AM Radio Modulator / Short Range Transmitter

This is a very low power AM transmitter, I refer to it as a modulator, since it only transmits 1-2 metres with good quality signal using the suggested 1m aerial.

it is mostly designed for testing purposes and personnel listening.

However technically it is a transmitter and may be subject to restrictions and laws where you live. You must check before use.

Notes:

Please be aware, Some components on the board can be damaged by reverse polarity, once shorted they remain short circuit - even when the power is re-connected correctly! With modern batteries this will short the battery and may cause it to overheat. I suggest you connect the battery ONE terminal at a time to reduce the risk of accidentally reversing the power by mistake.

I have tested and measured the output of this design using minimal test equipment and it seems to be within the popular FCC 15 regulations for at least the main points. In that the input power to the aerial is less than 100mw (around 90mw depending on battery power) and the second harmonic is 20db lower that the broadcast frequency. Some caveats;

1, My equipment for measuring is very basic and may not be accurate enough.

2, the power was calculated by measuring the V/mA in the output stage.

3, The harmonics where observed using FFT (see later text) not a spectrum analyser.

4, I don't know the FCC regulations other than a quick look through, so I am subject to misunderstanding.

This board has been deliberately designed to have a lower Q output matching stage, this is to allow reproducible and easy to obtain output. With the above in mind, I have tested this board with a 3m aerial no earth connection, this was a basic end-fed length of wire (at a height of 2m) and when matched using a set of output matching values (see link below). It transmits around the entire house and down the road for some distance.

See https://www.instructables.com/Adding-a-Longer-Aeri...

As always with this sort of project - you use it at your own risk. You really must check your local laws etc before use.

Note: AM occasionally suffers from interference, sometimes a low drone sound, sometimes chatter or a whistle in the background. This can effect any or all of the stations on MW including this transmitter. It's normally caused by weather conditions or things like sun-spots etc. It usually clears after a few hours, sometimes moving your radio a little can help etc. It's not a fault.

To build on the PCB you will require the Gerber files from Github:

https://github.com/coopzone-dc/AMmodulator

Or you can build on strip board, I have not provided a layout for this as I assume most will use a PCB. Attached a photo of the prototype board as a suggested layout.

Main parts are:

PCB (see github link above, or check on ebay - I sometimes list a few if I have time)

LM386N x1

2n7000 x1

270r x1

10k x2

47r x3

100nf x3

47nf x1

4n7 x1

10uf x2

100uf x1

330pf x1

100pf x1 (high voltage type, 100v also can use a trimmer 50-170pf see text)

1mh x1 small 1/4w choke

330uh x1 small 1/4w choke

Xtal module x1 (see text)

78L05 x1

An audio cable to suit the player you intend to use. I used 1/2 a 1 metre 3.5mm to 3.5mm cable for compatibility with an old iPhone5

Notes:

The 100pf capacitor is nominal and can be fine tuned by using either a trimmer or multiple parts (ie 100pf+47pf to give 147pf etc). Unless you are trying to increase the distance the modulator will work at, keep to the 100pf. Also note this part needs to be a high voltage type (100v +).

Xtal module, these are common on many market place sites, normally sold as an "oscillator module", the PCB is designed for a device of size 7mm x 5mm or the full-size through hole version, the pads on the board are large and other sizes may fit as well. Check the data sheet. The pins are 1=N/C, 2=+5v, 3=OUTPUT, 4=GROUND.

A better, more readable, version of the above circuit can be found on the github site mentioned in the supplies section above.

NOTES:

Several components are optional or can be substituted. For example:

Q1 Suggested as a 2n7000, but many will work as a substitute. For example ZVN2110A. Try to avoid ant misfit that has a high input capacitance (Eg. IRF510 etc) these will load the oscillator module and will probably not work very well. You should select one that has at least 50-60v Vds.

C9 Optional, under most circumstances not required, it’s to help stop high voltage conditions that may occur with different aerial conditions. With the suggested 1m of wire it’s not needed. You may well have to try various values if you connect a longer wire.

L2 Again a nominal value of 330uh, For longer aerial wire a smaller value may be used (150uh for example).

C11 Nominal 100pf, this is used to loosely match the aerial to the output, a better solution would be a trimmer cap or and old variable capacitor from a vintage chassis. You should use a higher voltage component 100v+

R6/R7 and R5 form a low impedance input network suitable for most players (mp3, CD players etc) that use 50-150ohm headphones. You can increase R5 to 240r if you can’t get enough volume to modulate the signal.

The circuit shows two power connectors, this is to allow for different PCB layout - you only need one or the other.

If your using the PCB from the gerber files on github, you can proceed to suggested Steps bellow.

If your using a strip board layout, keep in mind the following rules:

1, Keep all RF Output as far away as possible from the audio input section

2, Keep all component leads as short as possible.

3, Trim any tracks that are not going to be used.

4, Keep de-coupler PSU caps close to the IC's they are used for.

Suggested Steps

1, Add the passive components first (resistors / coils/ capacitors etc.)

2, Add connectors next.

3, Add the Voltage regulator and check you get 5v on the VCC pin on the oscillator module.

4, Check the amount of current being drawn by the board. Should be low with just the 5v regulator <10ma.

5, Add the oscillator and recheck the current drawn. At most it should increase by 2-3ma.

6, If you have a scope check the output of the oscillator you should see a 1mhz square wave 5v p-p

7 You can optionally solder a temporary wire (1ft) to the output of the oscillator module and check with an AM radio tuned to 1mhz (299m). You should here the carrier signal.

8, Add in the LM386, check the supply current again it should increase by a few ma. Check the voltage on pin 5 it should be around 4.5v (assuming you are using a 9v battery).

9, Add the mosfet. The supply current should increase by around 10ma, especially if you add the aerial as well.

10, If you have a scope observe the signal at the aerial your looking for a 1mhz signal, around 40-50v p-p.

11, Check for the signal using your radio.

It's now time to connect an input signal to the audio input. I used a 3.5mm lead with one end cut off to solder to the board. The remaining 3.5mm jack fits my old iphone5's headphone socket (and was/is the standard for many other players).

Using your radio tuned to 1mhz (299m). Play your music track and plug in the 3.5mm jack. Adjust the volume for undistorted sound on the radio. You may also need to tune the radio a little around the 1mhz zone to get the best signal.

With the short (1m) aerial you will probably need to move the wire around the radio to get the best signal. Normally within a metre or so of the back of the radio.

This makes the ideal setup for testing on the workbench or listening to a single radio in a room.

The exact distances and signal quality depend on many things, local interference etc. Experiment a little to get the best performance.

Observed output from the board.

The first picture show the scpoe with no signal, note the first spike is part of the calculations for FFT (Fast Fourier transform) and not an actual signal.

The second picture shows the unmodulated signal and the results of FFT. In this case the 3rd spike along is the 2nd harmonic, as you see it's -20db less than the primary signal.

The last picture shows the modulated signal at the aerial. It's slightly distorted it's near 100% modulation. It certainly sound ok when heard on the radio. It's around 60v p-p that's why C11 needs a high rating!