一款优秀的分立元件发射机电路(英文)

文章摘要：CoilsAs 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

一款优秀的分立元件发射机电路(英文),标签：电路设计,http://www.88dzw.com

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.

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