, 06:40:20 pm
content: Quote from: DaveXRT on July 26, 2012, 05:49:49 pmDX = Long Distance.Don't want a system that works when the frequency is clear but then fails when a long distance signal suddenly comes in as the day turns into night.

author: SeanB
author_rank: Super Contributor
timestamp: July 26, 2012, 06:51:31 pm
content: Don't think that DX is only a MW problem, it affects FM and higher as well.I used to listen to Capital Radio on 604kHz ( the SA version not the UK one) at nights, as otherwise I was in the daytime skip zone. Evening and the signal would arrive out of the noise as it got darker.

author: G7OVK
author_rank: Newbie
timestamp: July 26, 2012, 09:19:53 pm
content: Quote from: SeanB on July 26, 2012, 06:51:31 pmYou are right. It can affect all frequencies. The lower frequencies are affected the most, more frequently. I personally wouldn't put any kind of remote control system on any broadcast band anyway, despite any skip issues, and definitely not in the MW or SW broadcast bands. I'd also not be relying on a remote system that could be triggered by a carrier. It would have to have some sort of access control, even as simple as a single tone that could be decoded by a tone decoder, or a DTMF system as a minimum.

author: asbokid
author_rank: Regular Contributor
timestamp: July 26, 2012, 09:45:57 pm
content: Quote from: G7OVK on July 26, 2012, 05:18:46 pmAM RF ingress causes loads of problems to xDSL services, currently up to 17MHz (G.993.2 Profile 17a) in the UK.The QLN (Quiet Line Noise) dataset of a Broadcom-chipset modem can illustrate the problem. There is fun to be had geo-locating a broadband subscriber, based only on his QLN profile