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 Post subject: Even Simpler Barebone AM TX
PostPosted: Aug Mon 31, 2020 8:20 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 398
Even simpler bare bone TX.
One switch used as oscillator and another as modulator. Third one is left unused. It was tested with a 1MHz crystal several years back. A LPF is recommended to filter out the harmonics.


It is a cool design! Use LM386 at a slightly higher voltage to feed the voltage divider. 3Vpp will result in 100% modulation


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 Post subject: Re: Even Simpler Barebone AM TX
PostPosted: Sep Tue 01, 2020 2:39 am 
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Joined: Aug Thu 06, 2015 2:20 am
Posts: 283
Sorry. I'm confused again by your pin numbering.

Doesn't Pin 6 have to be grounded to enable the switches? Also what state should Pin 7 be in?

Neither is shown on your schematic.

Have you gotten your computer fixed yet?

Another question. Why can't the output of the LM386 if run from the same 5V supply be connected directly to Pin 3?


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 Post subject: Re: Even Simpler Barebone AM TX
PostPosted: Sep Tue 01, 2020 5:29 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 398
Both pins 6 and 7 should be grounded. LM386's output can be connected directly but its ouput doesn't swing all the way down to 0V. It's necessary to achieve 100% modulation. So driving a voltage divider with LM386 is better. With LM386, 85% modulation is the limit at 12V but with the divider 100% mod depth is easily obtained. Distortion is also low and sound quality is on par with expensive AM Transmitters.


Last edited by Dare4444 on Sep Tue 01, 2020 8:07 am, edited 1 time in total.

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 Post subject: Re: Even Simpler Barebone AM TX
PostPosted: Sep Tue 01, 2020 5:51 am 
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Joined: Aug Thu 06, 2015 2:20 am
Posts: 283
Why not use the unused switch Pins 11, 12 and 13 to improve the efficiency and the modulation percentage?


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 Post subject: Re: Even Simpler Barebone AM TX
PostPosted: Sep Tue 01, 2020 6:22 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 398
It might increase input power beyond 100mW FCC limit so I played the safe side. I'll have to experiment again to check input current and output power levels but I have crystal in the range of 6.4 to 30MHz which I think should work fine with the HC version of 4053. It can also be clocked with a CD4060, 20MHz XTAL giving 1.25MHz output


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 Post subject: Re: Even Simpler Barebone AM TX
PostPosted: Sep Thu 03, 2020 7:12 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 398
I forgot to mention that both switches have been used for the oscillator part. The remaining one is the modulator. I connected output of lm386 directly to pin 3 and tested the circuit with a 10MHz xtal. Output power is 6mW without LPF into a 47ohm load. Modulation quality is good but range is gonna be limited very limited. I'm getting a few feet with 10 feet wire ant at 10MHz.

Modifications needed. We need to separate the oscillator section and use the whole chip as a modulator. Plus LPF also required at the output and if the modulation is asymmetrical then a complimentary pushpull RF amp would be needed.


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 Post subject: Re: Even Simpler Barebone AM TX
PostPosted: Sep Thu 03, 2020 11:06 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 398
I wired up this circuit and connected a 10 ft. wire antenna for testing. Range is 10 feet. Lowering the crystal frequency from 10MHz to 7.2MHz made it work without instability. So I guess it will work just fine with a lower frequency crystal, say 1MHz. Audio was distorted at 10MHz. The two feedback capacitors need to be increased to 33p for 1MHz crystal. Total DC input to pin 1 of 7805 is 15ma at 12V. It means the chip is drawing less than 100mW at 5V! A 7 pole LPF and antenna matching network should be added for maximum range. Sound quality is great!


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Last edited by Dare4444 on Sep Tue 22, 2020 1:21 am, edited 1 time in total.
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 Post subject: Re: Even Simpler Barebone AM TX
PostPosted: Sep Thu 03, 2020 4:11 pm 
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Joined: May Mon 25, 2020 7:37 pm
Posts: 227
Location: Boston, MA
Holy solder blob, Batman! :shock: :lol:


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