Radio Frequency Circuits

Variable RF attenuator with PIN diode

2009-09-23T09:17:00.000-07:00

Variable RF attenuators are often used to control the level of a radio frequency signal using a control voltage in RF design. These variable RF attenuators can even be used in programmable RF attenuators. Here the known voltage generated by a computer for example can be applied to the circuit and in this way create a programmable RF attenuator.

Often when designing or using variable or programmable RF attenuators, it is necessary to ensure that the RF attenuator retains a constant impedance over its operating range to ensure the correct operation of the interfacing circuitry. This RF attenuator circuit shown below provides a good match to 50 ohms over its operating range.

RF attenuator circuit description
The PIN diode variable attenuator is used to give attenuation over a range of about 20 dB and can be used in 50 ohm systems. The inductor L1 along with the capacitors C4 and C5 are included to prevent signal leakage from D1 to D2 that would impair the performance of the circuit.

The maximum attenuation is achieved when Vin is at a minimum. At this point current from the supply V+ turns the diodes D1 and D2 on effectively shorting the signal to ground. D3 is then reverse biased. When Vin is increased the diodes D1 and D2 become reverse biased, and D3 becomes forward biased, allowing the signal to pass through the circuit.

Typical values for the variable RF attenuator circuit might be: +V : 5 volts; Vin : 0 - 6 volts; D1 to D3 HP5082-3080 PIN diodes; R1 2k2; R2 : 1k; R3 2k7; L1 is self resonant above the operating frequency, but sufficient to give isolation between the diodes D1 and D2.

These values are only a starting point for an experimental design, and are only provided as such. The circuit may not be suitable in all instances.

Choice of PIN diode
Although in theory any diode could be used in variable RF attenuators, PIN diodes have a number of advantages. In the first place they are more linear than ordinary PN junction diodes. This means that in their action as a radio frequency switch they do not create as many spurious products and additionally as an attenuator they have a more useful curve. Secondly when reverse biased and switched off, the depletion layer is wider than with an ordinary diode and this provides for greater isolation when switching or providing higher levels of attenuation.

Source: PIN diode variable RF attenuator circuit

Power Supply with Short Circuit Protection 13.8V 30-40A

2009-09-23T08:58:00.000-07:00

Here's a safe power supply with short circuit protection. The power supply is build around the LM723 controller and four BUZ24 (or IRF150) power N-Channel FET transistors. FET transistors are used because of the simplicity of controlling the current through these transistors, it's simply voltage controlled, and because of the low power consumption of the controller board.

Project schematics, PCB layouts and data sheets

Source: 13.8V 30-40A Power Supply

RF Power Meter Reading by Digital Voltmeter

2009-09-23T04:56:00.000-07:00

The RF Power Meter circuit is based on the AD8313 Log Detector manufactured by Analog Devices. In GSM phones AD8313 is used as a Log Detector, part of the Power Control Loop circuit. Generally could be easy identified near the Power Amplifier module.

AD8313 is a Logarithmic Detector which can accurately convert an RF signal at its input to an equivalent decibel-scaled value at its DC output. The DC output is “linear in dB” with a basic slope of 20mV/dB. The slope can be adjusted in a range from 18mV/dB to 30mV/dB. The linear input range of AD8313 is between -60dBm and 0dBm, which corresponds to a DC output between 0.6V to 1.6V (pin 8).

The operational amplifiers LM324 are translating the DC output range of AD8313 (0.6V to 1.6V on Pin nr 8) to a scaled range read by the Voltmeter (-6V to 0V). The scaled range has a resolution of 100mV/dB.

For example the minimum input value (-60dBm) corresponds to a read voltage value of -6.0V, -59dBm corresponds to -5.9V, -58dBm corresponds to -5.8V, and so on up to 0V that corresponds to 0dBm (as in the table below).

The frequency range of AD8313 is between 100MHz to 2.5GHz, but the range that not requires a dynamic slope adjustment is between 100MHz to 1.4GHz. The resolution of the RF Power Meter is better than +/- 1dB; only near 0dBm power input, the resolution is approximately +/- 2dB. The RF input has an impedance of 50 ohms provided by the 53 ohms resistor in parallel with the internal impedance of the AD8313.

For calibration inject first at the input an 800MHz signal at -60dBm and adjust P2 for -6V reading on the output Voltmeter. After that increase the input level up to 0dBm and adjust P3 for 0V reading on the output Voltmeter. The slope can be adjusted by the P1 semi-resistor.

Careful design of the RF input layout should be done for minimizing parasitics which can produce un-wanted resonances that affects the linearity vs frequency of the log-detector. Tolerance of the resistors is +/-1%.

A calibrated attenuator at the input can be used to increase the maximum input power, without damaging the detector.

Source: RF Power Meter using for reading a standard Digital Voltmeter

Surveillance Transmitter Detector

2008-11-14T22:23:00.000-08:00

This surveillance equipment is fm bug detector. The circuit can be used to "sweep" an area or room and will indicate if a surveillance device is operative. The problem in making a suitable detector is to get its sensitivity just right; too much and it will respond to radio broadcasts, too little sensitivity and nothing will be heard.

This surveillance equipment project has few components. It can be made on veroboard and powered from a 9 volt battery for portability. The fm bug detector prototype shown below, worked OK on a Eurobreadboard.

Circuit operation of the surveillance equipment is simple. The inductor is a moulded RF coil, value of 0.389uH and is available from Maplin Electronics, order code UF68Y. The coil has a very high Q factor of about 170 and is untuned or broadband. With a test oscillator this circuit responded to frequencies from 70 MHz to 150 MHz, most of the FM bugs are designed to work in the commercial receiver range of 87-108 MHz.

The RF signal picked up the coil, and incidentally this unit will respond to AM or FM modulation or just a plain carrier wave, is rectified by the OA91 diode. This small DC voltage is enough to upset the bias of the FET, and give an indication on the meter. The FET may by MPF102 or 2N3819, the meter shown in the picture is again from Maplin Electronics, order code LB80B and has a 250 uA full scale deflection. Meters with an FSD of 50 or 100 uA may be used for higher sensitivity.

In use the preset is adjusted for a zero reading on the meter. The detector is then carried around a room, a small battery transmitter will deflect the meter from a few feet away.

Source: FM Bug Detector

continue reading: FM Power Amplifier

30W VHF Amplifier with 2SC1946A

2008-10-28T06:28:00.000-07:00

Here is an 30 watt FM amplifier schematic that provides an appropriate power boost with an input of 4 watt up to 6 watts (2SC1971 Driver Amplifier recomended). The circuit is designed to cover 88-108MHz VHF FM broadcast band. However, the circuit is very stable at my place and provides a clean-output through 7 elements Butterworth low pass filter.

The amplifier current can be over 5 amps. All the coils are made from 16 gauge laminated wire (or Silver copper wire can do best) and the RFC can be of HF toroid core (as shown in the picture) or 6 holes ferrite bead.C3 and R1 forms snubber circuit while R2 and C6 prevent the amplifier from self-oscillation at VHF, sometimes you need to add 180 ohms in parallel with L7.That will cause the amplifier to dissipate UNDESIRABLE VHF thereby reducing spurious level.

Tuning of the amplifier is not hard at all. You just have to connect the output to a good antenna with a transmission line (RG214) of 50 ohms. First match the output network, and then do the same to the input network for a maximum power output. By way of adjustment, you can increase the output at its operating frequency.

Source: 30W VHF Amplifier 2SC1946A

See more : FM Transmitter

40-900 MHz Signal Amplifier-Booster

2008-09-27T21:39:00.000-07:00

This is a small amplifier of signal that cover the frequencies from 40 to 900 MHz. Those frequencies include TV in VHF and UHF and also radio in the frequencies of 88 - 108 MHz, strip of FM. The signals in up to 20 dB, becoming possible to receive even the weakest signals. Below is the circuit diagram.

The circuit is built in turn of an UHF broadband transistor, BFR 90. This transistor can operate as loud as 1.6 GHz in frequencies, and it has an gain of 23 dB. The signal of the antenna is applied the input of the circuit and for C5 it is coupled to the base of the transistor. It is amplified and of the collector of BFR90, through C2 and C1 it is taken the input of RF of the radio or receiver of TV. The circuit operates with a battery of 9V small that, because of the very low consumption of the circuit, it will last plenty of time.

Here's Printed circuit boards and component layout of the signal amplifier

Characteristic and Specifications technique for signal amplifier-booster with BFR90

Answers of Frequency: 40 - 900 MHz
Gain: 20 dB
Maximum level of output: 90 µV
Input-output impedance: 75 ohm

Signal Amplifier Circuit's Parts List
R1 = 82 Kohm
R2 = 270 ohm
R3 = 1,5 Kohm
R4 = 120 ohm
C1,C3 = 100 PF
C2 = 2.2 pF
C4,C5 = 1 nF
D1,D2 = 1N4148 diode
Transistor = BFR90, BFR91, BFR90,
Several = PCB, solder, battery 9V, metallic box, clip to battery, etc.
L1, L2=diameter: 5 mm thread diameter:0,5 mm turns: 8

See more : Stereo FM Transmitter

FM Transmitter With BF494

2008-09-27T21:16:00.000-07:00

This is a small circuit fm transmitter using transistor BF494 . The reach of the transmitter will depend a lot on this antenna, of the feeding of the circuit and of the place where will happen your operation.

With a feeding of 6 V (4 piles AA) and an antenna of about 40 cm of length, we can arrive to the 200 meters. With a smaller antenna, or in place with many structures of iron (flagstones) the reach will be sensibly reduced. A common wall without metallic structure is not obstacle for this transmitter.

Below the circuit schematic and Printed circuit board fo the fm transmitter

Parts List of the micro fm transmitter

Transistors:
Q1 - BC558
Q2 - BF494
Resistors:
R1--4,7 KΩ
R2--220 KΩ
R3--22 KΩ
R4--10 KΩ
R5--6,8 KΩ
R6--47 Ω
Capacitors:
C1--10 µF/16 V - ELECTROLYTIC
C2--4,7 µF/16 V - ELECTROLYTIC
C3--10 nF - ceramic
C4--4,7 pF - ceramic
C5--100 nF - ceramic
CV--trimmer from 3-30 to 5-50 pF
Other:
MIC--microphone of electret of two terminals
L1--Reel - to see text
S1--simple Switch
B1--3 to 6 V - 2 to 4 small piles
The--antenna - to see text Printed circuit board or bridge of terminals, box for assembly or hollow book, support for 2 or 4 small piles, threads, it welds, etc.

Source: Toni Eletronica

See more : Stereo FM Transmitter

FM Transmitter 2N2218

2008-09-27T20:54:00.000-07:00

Here's simple FM transmitter circuit using transistor 2N2218. Micropohone is a microphone of electret of two terminals and the antenna should possess from 15 to 40 cm.

Below is schematic circuit of the fm transmitter

Printed Circuit Board (PCB) and Component Layout of the fm tranmitter

Parts List of the fm transmitter
Resistors
R1 = 2,2 kΩ
R2 = 4,7kΩ
R3=5,6kΩ
R4 = 47Ω
Ceramic Capacitors
C1=4,7 nF
C2=2,2 nF
C3 = 4.7pF
C4 = 100 nF
C5 = TRIMMER CV 3-30 PF.
Transistor:
T1 = 2n2218 or equivalent.
Other:
L1 = 5 turns of enameled thread 22 AWG, diameter of 10 mm.
B1 = Battery of 9 volts alkaline.
Mic1 = electret microphone.
Several = printed circuit board, antenna, box, etc.

Source: Toni Electronica

See more: FM Transmitter Circuit

RF Low Pass Filter 1.8-145 MHz

2008-09-16T09:22:00.000-07:00

This lowpass filter circuit is a very basic filter. It required to remove the harmonic frequency content of the transmitter and can be used on frequency 1.8 - 145 MHz.

The cut off frequency of the filter can be achieved by making C2 and C4 variable trimmer acurate filtering. All coils are 22 SWG wound on 10mm Air former spaced at 2mm.

For making coils of the lowpass filter, download this RF Calculation Software and the list of Wire Gauge below.

Source: Ajpotts

See more: FM Transmitter

Yagi Antenna 88-108 MHz

2008-08-29T23:33:00.000-07:00

This antenna below is a construction of an Yagi antenna calculated for the FM broadcasting band 88 - 108 MHz. The central frequency used for 100MHz. You can extrapolate easily below for others frequency grace to values of diagrams. This antenna is less difficult to construct than the one presented on the previous page.

Description
The goal of this page is to help you to construct an antenna, not to make the theory of this one, theory complex strong to staying it. Here are some definitions of basis therefore. The name of axis of an antenna call the BOOM. The antenna uses a dipole says " trombone " that is the only element a few delicate to achieve. Contrary to what lets believe diagrams above the two tubes of the trombone are not to dish " but disposed perpendicularly to the axis of the antenna. On the photo below taken perpendicularly to the antenna one doesn't see the two tubes, but in " alone appearance " one. Directors increase the directivity of the antenna (sense of the arrow) but as the gain, it is because of them that with 4 elements the antenna to a Gain of 6dB and 5 elements procures a Gain of 8dB. The reflector return the bundle of wave " collected " by directors toward the dipole, but his major role is to protect the antenna against the parasitic signals coming from the rear. In any case an antenna must be raised in relation to background to work well. 1 to 2 length of work wave is counseled. For F = 100 MHz, the Length of wave is 3Meters (9.85 feet).Source: Radio Meteor

See more: FM Transmitter

FM Stereo Encoder With RC4200 Multiplexer IC

2008-08-06T23:56:00.000-07:00

This circuit is stereo encoder for stereo modulation fm broadcast. The Basic component for stereo multiplexing is a single RC4200 IC. The textbook way of processing and encoding a stereo signal for FM transmission goes like this:

1) Take both channels and low-pass-filter them at 15kHz, with steep rolloff;
2) Apply pre-emphasis. Depending on the part of the world, it should have either a 75µs or a 50µs time constant;
3) Strictly limit the audio level to ensure that overdeviation cannot happen;
4) Create a stable, clean 38kHz sine wave;
5) Subtract the right channel from the left channel, and multiply the result with the 38kHz carrier;
6) Create a clean 19kHz sine wave, phase-locked to the 38kHz one;
7) Add the left channel, right channel, the (L-R)*38kHz signal, and the 19kHz signal, with specific amplitudes.

There are several ways to implement this algorithm. Modern factory made transmitters often do the whole thing digitally, in a DSP. But it's still less expensive and simpler to do in the analog domain. That can be done in various ways too, and far too many transmitters these days use ultra cheap, mediocre methods like hard-switched multipliers based on CMOS switches. They do work, but are very noisy! My design instead uses a true, high quality analog multiplier for that task. As a result, the signal from my transmitter is as good as the very best signals I can receive locally, and MUCH better than the bulk of them!

Here is the schematic diagram. You probably won't be able to read it at this resolution, so better click on it, save it in full resolution, print it, and refer to it for the following explanation. If you have trouble opening the large version, right-click on the diagram, so you can save it to disk, then open it using IrfanView or any other GOOD image viewer. This is valid for all drawings on this page. The full resolution drawings are large, and depending on the amount of memory in your computer, some web browsers cannot open them and will report a broken link.

The two single-ended line-level audio signals enter through feedthrough capacitors, and are welcomed by an LC low-pass filter to get rid of any RF that could be on them. In each channel there is a buffer stage, and then a combined pre-emphasis and soft limiter stage. The advantage of doing the limiting and the pre-emphasis in one step is that it avoids overdeviating from loud treble sounds, or having loud bass sounds flatten out the treble, without the need of a multiband limiter. The gain of the non-limited portion of the audio signals is adjustable by means of trimpots. Then comes a six-pole low pass filter that removes signals above 15kHz.

A 74HC4060 chip derives the 38kHz and 19kHz signals, as square waves, from a custom-made quartz crystal. Two resonant circuits using ferrite pot cores turn these square waves into very clean, low noise sine waves. Trimpots allow to set the levels, while the adjustable cores of the inductors allow precise tuning. Jumpers allow to disable each of these signals for testing and adjustment purposes.

A rather old fashioned, but low-noise and low-distortion analog multiplier chip modulates the L-R signal, produced by an op amp differential amplifier, onto the 38kHz subcarrier. This circuit has three adjustments for balance. Its output level is adjustable too. The signals that are necessary only for stereo can be disconnected for testing by means of a jumper.

The output adder combines the L signal, R signal, (L-R)*38kHz signal, and the pilot tone. The first two signals are fixed at this stage, while the (L-R)*38kHz can be adjusted by its own trimpot, and the pilot tone by the trimpot before its LC circuit. Then there is a final level adjustment, used to set the deviation of the transmitter, and then a buffer stage with low output impedance, that drives the output through a resistor to avoid instability from capacitive loads.

There is an additional circuit which consists basically of a dual superdiode detector with a time constant and driver with adjustable output. This circuit picks up the complete multiplex signal just before the final level control, and produces a DC signal to directly drive a small meter, for deviation indication. This is a most important tool for the transmitter operator to set the proper audio level during routine operation!

Here is the printed circuit board. Click on it to get it in high resolution.... It's seen "through the board", so you can print it directly and place the ink in contact with the copper to get a correct sided copper pattern.

The entire circuit is built on this single-sided PCB. Only a few jumper wires are necessary, so it's not worthwhile making a dual sided PCB for this.

And this is a crude parts overlay, just to see where a part goes. Exactly which part goes where, is something you will have to work out with the schematic! Don't be lazy! Source: FM Transmitter 80W

You may wanna reading Transistor FM Transmitter

C-Quam AM Stereo Decoder MC13028

2008-07-18T06:45:00.000-07:00

This electronic circuit is AM stereo decoder that accepts the signal from the IF of a receiver (450 or 455 KHz) and decodes stereo medium wave broadcasts. It operates at 8-15VDC and provides standard level left and right audio output for direct connection to the audio amplifier.

It may be used to support 30Hz-15KHz audio but it depends on the bandwidth of the receiver's IF.

AM Stereo Decoder Circuit Schematic

Printed Circuit Board (PCB)

Layout PCB

You may wanna reading: FM Transmitter

5 MHz Bandwith Video Signal Amplifier

2008-07-17T22:32:00.000-07:00

This video signal amplifier circuit is a broad band amplifier which will take the video signals from your VCR and will amplify them sufficiently to drive up to 3 monitors, TV sets (provided that they can accept direct video signals), or other VCR’s for recording from one video to up to three others. It will also make possible to record from one video to two others and at the same time have a monitor connected to check what you are recording. The amplifier is also very useful if the video recorder is far from the monitor.

Video Signal Amplifier schematic Circuit

Printed Circuit Board (PCB)

Parts List
R1,R14-R16 - 150
R18-R21 - 150
R2 - 10K
R3 - 1K5
R4,R8,R9 - 1K
R5 - 330
R6 - 3K3
R7 - 390K
R10,R13 - 2K2
R17 - 560
R11,R12 - 27
C1,C3 100u/16v
C2,C4,C6,C8,C10,C11 100n
C5,C7,C9 - 470u/16v
Q1-Q3 - BC548
Q2 - Bc558
Q4 - BC338
Q5 - BC558

You may wanna reading FM Transmitter

50W Linear FM Amplifier with BLY90

2008-07-17T22:09:00.000-07:00

This RF FM Amplifier is always essential for the amateur that wants it strengthens some small transmitter. The present circuit of BLY90 can give 50-60W power out with input control 15-20W in FM band II 88-108 MHz.

FM amplifier schematic Circuit

Printed Circuit Board

FM Amplifier Parts List
C1-C4 = 10-80pF
C5 = 10nF
C6 = 1000pF
C7 = 100nF
C8 = 2200mF/35V
TR1 = BLY90
L1 = 1 Turn of diameter of 10mms, 1mm
L2 = 7 Turns of diameter of 10mms, 0,8mms
L3 = 3 Turns of diameter of 10mms, 1mm

Source: 50W FM Amplifier BLY90

Continue reading AV Modulator

4 Watt PLL FM Transmitter

2008-07-12T05:04:00.000-07:00

This transmitter circuit uses PLL method to generate stable frequency and the output power is selectable from 1 to 4 watts via an on-board jumper switch. It's designed using wide-band power amplifier technology, this eliminates the need to tune up the transmitter to "peak the power".

This PLL FM transmitter has an "Out Of Lock Power Down" circuit that will automatically reduce the output power to zero so that frequency adjacent radio stations will NOT be affected. A clean start up every time is assured without any disturbance to other channels.

The sound quality of the FM transmitter will match the most expensive exciters and walk over the rest! The response of audio frequency is absolutely level and linear to provide only the highest transmitter FM modulation quality. (PLL FM Transmitter Technical Manual)

Continue reading Audio Video Modulator

FM Stereo Encoder NJM2035

2008-07-09T10:40:00.000-07:00

This stereo encoder is the perfect solution for those looking for a high quality stereo sound transmission at a low cost. This stereo encoder produces an excellent crystal clear stereo sound and very good channel separation that can match with many more expensive stereo encoders that are available on the market. It is all possible thanks to a 38KHz quartz crystal that controls the 19kHz pilot tone, so you will never have to calibrate or re-adjust the circuit.

NJM2035 offers superb quality and is manufactured by NJR CORPORATION (JRC), a subsidiary of New Japan Radio, a company that is known as the world’s best manufacturer of high end professional audio semiconductors.

Source: NJM2035 Stereo Encoder

Continue reading Audio Video Modulator

FM Stereo Encoder BA1404

2008-07-09T10:20:00.000-07:00

This FM stereo encoder uses IC BA1404 for transmiting on FM band broadcast. Sub carrier frequency generated from 38 KHz Crystal. FM Power output is around 250mW. For input preamplifier used a couple of IC 741. You need a fm amplifier for boosting rf signal.

Stereo Encoder Schematic circuit

BA1404 datasheet

Another Stereo Encoder Circuit

You may wanna continue reading Audio Video Distribution Amplifier

VHF Audio Video Transmitter

2008-05-19T15:44:00.000-07:00

The circuit presented here is a simple audio video transmitter with a range of 3 to 5 metres. The A/V signal source for the circuit may be a VCR, a satellite receiver or a video game etc. A mixer which also operates as an oscillator at VHF (H) channel 5 TV frequency is amplitude modulated by video signal and mixed with frequency modulation, contains video carrier frequency of 175.25 MHz and audio carrier frequency of 180.75 MHz. Then, the transmitter is a B-System of CCIR compatible.

The circuit consists of transistor Q1 with its resonant tuned tank circuit formed by inductor L1 and trimmer capacitor VC1, oscillating at VHF (H) channel 5 frequency. Transistor Q2 with its tuned circuit formed using SIF coil and inbuilt capacitor forms oscillator. The audio signal applied at the input to Q2 results into frequency modulation of 5.5 Mhz oscillator signal. The output of 5.5 Mhz FM stage is coupled to the mixer stage through capacitor C8 while the video signal is coupled to the emitter of Q1 via capacitor C4 and variable resistor Inductor L1 can be wound on a 3mm core using 24SWG enamelled wire by just giving 4 turns. Calibration/adjustment of the circuit is also not very difficult. After providing 12V DC power supply to the circuit and tuning your TV set for VHF (H) channel 5 reception, tune trimmer VC1.

You may wanna continue reading Audio Video Amplifier

Video to RF Modulator

2008-05-19T15:39:00.000-07:00

This circuit is a RF modulator which can be used for modulating of video signals. The P1 controls the Light and the P2 controls the contrast of video signal. The RF modulated output signal can be received on the VHF band (about 189 MHz). Note that the circuit don't modulate the audio signal of the pictures.

See more : Audio Video Distribution Amplifier

Accurate LC Meter

2008-05-19T15:29:00.000-07:00

This is one of the most accurate and simplest LC inductance / capacitance Meters that one can find, yet one that you can easily build yourself. This LC Meter allows to measure incredibly small inductances starting from 10nH to 1000nH, 1uH to 1000uH, 1mH to 100mH and capacitance from 0.1pF up to 900nF. LC Meter's circuit uses an auto ranging system so that way you do not need to spend time selecting ranges manually. Another neat function is the "Zero Out" switch that will reset the initial inductance / capacitance, making sure that the final readings of the LC Meter are as accurate as possible.

To be able to determine the value of an unknown inductor / capacitor we can use the frequency formula given below.

Note that there are three variables that we can work with; f, L and C (f represents a frequency, L inductance and C capacitance). If we know the values of the two variables we may calculate the value of the third variable.

Lets say we want to determine the value of an unknown inductor with X inductance. We plug X inductance into the formula and we also use value of a known capacitor. Using this data we can calculate the frequency. Once we know the frequency we can use the power of the algebra and rewrite the above formula to solve for L (inductance). This time we will use the calculated frequency and a value of a known capacitor to calculate the inductance.

Isn't this amazing? We just calculated the value of unknown inductor, and we may use the same technique to solve for the unknown capacitance and even frequency. Visit: electronics-diy.com

You may wanna continue reading : Band Pass Filter for FM

1W RF Power Amplifier For FM

2008-05-19T15:23:00.000-07:00

This RF Power Amplifier is used for boosting small fm transmiters and bugs. It use two Philips 2N4427 and its power is about 1Watt. At the output you can drive any power amplifier with BGY133 or BLY87 and so on. Its power supply has to give 500mA current at 12 Volts.

More voltage can boost the distance but the transistors will be burned much earlier than usual.! In any case do not exceed the 15Volts. The RF Amplifier offers 15 dB in the area of 80 Mhz to 110 Mhz. L4, L5, and L6 are 5mm diameter air coils, 8 turns, with wire 1mm wire diameter.An easy project, with great results.

See more : Adjustable Band Pass Filter for FM

HI - FI Stereo FM Transmitter

2008-05-19T15:17:00.000-07:00

Whether you want to create your own radio station, transmit the music around the house, or simply create a wireless link between your iPod and a receiver in your car, this transmitter will let you do these things easily. With BA1404 HI-FI Stereo transmitter you will be able to transmit MP3 music from your iPod, computer, discman, walkman, TV/SAT receiver, and many other audio sources.

The above FM transmitter design is a result of many hours of testing and tweaking. The goal was simple; to test many existing BA1404 transmitter designs, compare their performance, identify weaknesses and come up with a new BA1404 transmitter design that improves sound quality, has very good frequency stability, maximizes transmitter's range, and is fairly simple for everyone to build. We are happy to announce that this goal and expectations have been met and even exceeded.

The transmitter can work from a single 1.5V cell battery and provide excellent crystal clear stereo sound. It can also be supplied from two 1.5V battery cells to provide the maximum range.

You may wanna filtering your FM Transmitter

FM Transmitter From David Sayles

2008-05-19T04:50:00.000-07:00

A small FM voice transmitter for Band 2 VHF. This fm transmitter circuit is simple, output frequency tuned by regulating on coil (L) that parallel with 15pf capacitor. Supply voltage needed for operate this circuit is 9 volt.

This small transmitter uses a hartley type oscillator. Normally the capacitor in the tank circuit would connect at the base of the transistor, but at VHF the base emitter capacitance of the transistor acts as a short circuit, so in effect, it still is. The coil is four turns of 18 swg wire wound around a quarter inch former. The aerial tap is about one and a half turns from the supply end. Audio sensitivity is very good when used with an ECM type microphone insert.

Continue reading : FM amplifier for ipod Transmitter

Transitor FM Transmitter Circuit

2008-05-19T04:46:00.000-07:00

This Transmitter circuit is a simple two transistor (2N2222) FM transmitter. No license is required for this transmitter according to FCC regulations regarding wireless microphones. If powered by a 9 volt battery and used with an antenna no longer than 12 inches, the transmitter will be within the FCC limits.

The microphone is amplified by Q1. Q2, C5, and L1 form an oscillator that operates in the 80 to 130 MHz range. The oscillator is voltage controlled, so it is modulated by the audio signal that is applied to the base of Q2. R6 limits the input to the RF section, and it's value can be adjusted as necessary to limit the volume of the input. L1 and C6 can be made with wire and a pencil. The inductor (L1) is made by winding two pieces of 24 gauge insulated wire, laid side by side, around a pencil six times. Remove the coil you have formed and unscrew the two coils apart from each other. One of these coils (the better looking of the two) will be used in the tank circuit, and the other can be used in the next one you build.

The antenna (24 gauge wire) should be soldered to the coil you made, about 2 turns up from the bottom, on the transistor side, and should be 8-12 inches long. To make C6, take a 4 inch piece of 24 gauge insulated wire, bend it over double and, beginning 1/2" from the open end, twist the wire as if you were forming a rope. When you have about 1" of twisted wire, stop and cut the looped end off, leaving about 1/2" of twisted wire (this forms the capacitor) and 1/2" of untwisted wire for leads.

You may continue reading : FM Power Amplifier for your FM Amplifier for ipod

Audio Video Modulator

2008-05-16T11:47:00.000-07:00

If you want to connect a video signal originating from a camera or other video source to a normal TV set, you will need Audio Video Modulator.

The audio and video signal is converted into a UHF TV signal so that the signal can be received through the TV antenna input. In certain countries,it is legal to use this modulator as a mini transmitter by connecting a small antenna.
This makes it possible to receive the signal from the video recorder or camera in another room of your home (range approx 30m, 98').

This modulator circuit provides you with wireless audio and visual transmission to a TV. The TV acts as a receiver, eliminating the need to buy a separate monitor. You can also hook it up to a VCR or CCD Camera, and even set up a remote CCTV security system!

Circuit Analysis:

Q3, VC1, C13, C16 and L3 all make up a colpitts oscillator circuit that fluctuates form 220~250MHz. You can regulate the frequency to any value within this threshold by tuning VC1 or L3. C13 modulates the signal rate. When the capacitance increases, so does the modulation. R9 and C16 bias the local oscillation. If you lower R9's frequency to 680W the oscillator's output level will increase.

Q2 and L2 act as a frequency doubler. C7, along with FCZ7S3R5 (IF transformer), the Q4 transistor, C14, C19 and R12 all make up the mixer. This mixer takes both audio and visual signals together and "mix" them into one and passes through RF Amplifier Q1 to transmit the signal to the antenna.

Testing:

Turning the blue component's trimmer on VC1 varies the frequency. When we turn the trimmer, the television's channel has to be changed accordingly. It is easier to tune the A/V Sender if you have a spectrum analyzer to help you find the correct frequencies. If the frequency is tuned to 474 MHz then this would be the equivalent of your TV's channel 14 UHF band.
The IF transformer is used to synchronize the audio and video frequency's level radio. If the TV's image is too blurry then you can adjust the IFT to fine-tune the image.
SVR1 controls the video signal input ratio, while SVR2 controls the audio portion. You can tune these components according to your needs.

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