AM Radio Modulator / Short Range Transmitter V2

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, the CD4060 and the 100uf capacitor 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.

In the later versions of the board/circuit I have added a diode that prevents the above issue.

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.

Circuit PDF and Gerber files on Github: https://github.com/coopzone-dc/AM-Modulator-v2/

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, or check on ebay - I sometimes list a few if I have time)

100nf C1, C5, C11

22pf C2, C3

22nf C4

10uf C6

220pf C7 Optional, may not be required.

47n C9

470uf C10

680nf C12

4n7 C13

270pf C14 See text

PWR two pin 2.54

PWR2 two pin JST

Audio 3 pin 2.54

3.5mm Socket 4 Pin PJ-320A (optional)

1mh L1 mini RF choke

330uh L2 mini RF choke

2N7000 Q1

1M R1

120R R4

10k R5

47R R7, R6

CD4060BE U1

LM386 U2

1N5819 Diode

ANT connector 2-pin screw terminal

10mhz X1 or whatever you choose (see table later)

These boards are available ready made on ebay. Or if you want advice on suppliers for above parts contact me.

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.

C11 and R10 are alternatives.

C7 and C4 are either or both to give better values are normally from 200pf to 550pf

C14 and C15 are either or both to give better values are normally from 10pf to 270pf 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+ SEE NEXT SECTION for more details.

L2 and L3 are either or both normally a nominal value of 330uh. You could also use 2x680uh giving 340uh for example.

U1, has several divider outputs Q4,Q5 (Q6 on some pcb's). Depending on X1 frequency you can choose these on jumper pads to get 625khz or 1mhz. See X1 below. In theory you should be able to use X1 between 9mhz and 20mhz and select a suitable output for the MW band from Q4, Q5 or Q6.

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.

C7 Optional, under most circumstances not required, it’s to help stop high voltage conditions that may occur with different aerial setups. With the suggested 1m of wire it’s not needed. You may well have to try various values if you connect a longer aerial.

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.

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 1mhz signal output. You will need to adjust L2 to around 150uh and reduce C14 (100pf approx). See U1 above.

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

In addition the two capacitors C4/C7, that form one value between them needs a mention. In general this should be between 200pf - 550pf. It does not really make a big difference and generally I use 270pf. The larger the value the less harmonic output but you also reduce the output power very slightly.

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 		Q4 			Q5 				Metres
20mhz 			1.25 mhz 		625 khz 		240m/480m
16mhz 			1 mhz 			-- 				300m
12mhz 			750 khz 		-- 				400m
10mhz 			625 khz 		-- 				480m

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 1M 			Aerial 3M
1.25mhz 		35pf 				27pf
1mhz 			100pf 				68pf
750khz 			160pf 				145pf
625khz 			268pf 				250pf

You can "make up" values using C14/C15, for example the 81pf could be 33pf + 47pf giving 80pf ( within 1% ). Some of the values you can just use C14 for example the 268pf can just be a 270pf ( again within 1%) .

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, If you have a scope check the output MOD on the circuit. It should be a 625khz 9v p-p square wave.

7 You can optionally solder a temporary wire (1ft) to the output MOD and check with an AM radio tuned to 625khz (479m). 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 15ma, especially if you add the aerial as well.

10, If you have a scope observe the signal at the aerial your looking for a 625khz 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 625khz (479m). 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 625khz zone to get the best signal. As many old radios have errors in the dial scale or are out of alignment.

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 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.

Observed output from the board.

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 sound ok when heard on the radio. It's around 60v p-p that's why C14 needs a high rating!

This is a very short video showing the board working wit a vintage radio, Bush Dac10 (not with origanal case, it was damaged when i got it.)