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I am trying to build a simple AM transmitter, transmitting only a carrier wave and thus silencing the noise on my radio standing next to my circuit.

I built this circuit on a breadboard.

circuit

It's a simple positive square wave oscillator designed to produce a frequency between 618kHz and 1934kHz, roughly the range of my radio. When simulating, this indeed produces the square wave intended.

At the output of my oscillator I simply put a wire on my breadboard acting as an antenna.

This however doesn't influence the audio coming from the receiver in any way.

What did I do wrong?

Update: It is not per se my intention to just jam a nearby radio. I try to get this circuit or something similar to work, as an early step in establishing a solid understanding about RF-communication. I try to radiate something controlled at a variable frequency. Just a carrier wave and thus silence seems the simplest to do.

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  • \$\begingroup\$ You're probably not transmitting on the frequency the radio is tuned to. Square waves are a very rude thing to put on an antenna, as they have substantial energy at overtone frequencies as well. Even if you had a circuit which (unlike this) produced a clean signal on the desired frequency, your goal is not very legitimate. Perhaps you want a receiver with a carrier, or code or subcarrier activated squelch. \$\endgroup\$
    – Chris Stratton
    Commented Jan 19, 2019 at 18:28
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    \$\begingroup\$ I would question whether it is oscillating at all. The gain bandwidth product on the TL072 is only 3MHz. That doesn't leave you much gain at the frequencies you want too look at. \$\endgroup\$
    – JRE
    Commented Jan 19, 2019 at 18:31
  • \$\begingroup\$ Do you have an oscilloscope? If so, check the output of your circuit. \$\endgroup\$
    – JRE
    Commented Jan 19, 2019 at 18:31
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    \$\begingroup\$ Your breadboard may have 5pf capacitance between each connection strip. These capacitances make breadboard circuit construction prone to not-oscillating. That's only one reason why simulation works while your construction doesn't. \$\endgroup\$
    – glen_geek
    Commented Jan 19, 2019 at 19:14
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    \$\begingroup\$ 9V might work... but my above link is much better,, Many CMOS Schmitt triggers to choose from in CMOS \$\endgroup\$
    – D.A.S.
    Commented Jan 19, 2019 at 19:24

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What did I do wrong?

enter image description here

The minimum recommended supply is 10 volts between positive supply pin and negative supply pin. You are using 5 volts according to your circuit.

Also, to make an effective monopole antenna you will need a ground connection or at least a few hundred pF capacitance to real ground.

Check that the circuit oscillates using an oscilloscope. I doubt that it will with a TL072 and a 5 volt supply. Be also aware that when you do get it working you might really annoy neighbours who are listening to their radio and you might contravene local laws on illegal radio transmission. I would suggest you use a ferrite rod and coil as you transmitter antenna as this has very poor penetration range due to it only producing a magnetic field and not a full-blown EM field.

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  • \$\begingroup\$ good read on specs. \$\endgroup\$
    – D.A.S.
    Commented Jan 19, 2019 at 19:24
  • \$\begingroup\$ Also note that the reference point established by R1/R2 needs to be grounded. There is no guarantee that the circuit will work unless you do. \$\endgroup\$
    – WhatRoughBeast
    Commented Jan 20, 2019 at 0:28
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When simulating, this indeed produces the square wave intended.

A simulation is only as good as the models used. I simulated your circuit using LTSpice and this model of the TL072. It required a minimum supply voltage of 18V to oscillate, and the maximum frequency obtainable (100k pot set to zero) was ~240kHz.

enter image description here

enter image description here

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AM transmissions add together in an AM radio which is why AM is used for aeronautical communications, then a pilot can MAYDAY his call over a channel already being used by another pilot. AM has lots of click, pop and snap interference that is common.

FM radios have a "capture ratio" where the strongest signal captures and uses a channel and other lower level interference on that channel is not heard.

A TL072 is an audio opamp that works poorly at the radio frequencies you need. Make a radio oscillator with radio parts. You probably need an illegal very high power AM carrier to swamp out your AM interference.

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Once you get the power supply correct ( + and - 5v) there is no means for this to oscillate. Once the op amp output reaches 5 volts R5 will hold it there in saturation. If your intent was to make a phase shift osc. try grounding the positive input to the op amp. That way there is reference 0 voltage at the input. second make a three section rc network to achieve 180 deg phase shift at the frequency you want. calculate RC phase shift of 60 deg for each and connect in series. start with the capacitor then the resistor to ground. this blocks the DC voltage from the output. use the op amp output to drive the network and connect the other end of the network to the neg input of the op amp. R3 and R4 set the gain but any gain above one will oscillate. C2 is not needed. R1 and R2 provide a 2.5 volt reference which has no effect. Any long wire antenna will radiate some. if you match the op amp output impedance to the antenna you will get maximum power transfer. A vertical whip over a ground plane is ~ 75 ohms impedance. Whip length ( 1/4 wave length) determines resonant frequency. Short antennas need a loading coil to get the impedance you need. Also, square waves are composed of harmonic content. check Fourier analysis of a square wave. I hope you have some EE background because you will otherwise be lost.

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  • \$\begingroup\$ You say "Once the op amp output reaches 5 volts R5 will hold it there in saturation", in that case the R1-R2 junction is at ~3V and V+ is at ~4V. R4 will charge C2 until V- > 4V at which point the the output will flip to -5V, etc. \$\endgroup\$
    – james
    Commented Jan 19, 2019 at 22:10
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    \$\begingroup\$ Use 2 x Enter to add paragraph breaks into your wall of text. Capitalise properly for legibility. Both will result in increased readership and, maybe, upvoting. Welcome to EE.SE. \$\endgroup\$
    – Transistor
    Commented Jan 19, 2019 at 23:58
  • \$\begingroup\$ James, I stand corrected. It does look like it will oscillate. However, pushing the op amp into saturation will add delay that is not shown in the schematic. The delay will determine the frequency, not the 5 pf cap, which the real circuit will likely have as stray capacitance. Pushing the op amp into saturation is not good design because the op amp parameters are not specified. \$\endgroup\$
    – charlesV
    Commented Jan 21, 2019 at 14:00

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