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电路设计->通信电路图->无线发射电路图->一款优秀的分立元件发射机电路(英文)

一款优秀的分立元件发射机电路(英文)

作者:dolphin时间:2009-07-06

BC547 2N3866 2N4427

WBFM TX V7

by Harry Lythall

Introduction

Ok, so I have done a lot of playing around with the domestic FM band. This will probably be one of the last transmitters for the 88MHz to 108MHz band. This particular TX is of special interest to those wishing to build low power Power Amplifiers for the VHF bands since it used impedance matching, power amplifier and antenna filtering, all of which should be used by radio constructors, whether it be for amateur radio or any other form of radio. The features of this project are:Higher output power - 150mW min (at 9v) and 300mW (at 12.5v) Very pure output signal due to carefull design and filtering VARICAP modulation - possiblity to add a synthesiser Single sided Printed Circuit Board, only 40mm x 72mm Covers the domestic FM band - 88MHz to 108MHz Easy to build, but coil winding experience IS required

NOTE - This project is illegal to use or build in many countries. I accept no responsibility what-so-ever for any ilegal use. This circuit is provided solely as an educational project. Now I have got that off my chest, let me get on with the project.

The Circuit

The circuit itself is fairly conventional, with a couple of small refinements. It all begins with TR1 (BC547) in an inverted Hartley oscillator configuration. The feedback to the Base of TR1 is via a small 4.7pf capacitor to help keep the oscillations as weak as possible whilst allowing the oscillator to be a reliable starter. The frequency of the oscillator is determined by L1 and the 22pf trimmer capacitor and functions in the range of about 76MHz to 119MHz using the PCB I have made.

TR1 = BC547TR2 = BC547TR3 = 2N3866 or 2N4427

The 15pf capacitor couples the top of L1 to the varicap diode which serves to add more capacitance to the tuned circuit to alter the frequency. R1 adds the supply voltage to the varicap, with a little noise decoupling (the 0.1uf capacitor). If you are to use synthesiser control then it is important to remove R1 from the circuit, then connect the synthesiser loop filter output to the terminal marked "Ctrl". Audio is coupled to the BB105 via a 47K resistor. There is only 47pf of decoupling in order not to restrict the AF bandwidth of the complete transmitter. The AF bandwidth is flat from 3Hz to about 72KHz, but if we look beyond these limits, there is an increase of 6dB at DC. This is because the two 47K resistors divide the AF input voltage by 2, but at DC the 0.1uf capacitor has time to charge, the two 47K resistors do not therefore divide.

TR2 (BC547) is both biassed and directly connected to the Emitter of TR1, which is a little unconventional in a VHF circuit. I needed to get a good input to TR2 and cut down on components. There are already far too many coils as it is in this circuit. Remember that the BC547 is an audio transistor but works well at VHF. The inductor in the Emitter of TR2 helps to extend the response a little to give a bit more signal to drive the final power amplifier transistor (TR3). TR2 gives no voltage gain; it is current we need to drive TR3. We already have enough volts from the oscillator.

22pf and L3 couple TR2 output into the Base of TR3. These components match the impedances so we get the maximum power possible into TR3 Base. The signal level, however is still quite low, so some DC biasing has been added to turn TR3 ON a bit. The transistor should draw about 5mA with no signal. This is not enough to make it become linear, but it is operating around class "B". This would make a very poor frequency multiplier, so harmonics are also reduced a little by the DC bias. Note that NO emitter resistor has been used. The prototype units all worked well without one and the drive level is not enough to cause the transistor to conduct very much. The small standing DC bias of 5mA doesn



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