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 Post subject: New AM Transmitter Circuit Idea (Schematics).
PostPosted: Sep Tue 21, 2021 2:23 pm 
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An IN4007 diode added to increase voltage swing of LM386.
Driver is a low noise 2n4401. The final is a robust 2n2222A. Here both transistors are modulated by the same audio voltage so expect 95% modulation depth. The oscillator should be able to provide a square wave. Choose resistor R value to keep it legal. Antenna matching network is not present. Connect a 6 foot wire antenna directly to the 100n ant. coupling capacitor if you wish so. The oscillator section and output matching network can be directly copied from Vectronics 1290K AM transmitter kit.


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Last edited by Dare4444 on Sep Tue 21, 2021 3:54 pm, edited 2 times in total.
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 Post subject: Re: New AM Transmitter Circuit Idea (Schematic).
PostPosted: Sep Tue 21, 2021 3:31 pm 
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I've found that by placing an inductor in the emitter of a colpitts oscillator, the AM modulation depth can be increased from 50% to 100%. The current must be pretty high to supply power during modulation peaks, prevent flattening, and distortion. It's achieved by using only 50ohm emitter resistance or two 100ohm resistors in parallel. C2 and C3 are feedback capacitors and their values can vary anywhere from 470pF to 1000pF.

RFC 1 is for RF blocking purpose and not let it interfere with LM386 modulator chip. Typical value is 100uH readymade axial inductor. It's better if RFC2 is wound on a ferrite toroid or binocular core. It helps trap and store the RF energy and allows modulation level to reach 100% just by modulating the oscillator. The audio quality remains surprisingly good. Buffer T2 amplifies the AM signal from T1. It could be a 2n3053, 2n2219A, 2n2222A, 2n3866, etc. Resistor R5 should be 390ohm. T2's emitter is at 4.6V so 12V power supply - 4.6V = 7.4V * 11.8ma = 88mW of DC input power to the final stage. Connect directly a 6 foot wire antenna or develop an impedance matching network to convert the 50ohm output impedance present at C4 to several thousand ohms for the wire antenna to get a better range. The two crystals can be of any value from 600KHz to 1.6MHz. The two bit switches selects one frequency for transmission. 1MHz crystals are readily available and can be used here. Pin 3 of LM386 can be disconnected from R7 and Mic1 and audio input directly fed to C8. Increase C8 to 820n when feeding audio from an external source. Use 12V regulated power supply. T2 does not need a heatsink.


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Last edited by Dare4444 on Sep Tue 21, 2021 5:35 pm, edited 2 times in total.
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 Post subject: Re: New AM Transmitter Circuit Idea (Schematics).
PostPosted: Sep Tue 21, 2021 4:37 pm 
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Joined: Mar Thu 01, 2018 1:30 am
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https://www.mouser.com/Passive-Componen ... ?P=1z0wnui

1MHZ XTAL


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 Post subject: Re: New AM Transmitter Circuit Idea (Schematic).
PostPosted: Sep Tue 21, 2021 5:34 pm 
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 Post subject: Re: New AM Transmitter Circuit Idea (Schematics).
PostPosted: Sep Tue 21, 2021 8:50 pm 
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How's the second design in yellow? Opinions out there? Anyone gonna try? It was tested on 7MHz and worked fine.


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 Post subject: Re: New AM Transmitter Circuit Idea (Schematics).
PostPosted: Sep Wed 22, 2021 5:07 pm 
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Bit switches S1 to S8 covers eight different MW radio frequencies. Number of crystals required is five. A single 2n3904 colpitts oscillator should drive the divider IC CD4040. A ferrite rod with untuned primary and tuned secondary will decide the performance of this circuit.


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 Post subject: Re: New AM Transmitter Circuit Idea (Schematics).
PostPosted: Sep Wed 22, 2021 9:01 pm 
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Joined: Jan Sat 25, 2020 5:23 am
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Location: Colorado Springs, CO 80917
Quote:
How's the second design in yellow? Opinions out there? Anyone gonna try? It was tested on 7MHz and worked fine.


There might be a glut of BCB transmitter ideas, more ideas than people still needing something to experiment on.
Where was this tested? Did you build one? There seems to be no tuned circuits at all in the design until the antenna (which may not be tuned either). That seems like a poor practice for RF, don't you think?


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 Post subject: Re: New AM Transmitter Circuit Idea (Schematics).
PostPosted: Sep Wed 22, 2021 11:27 pm 
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Joined: Mar Thu 01, 2018 1:30 am
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hwhall wrote:
Quote:
How's the second design in yellow? Opinions out there? Anyone gonna try? It was tested on 7MHz and worked fine.


There might be a glut of BCB transmitter ideas, more ideas than people still needing something to experiment on.
Where was this tested? Did you build one? There seems to be no tuned circuits at all in the design until the antenna (which may not be tuned either). That seems like a poor practice for RF, don't you think?


It was tested on 7.2MHz with a 50ohn LPF at its output. 100% modulation was obtained with room to spare.


Last edited by Dare4444 on Sep Thu 23, 2021 9:53 am, edited 2 times in total.

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 Post subject: Re: New AM Transmitter Circuit Idea (Schematics).
PostPosted: Sep Wed 22, 2021 11:30 pm 
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Joined: Mar Thu 01, 2018 1:30 am
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Tested and published.


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