AM Transmitter / Modulator for Long Wave (LW)

This is a very low power AM transmitter, I refer to it as a modulator, since it only transmits up to 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.

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 in the output matching stage, this allows it to be easily reproducible with cheap components.

With the values used it's limited to 2 metre, on the bench, distance so it's ideal for testing and personal listening. I have also used it with a much longer areal (4m) and re-tuned the output matching circuit and it is possible to extend the range to 20 metres around the house. How ever at the lower frequencies used on LW it requires some experimentation to achieve this.

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

Circuit PDF and Gerber files on Github: https://github.com/coopzone-dc/AM-Modulator-LW/

(https://github.com/coopzone-dc/AM-Modulator-LW/)

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.

PCB (see github link above) I also sell ready to use assembled PCB's on Ebay

270R R2,R10

39R R3

PWR S1

IN5817 D1

PJ-320A J5

ANT J4

330pf C4

2N7000 Q1

100nf C5

47n C9

4n7 C10

10uf C12,C13

Trimer 0-120pf See text C14

Cx See text C15

680nf C16

10/12/16mhz X1

CD4060BE DC U1

100nf C1,C6

22pf C2,C3

100uf C18

PWR J1

PWR2 J2

Audio J3

1mh L1

4.7mh L4/L5 in toal 4.7mh for longer aerials may need less 3.3mh for example

1M R1

10k R4,R9

120R R5

47R R6,R7

LM386 U2

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, the PCB design has duplicate components in order to make this easier to do.

R2/R3 Can reduce the power of the circuit. But be careful as it can cause distortion in the modulation if you use to low a value. Normally from 120R - 390R.

C14 Trimer 0-120pf (these can be tricky to get, so see C15 below if you use a smaller value here)

C15 Sometimes you may need to add in an extra capacitor to adjust the to the aerial length or if C14 is a smaller value trimmer. Normally between 20pf-100pf. Some trial and error testing may be needed.

L2 and L4 are either or both normally a nominal value of 4.7mh. You could also use other values again depending on frequency.

U1, has several divider outputs Q4,Q5 (Q6 on some pcb's). Depending on X1 frequency you can choose these on jumper pads under the chip.

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

R8/R7 and R6 form a low impedance input network suitable for most players (mp3, CD players etc) that use 50-150ohm headphones. You can increase R6 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.

X1 it's possible to use other values for X1, for example 16mhz will give a 250khz signal output. You will need to adjust L2/C4 for different frequencies.

NOTE: Currently incomplete info on this page more to follow soon.

Below are a couple of tables shows some suggested values for the matching output capacitor / aerial length

First table shows the frequency setting depending on Xtal and the output of the 4060. The values are rounded where it's converted to metres. Also note the 20mhz xtal provides two frequencies in the MW band, but if using breadboard can sometimes fail to start.

Xtal Freq Q6 Q5 Metres

16mhz 250khz -- 1200m

12mhz 187.5 khz -- 1598m

10mhz -- 250khz 1200m

The second table gives approximate values for C14/C15 with either a 1m aerial or a 3m aerial. To get the best results and maximise output you really should use a variable capacitor in place of them (0-300pf or more). But these are expensive and unless you want to tweak the output these fixed values are good enough. Try to use 5% of better and at least 50v preferable higher.

Freq (Aerial 1.5m)

250khz around 30pf on trimer.

187.5khz around 55pf on trimer

Using the trimmer adjust as needed, you may need to add in a small capacitor as well depending on the trimmer used.

If you are using the PCB from the gerber fles 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, Check you get 9v on the VCC pin on the CD4060BE, add the IC.

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

5, Add the oscillator Xtal and recheck the current drawn.

6, 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).

6, If you have a scope check the output MOD on the circuit. It should be a 250khz 9v p-p square wave at PAD Q6/Q5

7, Add the mosfet. The supply current should increase by around 15ma, especially if you add the aerial as well.

8, If you have a scope observe the signal at the aerial your looking for a 250khz signal, around 30-50v p-p. at the junction of L1/Q1 (2n7000)

11, Check for the signal using your radio.

It's now time to connect an input signal to the audio input. The 3.5mm jack lead fits my old iphone5's headphone socket (and was/is the standard for many other players).

Using your radio tuned to 250khz (1200m). 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 250khz zone to get the best signal. As many old radios have errors in the dial scale or are out of alignment.

With the short (1.5m) aerial you will probably need to move the wire around the radio to get the best signal. Normally within a metre or so off 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.

The first 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 near 95% modulation. It certainly sounds ok when heard on the radio.

TBA