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Here are my constraints:

  • I need to build an am transmitter, but it has to fit in a 1 inch diameter and ~1 inch height box.
  • Since the box is very small, I plan to use a CR2032 battery, which is also a considerable limit.
  • Plus, I plan to use a carrier frequency of 1.52MHz (I can get a crystal to that frequency).
  • The signal I want to transmit come from an electret microphone.

So far, I have tried to make this work with 555 and a 9V battery, here is what I came up with:

enter image description here

The aim is to mix the 555 output and the incoming sound together, so the antenna goes between R3 and R4. I had to change a bit the values of R2 and R1 to make the 555 oscillate at 304kHz, so that the 5th harmonic is at 1.52MHz.

Using an old alarm clock, I was able to get a very weak sound. I added a class A amplifier to get more power through the antenna but I am getting a weird effect. I can hear a sort of whistle added to the actual sound. The whistle frequency seems to change (it sweeps slowly) even if I'm not touching anything. Is it because of the square wave (and not sine) carrier ?

Despite this effect, I was planning to do the following:

  • Use a flat CR20232 3V battery
  • Use an ICM7555 instead of a regular 555

At least, it fits in the box... Any better idea ?

By the way, I'd like to use only DIP packages (max 8 pins) and discrete components.

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    \$\begingroup\$ You appear to be adding the audio to the carrier, rather than multiplying (mixing) the two of them together. All you're hearing on the receiver is the carrier wave beating with some nearby AM broadcast station. \$\endgroup\$ – Dave Tweed Apr 4 '14 at 22:27
  • \$\begingroup\$ Okay, thanks for the clarification. I did not know how to make a voltage multiplier without using a dedicated chip or op amp. That's why I did a passive summing without really being aware of the problems it might involve... \$\endgroup\$ – Spin Apr 4 '14 at 22:43
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    \$\begingroup\$ probably the smallest audio transmitter is using FM where the microphone diaphragm movement acts as a variable capacitor and modulates a colpitts oscillator directly. Used as bugs during the cold war I'm informed. \$\endgroup\$ – Andy aka Apr 5 '14 at 0:48
  • \$\begingroup\$ Thanks to the both of you. Maybe I should consider an FM emitter instead... There plenty of tiny ones with one or two transistors. \$\endgroup\$ – Spin Apr 5 '14 at 11:23
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You want a very basic AM transmitter. The old Knight-Kit C100 walkie-talkie has a very simple design. It used a super-regenerative receiver, to minimize parts count. It had one crystal, to set the transmitting frequency, and three transistors. You could probably simplify that, to make just a transmitter. The schematic USED to be available on the Web, but it appears to have vanished.

OLD copies of the Radio Amateur's Handbook should have circuits that could be adapted to a simple transistor AM transmitter.

If I were going to try to throw something together, I'd probably go DSB rather than AM, and use an SA612AN (NE602 replacemement). Instead of a VFO, I'd hang the crystal on the SA612AN, and I'd probably try a plastic 2N2222A, either barefoot or driving a 2N3866. I'm not sure what I'd use for an audio preamp. Here's a more elaborate transceiver, that illustrates the basic principles.

Now for the bad news. There's no way you're going to power this from a CR2032 coin battery. Look at camera batteries, like a Kodak 2CR5 6V lithium battery.

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  • \$\begingroup\$ Thanks, I'm always more confident in using a dedicated circuit. And it's even better if I can use a crystal as for the oscillator. \$\endgroup\$ – Spin Apr 5 '14 at 20:16
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There is probably no real answer to this question, as the 'smallest' is subjective.

What you are looking for is an RF Variable Gain Amplifier (RF VGA). A Google image search of 'FET Variable Gain Amplifier' yields this as one which is simple, need no -ve voltage bias and will probably work.

http://www.seekic.com/circuit_diagram/Automotive_Circuit/LM307_voltage_controlled_gain_amplifier_circuit.html

You might want to change the LM307 for something with some sort of guaranteed bandwidth of at least 3MHz.

You connect your oscillator (crystal or resonant circuit) ideally through a filter reduce harmonics then pass the resulting carrier sine wave to the VGA which drive the antenna.

If you can use FM rather than AM the circuit becomes simpler. If you do as suggested and use the microphone you will need to use a capacitive 'condenser' microphone which might be difficult to get hold of these days. (Most have built in amplifiers which hide the capcaitive output and produce a voltage) A capacitive modulated oscillator produces an FM output directly, which is why it is so simple.

An RF VCO is still simple to make, what you need is an RF Voltage Controlled Amplifier (RF VCO)

A google image search of "FET Voltage Controlled Oscillator" yields hundreds.

A good 'small one' one might be Question 9, here http://www.allaboutcircuits.com/worksheets/modulate.html

Obviously you need to choose the tank (resonant) circuit component to make it oscillate at 1.52Mhz

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  • \$\begingroup\$ A VGA is a very good idea, but the circuit you found on google requires a symmetric power supply. Anyway, it gives me the following idea: a voltage divider where one of the resistors is an actual resistor in series with a FET on which the input is shifted to a negative voltage thanks to some diodes (2 or 3). I have the following components in mind: 2N3819 and 1N4148. Do you think this is relevant ? \$\endgroup\$ – Spin Apr 5 '14 at 11:31
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    \$\begingroup\$ An RF VGA is overkill for this, about like swatting flies with an orbiting X-ray laser station. What he wants is an AM transmitter. "Plate modulation" for AM has been around for decades. Refer to any CB photofact for the basic concept: Basically, you use the audio amplifier to modulate the power supply to the RF final. On a CHEAP super-regen walkie talkie, there was generally one RF transistor that doubled as a crystal oscillator on transmit and the RF front end on receive. \$\endgroup\$ – John R. Strohm Apr 5 '14 at 14:27
  • \$\begingroup\$ The most common traditional plate modulation scheme feeds the plate (collector) of the final through the secondary winding of an audio output transformer. The microphone audio chain drives the primary. It does take some power from the audio driver. This scheme is routinely used in AM CB radios, and it is still commonly called plate modulation even in solid-state rigs. \$\endgroup\$ – John R. Strohm Apr 5 '14 at 22:54

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