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Old 19th Dec 2020, 7:22 pm   #1
buggies
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Default An Elektor reader's AM TX (inductive)

An email from Elector yesterday included this pointer:-
Arduino-Powered AM Transmitter: Broadcast the Inductive Way on Medium Wave
by Burkhard Kainka | December 17, 2020
The url is:-
https://www.elektormagazine.com/maga...r-201505/27787
where there is a link to download the article (pdf attached).

What struck me on reading it was:-
The Arduino is only used as a square wave generator ( with open drain FET output).
It requires either BASCON-AVR on the PC in order to modify/upload the program or an ISP to program the supplied hex file..
The resultant modulation would only be on the positive half cycles of the carrier.

I completed a rough prototype today and confirmed the modulation was "one-sided" - see the attached trace.
The coil was about 6 metres of wire wound round a Tesco 1 litre ice cream tub.
With this small coil, the signal was only usable up to about 18 inches away ( three radios tried with ferrite rod aerials.)

Even with the modulation depth shown, with 2 volt p-p audio, the received audio level (of music ) was lower than adjacent broadcast stations.

Would anyone like to comment on the effect of that odd modulation at the normal AM detector diode?
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Old 19th Dec 2020, 7:43 pm   #2
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Default Re: An Elektor reader's AM TX (inductive)

The modulation as shown in the first pic has the big-issue that there's a standing-level of carrier and the modulation goes a-bit-up-and-down on top of this.

The standing carrier will mean that a 'traditional' AM receiver's AGC will respond to the standing-carrier and so reduce the receiver's gain - meaning it will sound 'oddly quiet' when compared to current-generation AM broadcasts [which often electronically-massage the positive-peaks of the modulating waveform].

I wonder what would happen if you shoved more audio-signal into it? Can you get the bottom-parts of the waveform shown in the top of the first photo to actually hit the zero-line?
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Old 19th Dec 2020, 10:04 pm   #3
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Default Re: An Elektor reader's AM TX (inductive)

It seems a pointless exercise. I know Arduinos are cheap, but they're not that cheap.
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Old 20th Dec 2020, 8:43 pm   #4
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Default Re: An Elektor reader's AM TX (inductive)

Yes - a bit of a hammer to crack a nut.
My interest was the passive modulation circuit which I thought could be used with a simpler 8 pin oscillator and DIL switch circuit.
With regard to increasing the modulation depth, I tried the effect using an audio oscillator instead of a PC soundcard output.
With the modulation almost touching the baseline the received sound was about the same level as a broadcast station and the detected waveform did not clip until it touched the baseline. This was with about 4.8v P-P sinewave.
Tried with the earphone output of a Sony MP3 player but even at full volume the output was insufficient.
I think the passive modulator is a no-go and an amplifier/buffer would be needed - so the simplicity is lost.
File under "could have been handy"...
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Old 22nd Dec 2020, 3:13 pm   #5
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Default Re: An Elektor reader's AM TX (inductive)

It does seem a bit involved for a simple project to me but may be more straightforward for an Arduino enthusiast. A point to remember is that there are variations of the Arduino and programmed chips are available.

Arduino Nano's are available for between £3 and £4 each on the usual suppliers, postage paid. They are useful little units and can replace the larger Uno in many applications.
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Old 22nd Dec 2020, 3:23 pm   #6
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Default Re: An Elektor reader's AM TX (inductive)

On further glance I would probably use the Arduino for the RF frequency and a single transistor modulator with a fixed tuned circuit on the output. More traditional and not quite in keeping with this article but a simple circuit (in basic form) which would be cheap in parts and re-tuneable in the fixed steps of Arduino timer output. With the tuned circuit frequency multiplication would be possible with reasonable harmonic performance at low output levels.
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Old 23rd Dec 2020, 3:38 pm   #7
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Default Re: An Elektor reader's AM TX (inductive)

The AD9850 DDS chips have an analogue ref input which modulates the output amplitude. So, with the DDS generating the carrier and audio into the ref, it might make an interesting pantry tx. Might even try it !
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Old 24th Dec 2020, 4:15 pm   #8
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Default Re: An Elektor reader's AM TX (inductive)

I think I might give that a go as well as I have four or five of those AD9850 boards from China.
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Old 24th Dec 2020, 6:49 pm   #9
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Default Re: An Elektor reader's AM TX (inductive)

AN-423 from Analog Devices gives some ideas.
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Old 17th Jun 2021, 6:56 pm   #10
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Default Re: An Elektor reader's AM TX (inductive)

Quote:
Originally Posted by See_Mos View Post
I think I might give that a go as well as I have four or five of those AD9850 boards from China.
I had a little play with one of the Chinese AD9850 boards for AM, programmed by a Raspberry Pi 4. The level control input had a capacitor across it, seemingly to prevent rapid changes in carrier level - so I removed that, and it became capable of proper AM. With the resistive pad I put in the way, I got a sensitivity of 1.55V p-p for 100% mod. In reality, it measured at 98%, but it was close enough.

As an initial experiment, I sat the board on top of a Sony ICF7600D and got reasonably good audio through it, though the field strength was very poor. The response of the modulator extended way beyond the audio region, suggesting that some filtering will be necessary for the audio input.

The next addition to the lash-up was an RF "linear" amplifier. My first attempt used a BF244B FET feeding into a 2N2222, using the FET to provide a high input impedance so as not to load the DDS chip, and the bipolar transistor to provide some current gain. This fed a 1m length of wire as the antenna. The range increased to a few metres, but the audio was distorted. Judicious application of negative feedback cleaned up the audio, so that now I have a range of several metres and good modulation quality.

At some point in the next few days, I plan to:

1. Write some PIC code to load the required data into the DDS board

2. Add an audio lowpass filter (and possibly a compressor / limiter) so as to keep the modulation bandwidth within sane bounds.

3. Develop a more powerful amplifier and lowpass filter for the RF output so that it can be matched into a "proper" antenna and I can cover my whole property with a reasonable signal

4. Add a proper regulated and filtered power supply so that I can do away with all the various batteries and power supplies that I'm using to power the lash-up.

When I have it working to my satisfaction, I'll post the "cookery instructions" including the PIC code so that it can be replicated by anyone else interested in building a highly frequency stable, reliable MW pantry rig!

This looks like a good way of building an "updated Minimod"

Last edited by mictester; 17th Jun 2021 at 7:05 pm.
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