Posted on Tuesday, April 16, 2013 • Category: FM Transmitters
Long range, very stable, harmonic free, FM transmitter circuit which can be used for FM frequencies between 88 and 108 MHz. With good antenna transmitter can cover 5km range. It has a very stable oscillator because it uses LM7809 voltage regulator which is a 9V stabilized power supply for T1 transistor. Frequency adjustment is achieved by using the 10K linear potentiometer. The output power of this long range RF transmitter is around 1W but can be higher if you use transistors like KT920A, BLX65, BLY81, 2N3553, 2SC1970 or 2SC1971.
Posted on Friday, April 12, 2013 • Category: FM Transmitters
Building two stage 40 Watt FM Transmitter Amplifier. RF input power should be between 0.5 and 1 watt. Amplifier is powered by 28V power supply. The diagram shows a 2N3375 driving a 2N5643 but there are many other transistors that will work. I used these two transistors just because they were cheap at the time. If any of the variable capacitors are at full capacitance you can pad them out with a fixed ceramic capacitor of suitable value. Extra capacitance also might be needed on the base of the transistors (i had to add 3 100pF capacitors on the base of the 2N5643). The transistors are bolted to a piece of right angle aluminum which is fixed to the metal chassis to dissipate heat effectively.
Posted on Thursday, April 4, 2013 • Category: Frequency Wave Generators
Presented here is XR2206 function generator with multiple waveform selection and a frequency readout display. The diagram on the right shows the internal workings of the XR2206 in the form of a block diagram. Essentially the chip contains A VCO (voltage controller oscillator), wave shaper and buffer. The XR2206 frequency generator diagram frequency of the VCO is set with a capacitor and a resistor. The capacitor sets the frequency range whilst a variable resistor can be used to vary the frequency in the set range. The frequency is defined by ƒ = 1/(RC). For a starting point for the design of the frequency generator I used the test circuit from the XR2206 datasheet. I built this on bread board and experimented with the timing resistor and capacitor and managed to get the frequency up to 4MHz.
Posted on Saturday, March 30, 2013 • Category: FM Transmitters
This is 150W FM transmitter amplifier for 88-108MHz band. The amplifier has two stages using BLF244 mosfet transistor for the first stage which requires 0.5 - 1Watt of RF input to get about 20watts to drive the final stage SD1407 which can push nearly 200 Watts on this design.
This design is more or less broadband however I added two variable capacitors after each stage for optimum matching and power output. Make sure the trimmer and the capacitors after the final stage SD1407 are a high voltage types with at least 200V rating. The power on this amplifier can be varied by adjusting the bias voltage using the white pot to the BLF244 mosfet. I added a zener diode onto the bias supply to protect the transistor from too much bias voltage.
Posted on Sunday, March 24, 2013 • Category: Power Supplies
Here's how to build your own adjustable power supply based on LM317. The IC LM317 is so versatile that an almost unlimited number of different, small, high grade power supply circuits can be built using it. The configurations can be introduced for different applications for upgrading an existing unit with features that would virtually make it indestructible.
A few useful application circuits using IC LM317, collected from National Semiconductor's PDF datasheet are meticulously explained in this section with the help of the relevant circuit diagrams. All the circuits discussed below require an unregulated input voltage (max. 35 Volts) from any standard transformer/bridge/capacitor network.
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