Thoughts on this AM Transmitter

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MiXiN
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Thoughts on this AM Transmitter

Post by MiXiN » Tue Dec 05, 2017 12:04 am

I've just been looking on eBay and found this; https://www.ebay.co.uk/itm/Trasmettitor ... ect=mobile

Am I right in thinking it won't do 1W and will drift due to the fact there's no PLL/Xtal?

MiXiN
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Re: Thoughts on this AM Transmitter

Post by MiXiN » Tue Dec 05, 2017 1:18 am

I was too late to edit my first post, so I'll have to mention it in this one; There's a circuit diagram shown on the eBay listing, and it's in the 2nd image.

Albert H
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Re: Thoughts on this AM Transmitter

Post by Albert H » Tue Dec 05, 2017 10:40 am

It's a free running oscillator, so it will drift. They shouldn't drive the logic output straight into the gate of the output FET - it's NOT the way to do it. Finally - there's no way this will do "1 Watt". It's designed to operate into a short bit of wire as the aerial. It'll give a reasonable signal around your house, but won't go any further than that.

It's easy to generate a PLL-locked carrier for medium wave - which won't drift! There's a standard crystal (4608kHz) that gives the 9kHz steps you need, and three cheap CMOS chips (4060, 40103, 4046) do the job. This will give you a squarewave at the carrier frequency. If you buffer this with a 4049 (another standard CMOS chip) with all the gates paralleled, you can drive the gate of a medium power FET - an IRF640 is a good choice. The drain of the output FET needs a reasonably big choke to its supply. The supply for the final FET should be from the output of an audio power amplifier IC - I use the TDA2030. The audio IC provides half supply without modulation. I use a small RF transformer to convert the low output impedance from the FET up to around 50Ω for the output lowpass filter - wound on a ferrite toroid.

It's also worth including a basic compressor / limiter into the audio input, so that the rig will handle a range of levels without the need to tweak. Audio lowpass filtering is also essential to keep the bandwidth sensible. The circuit I use has a quad op-amp and a couple of audio FETs and keeps the modulation pretty tight.

Medium wave aerials (if they're to be any use) have to be quite long. A quarter-wavelength is best. You also need a good earth - I used to use a huge area of buried chicken-wire!

To keep the impedances sensible, you want a supply of about three to four times the output power: a 10 Watt carrier rig works well with a couple of car batteries in series, and will work all weekend without a recharge! 10 Watts carrier is 40 Watts peak, and will cover quite a big area with a reasonably good aerial and earth in the day time.
Is it meant to smoke like that? :shock:

doc retro
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Re: Thoughts on this AM Transmitter

Post by doc retro » Thu Dec 07, 2017 8:33 am

So Albert t h .
Will a 1 watt am rig have a peak to peak of around 4 watts and as for antenna build what's the best 1/4 wave build that can be condensed down to make a smaller foot print but still resonate quite efficiently.?

geegosh
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Re: Thoughts on this AM Transmitter

Post by geegosh » Sat Dec 09, 2017 10:02 pm

Have just purchased one of his medium wave TX's - will try and report back when I've had a play with it.

Don't get upset about the lack of PLL - engineer's are still turning dials to keep huge medium wave transmitters aligned in some sites to this day, and back in the 1960's on the ships nobody would've dreamt of all this Class E solid state malarky.

I'm interested to see the advertised RF power and quality as the designs are pretty simple, for the price I'd snap one up and see for yourself too

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Re: Thoughts on this AM Transmitter

Post by Analyser » Sun Dec 10, 2017 7:18 am

It might be just that you're operating the amp in a non-linear mode. You can try increasing the voltage on the bias pin of the RA module to see if that helps the issue.

OgreVorbis
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Re: Thoughts on this AM Transmitter

Post by OgreVorbis » Sun Dec 10, 2017 7:18 pm

I don't really see the point in buying something like that unless it's just for novelty.

If you want a micro AM transmitter, DDS is the best way to go. PLL/VCO technology is still better at FM frequencies because it generates less noise and is more cost effective because DDS is still pretty noisy up there in the higher frequencies. However the AD9850 is a really clean DDS that works at these lower frequencies.

You can find some AD9850 modules on ebay, but those require an external MCU (such as arduino, etc.), but there are also a few that come with all that integrated and an LCD screen for changing frequency. Some only do SSB mode though, so you need to look carefully.

Albert H
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Re: Thoughts on this AM Transmitter

Post by Albert H » Wed Dec 13, 2017 3:21 am

A PLL MW rig is really easy if you just want a few Watts. The cheap (<£1) IRF FETs do the job easily. If you use an audio power amp IC like the LM1875 or TDA2030, your modulator is trivial.....
Is it meant to smoke like that? :shock:

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