AM Transmitter - Page 1
Forum section: electronics / beginners

author: Polybus
author_rank: Contributor
timestamp: May 10, 2015, 03:19:08 am
content: Howdy,I'm having a go at building the simple AM Transmitter as shown athttp://sci-toys.com/scitoys/scitoys/radio/am_transmitter.htmlMy Beginner question is this:I can't easily get hold of the Crystal Oscillator they specify (I'm in Australia) -http://sci-toys.com/scitoys/scitoys/radio/fingers_1_mhz.jpgorhttp://www.scitoyscatalog.com/mm5/merchant.mvc?Screen=PROD&Store_Code=SC&Product_Code=OSC2&Category_Code=RBut I can get a SG8002DCPHB1MHZ, Crystal Oscillator found here:http://au.rs-online.com/web/p/crystal-oscillators/4785749/Will the SG replace the two specified oscillators?I have searched the web and found specs for each - and to a newby they seem compatible - but before I order them - any comments??And a similar question with the Transformer - they specify this one:http://sci-toys.com/scitoys/scitoys/radio/xformer.jpgAgain - I have looked them up - and compared them - and I am sure that this one will replace it OK:http://www.jaycar.com.au/Power-Products-Electrical/Power-Conversion-%26-Transformation/AC-AC-Transformers/1K-ohm-Centre-Tapped---8-ohm-Output-Transformer/p/MM2532Anyone wish to enlighten a struggling newby?Thanks heaps.Polybus

author: c4757p
author_rank: Super Contributor
timestamp: May 10, 2015, 03:33:16 am
content: In general, it's hard to promise that a given replacement for the oscillator will work, since they're using it for such an unusual purpose - but having tried this myself, I found most worked.However, you've got to check the voltage rating. They're applying 9V, but the one you chose has a maximum supply voltage of 5.5V. You might be able to make it work on the lower voltage, but otherwise you'll have to find one with a higher maximum.Transformer looks fine. Not much to say about those.*Aside: the way they label the power input "+9 volts" and "-9 volts" is silly and misleading. Please try not to learn from that example... It should be "+9V" and "0V".

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 10, 2015, 04:17:47 am
content: Yes, I concur with @c4757p.Since the crystal oscillator is being "abused", there is really no predicting that ANY particular oscillator can be AM modulated as shown.All you can do is try it and see if it works.Also agree that is a quite suitable alternate transformer for that circuit.

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: May 10, 2015, 04:36:27 am
content: Crystal oscillator modules are great fun and you should get some results out of them.1 MHz will be hard to build an efficient antenna for.Also make sure there's no local broadcast stations on 1 MHz in your area otherwise you'll battle to hear it.Once I keyed one (sending morse code on 28 MHz) and got a 5km range.Another thing that's worth trying (if you've got a portable UHF receiver) is to modulate one of those cheap 433 MHz data modules. The transmission is a mix of AM and FM, though mostly FM. They can give good range up to >1km and depending on where you are no licence is required. Eg

author: Polybus
author_rank: Contributor
timestamp: May 10, 2015, 04:56:05 am
content: RIGHT!!!Thanks for the info guys.I've placed the order and now all I've got to do it wait.....and wait.I live quite remote in Far North Queensland - with almost no AM stations so I should be right with the 1MHz.I'm building this little transmitter to test my Crystal Radio - no nearby AM Stations to listen toThanks again.Polybus

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: May 10, 2015, 07:43:57 am
content: Quote from: Polybus on May 10, 2015, 04:56:05 amOnce done, thought about a shortwave crystal radio?You may be able to get Radio Australia and even further on it.Possibly best to try an audio amplifier first - though very strong signals will be receivable without it.

author: Polybus
author_rank: Contributor
timestamp: May 10, 2015, 08:37:07 am
content: I have seen a few Short Wave Crystal Radios on teh web - do you have a favourite one that is within the abilities of a beginner to understand and build?Polybus

author: Zero999
author_rank: Super Contributor
timestamp: May 10, 2015, 10:32:22 am
content: The type of oscillator used outputs a squarewave, rather than a sine, so the same thing will be transmitted on harmonics. You may even find you get better results with your crystal radio tuned to a harmonic, rather than the fundamental.

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: May 10, 2015, 11:49:09 am
content: Quote from: Polybus on May 10, 2015, 08:37:07 amThis one is pretty simple.http://homepage.ntlworld.com/lapthorn/xtal.htmNote that we don't have as many strong signals as other parts of the world so you might only hear 2 or 3 stations at a time.If you don't have a toroid you can use an air wound coil. This is one I made, that with the assistance of RF bias through a beat frequency oscillator, can hear SSB signals. Without the BFO it will pick up AM signals on shortwave. A 1 transistor audio amp & crystal earpiece is desirable, preferably switchable so you can test it with the amp out.The second version, with a different coil is

author: Unknown
author_rank: Unknown
timestamp: May 10, 2015, 11:55:14 am
content: Quote from: Polybus on May 10, 2015, 08:37:07 amThe design/circuit athttp://www.qsl.net/kc4iwt/xtal/SWMystery.htmwas the very first SW crystal set I ever built, as a kid back in the 70's. It's about as simple as it gets and, provided you give it a decent antenna / ground & use a hi-Z headset or crystal earpiece, should have no problem pulling in RA or other stations in the region (e.g. All India Radio would be a good bet in the mornings or evenings).It's also pretty easy to mod e.g. I built a couple versions with varicap tuning, & others with switchable fixed + variable caps that could tune in ranges from a couple of MHz up into the low VHF band.

author: Polybus
author_rank: Contributor
timestamp: May 23, 2015, 06:09:17 am
content: RIGHT!!!I've got all my bits and pieces and got the transmitter up and running!!Very happy - its all still on a breadboard but I'll sort that out later.At the moment, with a 6 inch antenna - it will transmit all of about 12 inchesbut I'm still a happy camper.But of course I have a bunch of questions now that I can't really find answers to on the web.My Transformer is supposed to be 1000 Ohms to 8 Ohms - but when I measure the primary and the secondary I get about 400 and 4 (approx those figures) - Im guessing that is because I'm measuring simple resistance, while they are quoting Impedance - Is that right?And I've been looking around at a bunch of different transformers and they seem to be described differently:Min is 1kOhm to 8 OhmsBut some are described as 5w to 8 Ohms - what's that all about? I can't find a simple explanationI know Ohms Law and P=VI etc, and I can do the algebra (Maths is not my weak point - electronics is!!) Does the 5w relate to any combination of V and I that will equal 5w, and can I then use any particular combination of V and I (lets just say 10v and 0.5A for the sake of the discussion) to calculate a resistance??Doesn't really make a lot of sense....for one -I'd be calculating Resistance - not Impedence.Anyone want to take pity of a poor Newby.....Polybus

author: Zero999
author_rank: Super Contributor
timestamp: May 23, 2015, 07:26:59 am
content: You're just measuring the DC resistances of the transformer.The transformer has an impedance of 1k across the primary, at audio frequencies, when an 8R load is connected to the secondary.A 5W transformer will be rated to 5 Watts.

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: May 23, 2015, 07:34:26 am
content: Quote from: Hero999 on May 23, 2015, 07:26:59 amFrom memory a 1k winding on a small transformer will measure about 60 or 70 ohms DC with a multimeter. The speaker connection is much less - 1 ohm or less.This is a good way of testing transformers to determine whether they're interstage coupling (ie both sides 500 ohm plus) or speaker (one side low resistance).

author: Unknown
author_rank: Unknown
timestamp: May 23, 2015, 08:00:02 am
content: Quote from: Polybus on May 23, 2015, 06:09:17 amThese are most likely PA transformers, meant for connecting an 8 ohm speaker to a 100v / 70v public address distribution line. Since the primary is assumed to be fed from a 100v (or 70v, usually via a tapped primary) audio signal, it's common for them to be described in terms of audio power rating (e.g. 5w) & the impedance (e.g. 16/8/4 ohms) of the speaker connected to the secondary.(This PDFgives a reasonable beginner's rundown on the history & reasoning behind the PA industry's use of "100v" & "70v".)