Single chip(MAX 2606) based FM transmitter circuit

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Here’s a single chip FM transmitter circuit using Maxim semiconductors IC MAX2606. The MAX2606 is a compact, high-performance intermediate frequency VCO specially designed for wireless communication circuits. They have monolithic construction with low-noise and a low-power operation in a compact 6-pin SOT23 packing .Th1s low-noise IC feature an on-chip varicap diode and feedback capacitances that avoid the need for external tuning components, making the MAX2606 perfect for portable systems. Only an external inductor is needed to set the oscillation frequency.In addition to this, an
integrated differential output buffer is also there for driving a mixer or prescaler.The MAX2606 can be operated from a single +2.8 V to +5.4V supply and consumes very less current .The chip can be operated from 45MHz to 650MHz .

In the circuit the nominal frequency is set to 100 Mhz by inductor L1, (390nH) . The left and right channel audio signals from your source are added by R3 and R4, and attenuated by the POT R2. R2 can be used as a volume control .POT R1 can be used to select a channel of transmission between 88Mhz and 108Mhz.Use 80 cm long wire as the antenna .

Notes

• Assemble the circuit on a good quality PCB or common board.
• Use a battery for powering the circuit.It will reduce noise.
• An FM antenna from a old radio is a better option than the wire antenna.

The output is between 88MHz and 108MHz, and allows transmitting audio signals to FM radios for remote listening. A schematic is provided with all component values to complete the project. Output power is ~ -21dBm, and the IC operates on 3V.

IC1 is a voltage-controlled oscillator with integrated varactor. Its nominal frequency of oscillation is set by inductor L1, and a 390nH value places that frequency at 100MHz. Potentiometer R1 then lets you select a channel by tuning over the FM band of 88MHz to 108MHz. Output power is about -21dBm into 50 (most countries accept emissions below 10dBm in the FM band).

The audio unit's left and right audio signals are summed by R3 and R4, and attenuated by the (optional) potentiometer R2. R2's wiper signal serves as a volume control by modulating the RF frequency. Signals above 60mV introduce distortion, so the pot attenuates down from that level.

In the absence of a standard FM radio antenna, 75cm (30 inches) of wire will suffice as a transmitting antenna. For best reception, it should be mounted parallel with the receiving antenna. The IC operates on a single supply voltage in the range 3V to 5V, but one must regulate the applied voltage to minimize frequency drift and noise.

Long Range FM Transmitter Circuit

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This circuit is a circuit diagram fm transmitter. This circuit is somewhat different from the previous fm transmitter circuit. Transmitter circuit described here has the additional RF power amplifier stage, after the oscillator stage, to increase the power output of 200-250 milliwatts. With a good matching 50-ohm ground plane antenna or multi-element yagi antenna, this transmitter can provide a good enough signal strength to a distance of about 2 kilometers. The circuit built around transistor T1 (BF494) is the basic low-power variable-frequency VHF oscillator. A varicap diode circuit is included to change the frequency of the transmitter and to provide frequency modulation by audio signals. The output of the oscillator is about 50 milliwatts.
Transistor T2 (2N3866) forms a VHF-class power amplifier. This increases the oscillator signals’ power four to five times. Thus, 200-250 milliwatts of power produced at the collector of transistor T2. For better results, assemble the circuit on a good quality glass epoxy board and house the transmitter in the case of aluminum. Shield the oscillator stage using aluminum sheets. Transistor T2 must be mounted on the heat sink. Do not switch on the transmitter without a matching antenna. Adjust both trimmers (VC1 and VC2) for maximum transmission power. Adjust potentiometer VR1 to set the fundamental frequency near 100 MHz.

Coil winding details are given below:
L1 – 4 changes of 20 SWG wire close wound over 8mm diameter plastic former.
L2 – 2 changes of 24 SWG wire near top end of L1.
(Note: There is no core (ie air core) is used to coil on top)
L3 – 7 changed from 24 SWG wire close wound with 4mm diameter air core.
L4 – 7 changed from 24 SWG wire-wound on ferrite beads (choking)
Potentiometer VR1 is used to change the fundamental frequency whereas potentiometer VR2 is used as power control.

preamplifier 88-108Mhz

This VHF amplifier working on Band 2 Radio Spectrum tuning approximately 88 - 108 Mhz

The Preamplifier circuit uses two 2N3819 FET's in cascade configuration. The lower FET operates in common source mode, while the upper FET, operates in common gate, realizing full high frequency gain. The bottom FET is tunable allowing a peak for a particular station.

Coil details follow:

L1 4 turns of 18swg air spaced with a 1cm diameter, the tap is one turn up from earth end...

L2 4 turns of 18swg air spaced with 1 cm diameter. The coupling coil is 1 turn interwound from the supply end. Enamel coated wire must be used. More info ...

FM Wireless Mike

FM Wireless Mike can transmit voice signals to any FM Radio receiver 100 meters away. The circuit is basically a frequency modulated transmitter working at 100 MHz. The frequency of the transmitter can be varied slightly by changing the trimmer C5. You can use ordinary condenser mike in this circuit. The transistors can be replaced by any low power transistors like BC148 or BF494.

The coil L1 is air core 6 turn 24 SWG. Third turn is tapped and connected to telescopic antenna. You can replace telescopic antenna with a small piece of wire.

Ta7378P FM wireless microphone circuit

TA7378P using FM radio integrated block the production of FM wireless microphone, external components small, simple and easy system, work stability, particularly suitable for the production of radio enthusiasts. Figure FM wireless microphone for the specific circuit, ICl (TA7378P) includes RF amplifiers, mixers, to enlarge the buffer, the local oscillator circuit and bias, the regulator circuit.

The circuit Qiaomiaodejiang combination of internal circuits, use of the circuit-and AFC (automatic frequency control) the use of a variable capacitor oscillator circuit to generate FM (FM) waves.

In order to reduce the impact of external, in the oscillation between the mixer and a buffer amplifier has. Finally, RF amplifiers zoom through launch FM radio antenna.

99.6MHZ J-power FM transmitter antenna

99.6MHZ J-Power FM transmitter antenna

99.6MHZ J-power FM transmitter antenna 99.6MHZ J-power FM transmitter antenna

10 mm diameter brass refrigeration production with a total length of 3.5 M 10 mm diameter brass refrigeration production with a total length of 3.5 M

15 W transmitter power amplifier 88-108MHZ

The power amplifier can be reactive 1-2 W ,88-108MHZ power FM transmitter As for the expansion of 10-15 W, a single C Larger and multi-level low pass filter components, has a high conversion efficiency and strong Yi-wave suppression.

Circuit see attached map shows, using high-power launch of C1972, its parameters are as follows: 175 MHZ, 4A, 25W, power gain ≥ 8.5 db, as shown by parameters, circuit work center frequency of about 98 MHZ, the importation of about 2 W of RF power , The rated output up to 15 W.

To maintain 88 ~ 108 MHZ with any frequency output reached rating, according to the level before the center frequency of some components to make suitable adjustments. May, when necessary, to reduce low-ball-series, to increase power output. The expansion of the power signals from three low-pass filter Yi-filtered high element of the transmitting antenna feed.

Components choice: In addition to electrolytic capacitor, the other tiles with high-frequency capacitors, C11, C12, C14 use high-frequency characteristics of a good, stable performance of adjustable capacitors, inductors Choke RFC1, RFC2 finished with inductors, must pay attention to the current RFC2 Carrying capacity, should use the coarse diameter Cores with the inductors.
L1-L6 available ø0.8mm the high-intensity enameled wire system, a diameter of about 5 MM, a few laps in the plans to "T" for the units indicated. Q1 ordinary Q9 socket, and supporting the use of plugs. Q2 used for 50 Ω RF output connectors, and then of resistance is smaller, more conducive to impedance matching.
Larger effective power more common for the launch of the C1972, of course, especially if you sufficient money to buy blocks C2538 contour of the gain, power will be even greater.

Debug circuit, be sure to pay attention, the power circuit, we must connect false load (I use 30 1 W, 1500 Ω high-precision metal film resistors made parallel), and there must be enough in the cooling devices, normal working hours Power Of not less than 2.5 A, the antenna impedance strictly equivalent to 50 Ω, can not be used Duanbang drawbars antenna, or a strong current of RF feedback circuit will create their own interference, most of RF energy to space and can not be convergence in the consumption of power, to overheating Damage must be launched for 50 Ω coax, tabled Reply to launch outdoor antenna.

Circuit the normal work of the key lies in whether the circuit debugging, the whole process had to very carefully.

Debugging, enter only the smaller the incentive power supply voltage drop to 9 V, using high-frequency voltage (can not use ordinary multimeter) monitoring false load at both ends of high-frequency voltage value, regulating C12, C14, L3, L4, L5, L6 So that the voltage range of 15-20 V around, and then adjust C11, L1 voltage to the largest.

And then gradually raise the voltage, each raising a voltage repeatedly adjusted C12, C14 and C11, L1 so that the maximum output voltage, noted that the input voltage and RF power simultaneously increasing incentives to ensure the accuracy of the results of debugging. Reach rating, 13.8 V supply voltage of about 2 A current work around, 50 Ω-load resistance at both ends voltage ≥ 40 V, RF power output of 15 W.

With the RF power amplifier with 50 Ω-wide umbrella to the vertical launch antenna (gain of about 2 dB), to ordinary FM radio test fired from the coverage of not less than 15 KM

88-108 MHz Preamplifier

This VHF amplifier working on Band 2 Radio Spectrum tuning approximately 88 - 108 Mhz

The Preamplifier circuit uses two 2N3819 FET's in cascade configuration. The lower FET operates in common source mode, while the upper FET, operates in common gate, realizing full high frequency gain. The bottom FET is tunable allowing a peak for a particular station.

Coil details follow:
L1 4 turns of 18swg air spaced with a 1cm diameter, the tap is one turn up from earth end...
L2 4 turns of 18swg air spaced with 1 cm diameter. The coupling coil is 1 turn interwound from the supply end. Enamel coated wire must be used.

FM Transmitter With Transistor 2SC1815

Will have to introduce a system of simple and small FM transmitter. Not only does it meet the requirements of the launch distance, but also at the same time using microphone and voice input signal line, background voice. Modulation circuit surveillance has also joined the table first, so that they can better control and the proper use of transmitters, the following circuit diagram, read briefly, whether it's just like a radio station equipment as many functions. Yes, this small transmitter can help you easily set up an amateur FM radio stations! 500 M in the coverage around. Echocardiography the bar, act immediately!

Principle small FM transmitter circuit components and options:

Signal to the microphone after intake, the C1 coupling into the BG1 and external circuit voltage of the single negative feedback amplifier, the voltage signal amplification weak enough to enlarge the extent of U1A and the line to enter the input signal with a mixed U1B. Mixed signal modulation way to the R17 from BG2 FET and the surrounding circuit composed of common than LC finishes the test, the test than the LC oscillator are characterized by a general three-capacitance oscillator simple, but also with high efficiency and high stability. Waveform, and modulation bandwidth, as this amateur production of FM transmitters appears to be particularly important.

Finally BG3 cast by the post-amplifier to enlarge the field by launching antenna fired by general can listen to FM radio broadcast. Another component of the signal through the amplifier to enlarge U1C after the wave rectifier D2 SR DC drive microamps table, Near and surveillance signal modulation rate. Use the system for general should not exceed 85 per cent suitable. GM and other integrated circuits using LM324 four operational amplifier and form a single power reverse input, both of the input voltage is set at half the supply voltage. Table surveillance for the first u-200, BG1.BG3 choose 2SC1815. BG2 three DO2 FET, D2 varactor diode using a S2267. Antenna require the use of 1 / 4 wavelength, such as the use of the best pull rod antenna not less than one metre. Otherwise effect will be less than overstaying launch. Final say about, please do not interfere with normal radio!

FM transmitter With uPC1651

The circuit has been produced by Japan's NEC as a major upc1651 IC devices, the high gain circuit, work stability, thereby ensuring the microphone performance. Use of FM transmitter. Used it 40 - 50 cm soft drag line for antenna, the effective range of more than 30 launch M. L carefully spacing adjustment and fine-tuning capacitor, will launch frequency coverage for 88 - 108 Mhz. Map L enameled wire with diameter of 0.51 mm in diameter, 4 mm cylindrical Tuitai from around 5 laps

Macsot MR-700 High Quality wireless microphone

This is collected by the Macsot MR-700 wireless microphone system launch of the circuit for maintenance reference. High Quality Audio on receiver output.

Active components:

• Microphone or Audio Preamplifier with IC MC4458
• Dynamic Audio processed with one NE571 From Philips
• Frequency Oscillator generated by one crystal 20.05 MHz and Varactor diode 2CB11A. No drift frequency is guaranted.
• 9V DC power supply or battery.

FM Stereo Transmitter With MC145151

This fm stereo transmitter use pll chip MC145151 from Motorola and stereo encoder chips BA1404 with vco built in. MC145151 IC used is to overcome the single-frequency instability in the BA1404. Modulation of the emission control BB910 frequency. B571C kept in control procedures, the procedures required to prepare another.

Parts List and Schematic

Component Values of the fm stereo transmitter you can find out the schematic. Please download schematic for enlarged image

Amplifier section of this FM Stereo transmitter uses a single 2sc2053 transistor with power out about 250 mWatt.

300mW FM Transmitter 2SC2538

The FM transmitter using a varactor diode way radio, plus a Class C amplifier, RF output power of up to 300 mW more open to more than one kilometer distance communications. Select the components of this fm transmitter: Q1 with ≥ 100 mA, Ft ≥ 300 MHz, β ≥ 100 tubes available 3 DG82, 3DG122, 3DG130, 2G711; Q2 with 2SC2538, 2SC1970, etc..

It must be noted in the fm transmitter assembly that RF Baffles circle L8, L9, L10; C13, C14, C17, C18, C19, and so can not be omitted, otherwise it would cause unnecessary self-excited oscillation. L1, L5 need to tap extraction, data such as icons, not otherwise due to impedance matching, the output power of less than maximum. Such as a battery-powered, Q3 (Darlington tube), C15, R6 can be omitted.

Debugging steps :

1. In the output termination of testing circuit to regulate the C7, C9, C12 to the largest multimeter readings,

2. Remove test circuit connected to one meter in length pole antenna to pull to fine-tune C12 simple reading of the largest field will be completed debugging.

500mW FM PLL Transmitter 88-108MHz

FM transmitters have always been fascinating and one can find thousand examples of them on the internet. Sadly most of them are full of error and miss leading information. Most of them also have low stability and frequency drift, many coils and components which are difficult to find. The output power is often set to several watts with just a transistor or two…..can't fool me. So therefore I decided to construct a simple transmitter with great performances.

Some contruction achievements I wanted were:

# Simple construction
# Commonly components
# High quality and stability
# Low number of coils
# High output power

The frequency of this transmitter can easy be changed with software and space/compress an air coil, simple don't you say? The basic hart of this transmitter is a colpitts oscillator. The oscillator is a VCO (voltage controlled oscillator) which is regulated by a PLL circuit and PIC micro controller. Don't get upset now…it is not that difficult after all. Let's check the schematic and I will explain the function.

Hardware and schematic

Click on the pic to see the large schematic. The main oscillator is based around the transistor T1. This oscillator is called Colpitts oscillator and it is voltage controlled to achieve FM (frequency modulation) and PLL control. T1 should be a HF transistor to work well, but in this case I have used a cheap and common BC817 transistor. The oscillator needs a LC tank to oscillate properly. In this case the LC tank consist of L1 with C1, C2, C3, and the varicap BB139. The coil is parallel with C1 and C2 which are in serial . The same with the varicap and C3. You can think that L is parallel with [ (C1//C2) + (Varicap//C3)] The value of C3 will set the VCO range. The large value of C3 the wider will the VCO range be. Since the capacitance of the varicap is dependent of the voltage over it, the capacitance will change with changed voltage. When the voltage change, so will the oscillating frequency. In this way you achieve a VCO function.

PLL and Microcontroller
The oscillator is made to work as "Voltage Controlled Oscillator" VCO.

To control the frequency a synthesizer circuit LMX 2306 has been added. The PLL circuit has a pickup coil (L2) connected to pin 6. This coil should be put close to the L1 coil for picking up some of the oscillating energy. The PLL in the LMX2306 will then use this frequency to regulate the VCO and lock it to desired frequency. The regulating system also need an external reference crystal. In this case I use 12.8 MHz.

At pin 2 of MX2306 you will find a PLL filter to form the Vout which is the regulating voltage of the VCO. The PLL try to regulate the Vout so the oscillator keeps the frequency locked to desired frequency. The desired frequency is programmed into the PIC EEPROM and is clocked into the synthesizer (LMX2306) at power up. I will below explain how to program the EEPROM for different frequencies. At pin14 of the synthesizer you have a control output. At this output you will find the reference frequency for testing. (I must warn you because the signal is not symetrical in shape. The positive pulse are only a few microsecond so you will have difficult to see it at oscilloscope.) I solved it by connecting it to a 74HC4020 (14-stage Binary Counter) to pin 10 Clock input. At Q0 (pin 9) you will have a symmetrical square wave with half frequency since the circuit is a counter. At Q1 pin 7 it will be divided by 4, see datasheets for more info.

LF input
The audio you wish to transmit should be connected to the Audio input (left side of schematic).

The signal will affect the varicap and thereby Frequency Modulate FM the RF carrier. A potentiometer P1 has been added to set the modulation depth (Wide FM or Narrow FM). You may have to play a bit with the value of P1 because it tends to modulate to much. You may have to add a 500k - 1M potentiometer instead. You test and find out yourself.

Buffer stage

Here you find another HF transistor and it is working in class C. The resistor R1 and the resistor Re2 set the DC current. In this case I found that 9.1k will give good output power and so the same with 150. If you wish to increase the power Re2 should be lower. You can add another 150 ohm resistor parallel.

In the table below I show you the output power with different voltages and values of resistor Re2. I advice you not to run this transmitter with to high output power. The transistor I use is a small one and tends to get hot. I advice you to run the unit from 0 - to 200mW. At 500mW the transistor will be in pain...*smiling* At the output you will find a T network. This "filter" will match the antenna impedance to the transmitter output stage. You have two variable capacitors 60pF to tune the transmitter for best performances.

The antenna I used I a 1/4 wave whip antenna (wire) about 75cm long. This type of antenna is smaller but not so good performance as a dipole. With a dipole you will be able to transmitter much longer distance.

How long can I transmit?
That is a very difficult question because the environment affect the transmitting distance very much. In a city environment with concrete buildings the transmitter will send maybe 200m.

I an open filed it will transmit 2000m. I did a filed test and with 70mW output power into the "bad" whip antenna placed indoors I could transmit 200-300m out into a park with no problem.

Output power

Table below show you the power measurements I have done. The Re2 is 150 ohm and in some test I connect a 50 ohm parallel. The output power in measured into a dummy load of 50 ohm.

Testing

The first thing you should test is that the oscillator is working. I disconnected the Vout from pin 2 of the PLL LMX2306. I then connected Vout to ground and check the oscillator. The oscillator should now oscillate at the lowest frequency. With my Wireless frequency counter I found that the oscillator was working at 100 MHz. I streatched the coil L1 a bit until it oscillated at 105 MHz. I then connected Vout to +5V and now the oscillator was oscillating at 108MHz. Great!, just as I wanted. By changing the Vout from 0 to +5V I could change the oscillating frequency from 105 to 108 MHz. I then reconnected the Vtune to the PLL.

Download PIC16F870 programs (INHX8M format)
The zip file contains several hex files made for different frequencies (88 to 108) MHz.

fm_500.zip( http://hem.passagen.se/communication/pic/fm_500.zip ) PLL software to FM transmitter (the hex files are zipped!).

Final word

This project is explaining how you can build a FM transmitter with great performances. I advice you not to use it because it is not legal. You can only use it with a dummy load, not with an antenna. If you choose to use an antenna I hope you will use it with good manner.

More Information --->( http://hem.passagen.se/communication/fm_500.html )

Phone FM Transmitter

This FM transmitter attaches in series to one of your phone lines. When there is a signal on the line (that is, when you pick up the handset) the circuit will transmit the conversation. In particular it will radiate from the phone line itself. It is a passive device - there is no battery. It uses the signal on the phone line for power. No aerial is needed - it feeds back the RF signal into the phone line which radiates it in the FM band.

The frequency of transmission may be adjusted by the trim cap. L1 is 6 turns of enameled wire, L2 is 8 turns and L3 is 6 turns. Spread out L3 coil about 1 mm apart. The coils should not touch. a solder connection (or tap) is required from the top of the first turn in the L3 coil to the pad next to the coil. Solder a piece of wire to the top of the first turn as shown on the overlay. Then solder the other end to the pad immediately next to the L3 coil. R1 & C4 act as a low pass filter. C3 is a high frequency shunt. L2 is a RFC (radio frequency shunt.) It decouples the power and audio from the transmitter amplifier circuit. L1 and C6 should be adjusted to match a frequency on your FM receiver. With C1 at 27p you will find that the kit tunes into the FM band in the 86 - 95 MHz area. With C1 at 22p the band is raised to about 90-95mhz (depending in the coil spacing.)

If you want to move this tunable area still higher to over 100MHz range then replace C1 by a 15pF or 10pF capacitor. You can experiment to get greater transmission range away from the phone line by adding an aerial (about 150 cm of 26 gauge wire) to the collector of T2.

FM Booster

Here is a low-cost circuit of an FM booster that can be used to listen to programmers from distant FM stations clearly. The circuit comprises a common-emitter tuned RF preamplifier wired around VHF/UHF transistor 2SC2570. (Only C2570 is annotated on the transistor body.)

Assemble the circuit on a good-quality PCB (preferably, glass-epoxy). Adjust input/ output trimmers (VC1/VC2) for maximum gain.

Input coil L1 consists of four turns of 20SWG enameled copper wire (slightly space wound) over 5mm diameter former.It is tapped at the first turn from ground lead side. Coil L2 is similar to L1, but hasonly three turns. Pin configuration of transistor2SC2570 is shown in the figure

Quality FM Transmitter

This FM transmitter for your stereo or any other amplifier provides a good signal strength up to a distance of 500 metres with a power output of about 200 mW. It works off a 9V battery. The audio-frequency modulation stage is built around transistor BF494 (T1), which is wired as a VHF oscillator and modulates the audio signal present at the base. Using preset VR1, you can adjust the audio signal level. The VHF frequency is decided by coil L1 and variable capacitor VC1. Reduce the value of VR2 to have a greater power output. The next stage is built around transistor BC548 (T2), which serves as a Class-A power amplifier. This stage is inductively coupled to the audio-frequency modulation stage. The antenna matching network consists of variable capacitor VC2 and capacitor C9. Adjust VC2 for the maximum transmission of power or signal strength at the receiver.

For frequency stability, use a regulated DC power supply and house the transmitter inside a metallic cabinet. For higher antenna gain, use a telescopic antenna in place of the simple wire. Coils L1 and L2 are to be wound over the same air core such that windings for coil L2 start from

the end point for coil L1. L1: 5 turns of 24 SWG wire closely wound over a 5mm dia. air core L2: 2 turns of 24 SWG wire closely wound over the 5mm dia. air core L3: 7 turns of 24 SWG wire closely wound over a 4mm dia. air core L4: 5 turns of 28 SWG wire on an intermediate-frequency transmitter (IFT) ferrite core

MOSFET-BASED PREAMPLIFIER FOR FM RADIO DXing

FM transmissions can be received within a range of 40 km. If you are in fringe areas, you may get a very weak signal. FM DXing refers to hearing distant stations (1500 km or more) on the FM band (88-108 MHz). The term ‘DX’ is borrowed from amateur radio operators. It means ‘distance unknown’; ‘D’ stands for ‘distance’ and ‘X’ stands for ‘unknown.’ For an FM receiver lacking gain, or having a poor signal-to-noise ratio, using an external preamplifier improves the signal level.

The dual-gate MOSFET preamplifier circuit shown in Fig. 1 gives an excellent gain of about 18 dB. It costs less and is simple to design. Field-effect transistors (FETs) are superior to bipolar transistors in many applications as these have a much higher gain—approaching that of a vacuum tube. These are classified into junction FETs and MOSFETs. On comparing the FETs with a vacuum tube, the gate implies the grid, the source implies the cathode, and the drain implies the plate.

In a transistor, the base implies the grid, the emitter implies the source, and the collector implies the drain. In dual-gate FETs, gate 1 is the signal gate and gate 2 is the control gate. The gates are effectively in series, making it easy to control the dynamic range of the device by varying the bias on gate 2. The MOSFET is more flexible because it can be controlled by a positive or negative voltage at gate 2. The resistance between the gate and rest of the device is extremely high because these are separated by a thin dielectric layer. Thus the MOSFET has an extremely high input impedance. Dual-gate MOSFETs (DG MOSFETs) are very popular among radio amateurs. These are being used in IF amplifiers, mixers, and preamplifiers in HF-VHF transceivers.
The isolation between the gates (G1 and G2) is relatively high in mixer applications. This reduces oscillator pulling and radiation. The oscillator pulling is troublesome particularly in shortwave communications. It is a characteristic in many unsophisticated frequency-changer stages, where the incoming signal, if large, pulls the oscillator frequency slightly off the frequency set by the tuning knob and towards a frequency favourable to the (large) incoming signal. A DG MOSFET can also be used for automatic gain control in RF amplifiers. DG MOSFET BF966S is an n-channel depletion-type MOSFET that is used for general-purpose FM and VHF applications.

In this configuration, it is used for FM radio band. The quadratic input characteristic of the FET input stage gives better results than the exponential characteristic of a bipolar transistor. Gate 1 is meant for input and gate 2 is for gain control. The input from the antenna is fed to gate G1 via C1 and L1. Trimmer VC1 is used to tune and select the input frequencies. Capacitor C4 (100 kpF) at the gain control electrode (gate 2) decouples any variation in G2 voltage at radio frequencies to maintain constant gain. Set preset VR (47k) to adjust the gain or connect a fixed resistor for fixed gain. The output of the circuit is obtained via capacitor C5 and fed to the FM receiver amplifier.

For indoor use, connect a ¼- wavelength whip antenna, ½-wavelength 1.5m wire antenna, or any other indoor antenna set-up with this circuit. You may use a 9V battery without the transformer and diode 1N4007, or any 6V-12V power supply to power the circuit (refer Fig. 1). The RF output can be taken directly through capacitor C5. For an improved input and output impedance, change C1 from 1 kpF to 22 pF and C5 from 1 kpF to 100 kpF. For outdoor use at top mast, like a TV booster, connect the C5 output to the power supply unit (PSU) line. Use RG58U/ RG11 or RG174 cable for feeding the power supply to the receiver amplifier. The PSU

for the circuit is the same as that of a TV booster. For TV boosters, two types of mountings are employed: The fixed tuned booster is mounted on the mast of the antenna. The tunable booster consisting of the PSU is placed near the TV set for gain control of various TV channels. (For details, refer ‘High-Gain 4-Stage TV Booster’ on page 72 of Electronics Projects Vol. 8.) Mount the DG MOSFET BF966S at the solder side of the PCB to keep parasitic capacitance as small as possible. Use an epoxy PCB. After soldering, clean the PCB with isopropyl alcohol. Use a suitable
enclosure for the circuit. All component leads must be small. Avoid shambled wiring to prevent poor gain or self oscillations. Connecting a single-element cubical quad antenna to the circuit results in ‘Open Sesam’ for DXing.

You can use a folded dipole or any other antenna. However, an excellent performance is obtained with a cubical quad antenna (refer Fig. 2) and Sangean ATS- 803 world-band receiver. In an amplifier, FET is immune to strong signal overloading. It produces less cross-modulation than a conventional transistor having negative temperature coefficient, doesn’t succumb to thermal runaway at high frequencies, and decreases noise. In VHF and UHF, the MOSFET produces less noise and is comparable with JFETs. DG FETs reduce the feedback capacitance as well as the noise power coupled to the gate from the channel, giving stable unneutralised power gain for wide-band applications. This circuit can be used for other frequency bands by changing the input
and the output LC networks. The table here gives details of the network components for DXing of stations at various frequency bands.

Long Range FM Transmitter

Description

The use of transmitters which have a more powerful output than the ‘flea-power’ are sometimes required when there are many obstacles in the path of the surveillance transmitter and monitoring station receiver, or the distance between them is too far so as to make a low powered devicefeasible. Whereas a typical microtransmitter will produce an RFpower in the order of just a few milliwatts, i.e. a few thousandths of awatt, the VHF-FM transmitter described has a poweroutput of between around a half and 2 watts, depending on the power source, which may be anywhere between 6 volts and 30 volts d.c. The battery or batteries should be of the alkaline high power type, sincethe current drain will be found to be relatively higher when comparedto microtransmitter current drain. The power output of this device is somewhat proportional to the current drain and so therefore both may be decreased by altering the value of R6 to a higher resistance, or a variable resistor with a value of around 1k may be introduced in series with the existing R6, so as to give a variable power output. Thevariable resistor must not be a wirewound device because this wouldact as an inductor which will cause feedback problems.

The audio input to the power oscillator, which incidentally is formed by TR2 and associated components, is derived from a piezoelectric microphone which drives the simple audio frequency amplifier TR1. The input of the audio amplifier is controlled by the gain pot R1, which selects the correct amount of voltage that is generated by the piezoelectric microphone, then connects this signal to the base of audio amplifier TR1 via C7. It may be found that there is insufficient housing space for a bulky piezoelectric microphone, so with a slight modification to the circuit, it is possible to employ an electret microphone insert as shown Since the RF field that is generated by this transmitter is relatively large, the problem of RF feedback may very well be encountered. This may be overcome by placing the transmitter inside a metal enclosure, keeping all internal wiring as short as possible and the aerial wire

Component listing for 1 watt transmitter

Resistors Semiconductors
R1 = 27k TR1 = BC547
R2 = 330k TR2 = 2N2219 fitted with heat sink
R3 = 5k6 MIC = piezoelectric microphone
R4, 5 = 10k
R6 = 100R

L = 6 turns 22 gauge enameled wire wound on 3⁄16″ former

Capacitors

C1, 2, 3, 8 = 330 pF
C4 = 2–10 pF trimmer
C5 = 4p7
C6 = 1 nF

Telephone FM Transmitter

Telephone FM Transmitter

This FM transmitter attaches in series to one of your phone lines. When there is a signal on the line (that is, when you pick up the handset) the circuit will transmit the conversation. In particular it will radiate from the phone line itself. It is a passive device - there is no battery. It uses the signal on the phone line for power. No aerial is needed - it feeds back the RF signal into the phone line which radiates it in the FM band. The frequency of transmission may be adjusted by the trimcap.L1 is 6 turns of enameled wire, L2 is 8 turns and L3 is 6 turns. Spread out L3 coil about 1 mm apart. The coils should not touch. A solder connection (or tap) is required from the top of the first turn in the L3 coil to the pad next to the coil. Solder a piece of wire to the top of the first turn as shown on the overlay. Then solder the other end to the pad immediately next to the L3 coil. R1 & C4 act as a low pass filter. C3 is a high frequency shunt. L2 is a RFC (radio frequency shunt.) It decouples the power and audio from the transmitter amplifier circuit. L1 and C6 should be adjusted to match a frequency on your FM receiver. With C1 at 27p you will find that the kit tunes into the FM band in the 86 - 95 MHz area. With C1 at 22p the band is raised to about 90-95mhz (depending in the coil spacing.) If you want to move this tunable area still higher to over 100MHz range then replace C1 by a 15pF or 10pF capacitor. You can experiment to get greater transmission range away from the phone line by adding an aerial (about 150 cm of 26 gauge wire) to the collector of T2.