Coils

As you will apreciate, L1 and the tuning capacitors all affect the frequency of the complete transmitter. Winding L1 has therefore NOT been considered. This would result in a spring-like affair that would cause instability, or more precisely, "microphony". This is an effect where the coil wobbles about with very small mechanical movements. In severe cases you can even talk to the the circuit, as any owner of a Marconi TF995 signal generator will testify. By using a coil etched on the PCB you will find that microphony has been eliminated. So has coil expansion with temperature. It must be remembered, however, that this circuit is STILL based upon an LC circuit and therefore subject to changes of frequency with changes of supply voltage and "hand capacitance", etc. I will cover the supply voltage changes shortly.

L2 is wound on small ferrite beads. L2 is placed in series with the emitter of the buffer transistor, TR2. In the interests of stability it is very important that this coil does NOT radiate like a loop antenna. It is composed of 4 turns of 0.15mm Dia. enamelled wire (magnet wire in the US). The grade of ferrite is unimportant, as long as it is a grey one. One complete turn is formed when the wire passes through the hole in the middle once. The ferrite is mounted vertically in the same manner as a resistor. The ferrite is 3mm outside diameter and the assembled coil looks like this:

L3 is wound using 3 turns of 0.8mm Dia enamelled copper wire with a 6mm inside diameter. Wind it on a drill bit to get the inside diameter correct. The coil is close-wound, that is to say that the turns are just about touching and shall not be spaced. Form the ends of the coil, bending them out then down so that the leads are 5mm apart. The coil should look like this:

L4 comprises 6 turns of 0.15mm Dia. enamelled wire (magnet wire in the US) wound on TWO ferrite beads, the same beads that were used for L2. The beads are placed side-by-side as a pair of biniculars. One complete turn is when the wire is threaded once though both beads.

L5 is 5 turns of 0.8mm Dia. enamelled copper wire with a 6mm inside diameter. Just as L3, wind it on a drill bit to get the inside diameter correct. The coil is also close-wound. Form the ends of the coil, bending them out then down so that the leads are 5mm apart.

L6 is 3 turns of 0.8mm Dia. enamelled copper wire with a 6mm inside diameter. Just as L3 and L5 the coil is also close-wound. Form the ends of the coil, bending them out then down so that the leads are 5mm apart. You can get a closer view of L5 and L6 in this picture. The position of the capacitor between them is very important, equal distance from both coils keeps the 2nd harmonic at the lowest level.

NOTE - L5 and L6 MUST be wound in the same direction. If you try to wind one of them backwards or in the other direction then the spurious outputs will increase.

Construction

The transmitter is constructed on a single-sided printed circuit board. I will place the PCB foil pattern on my DOWNLOAD section of the homepages. The board is only 40mm x 72mm. If you use any other construction method then you will need to change a few component values. If you use veroboard, prototype board then the project will probably not work. The board has been designed to compensate for the lack of a second copper ground land and has also "thermal breaks" around some of the component connections. This makes it easier to solder for new beginners (for me too, but I should not admit that!).

There are two wire links on the board, fit these first. I try to make my links and components as neat as possible with as short leads as possible. Look at the photograph of the finished transmitter to see how they lay. Form the leads and use a bit of masking tape to hold them in position when soldering. The link wires are made using off-cuts from the resistors.

You will note that the 15pf capacitor coupling L1 to the BB105 varicap diode is laying on the board. It磗 legs are so formed that it acts as a link. Assembly order is not particularly important, but it is easier if all horisontal components are mounted first, then the passive components (resistors/caps), transistors and the coils last. Neatness and attention to detail is particularly important. The vertically mounted resistors should all be mounted as shown on the component overlay. It DOES matter which way round they are. This is one of the prices for using a cheap single-sided board.

Testing

When all the components have been fitted, check your work thoroughly. I reccomend you shine a strong lamp behind the board component side and compare the tracks with the PCB foil pattern. This will allow you to check for solder bridges between tracks. Assuming all is well, connect a 50-Ohm resistor to the antenna (ANT) terminals. Two 100-Ohms in parallel will be fine. Now connect the board to a 9v supply in series with a 12v 3W torch lamp. If the lamp glows brightly then switch OFF and check your wiring because you have a fault. If there is no fault then the lamp should only glow dimly, if it glows at all. The complete transmitter should draw less than 100mA.

If all is well, switch ON an FM radio set tuned to somewhere around 108MHz. Adjust the tuning capacitor on the board so the plates are at around minimum capacitance and you will hear the transmitter on the radio. With the capacitor plates near maximum capacitance you should be able to tune the transmitter to 88MHz.

Now couple the AF IN terminals of the transmitter to the LINE OUT of a stereo ststem, your computer, or even the headphone terminals of your Sony Walkman. I prefer to use headphine terminals since the volume control will give you some control over the modulation depth. You can set the modulation depth by comparing it with another radio channel. Set your transmitter A LITTLE LOWER IN VOLUME than other channels, unless you have access to a modulation meter. Note that you may have to use a capacitor in series with the AF input wire. See the application data further down.

If your transmitter is working then you can remove the test lamp and connect the battery supply directly to the transmitter. Check that nothing is burning. TR3 should get a little warm, but comfortable to the touch. All other components should remain stone cold. TR3 may get a little warmer if you increase the supply voltage to 13.8v but in this case the transmitter will be delivering almost half a watt of output power.

Performance

I think that here I should give a little information about the actual measured values of the prototype. The target was to achiece a clean 100mW of output power at 9v. I also indended the transmitter to be equally stable at 13.8v DC since this is what most constructors seem to want. The target was exceeded on all counts. There are no spurious outputs visible from 500MHz upwards, so this spectrum analyser view is only from DC to 500MHz. It shows that there is a little 2nd and 3rd harmonic outputs, but the levels are so low that they are quite negligible. I could hardly believe my eyes when I built the first prototype, but after cleaning up the PCB the output was even better! The vertical scale is 10dB per division and the horisontal scale is 50MHz per division:

As you can see, the worst case is the 3rd harmonic at -60dBc. The carrier level was +23dBm (200mW) at 10v supply. This falls off a little to about 160mW at the ends of the bands. Brief specifications are given below. I have not been all that meticulous with the figures. When I got a reading of 73mA I rounded it up to 75mA to keep the figures simple. The figures are only a guide anyway.