The transmitting system consists of two tuned circuits such that the one containing the spark-gap is a persistent oscillator; the other, containing the aerial structure, is a free radiator maintained in oscillation by being coupled to the first. The transmitting system consists of two electrically coupled circuits, one of which, containing the air-gap, is a powerful but not persistent oscillator, being provided with a device for quenching the spark so soon as it has imparted sufficient energy to the other circuit containing the aerial structure, this second circuit then independently radiating the train of slightly damped waves at its own period.
For a fixed frequency transmitter one commonly used method is to use a resonant quartz crystal in a Crystal oscillator to fix the frequency. Where the frequency has to be variable, several options can be used.
For VHF transmitters, it is often not possible to operate the oscillator at the final output frequency.
In such cases, for reasons including frequency stability, it is better to multiply the frequency of the free running oscillator up to the final, required frequency. The task of many transmitters is to transmit some form of information using a radio signal (carrier wave) which has been modulated to carry the intelligence.
A few rare types of transmitter do not carry information: the RF generator in a microwave oven, electrosurgery, and induction heating. RF transmitters that do not carry information are required by law to operate in an ISM band. In many cases the carrier wave is mixed with another electrical signal to impose information upon it. This occurs in Amplitude modulation (AM). Amplitude Modulation: In Amplitude modulation the instantaneous change in the amplitude of the carrier Frequency with respect to the amplitude of the modulating or Base band signal.
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AM Broadcast Transmitter schematic
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In this circuit, a 74HC14 hex Schmitt trigger inverter is used as a square wave oscillator to drive a small signal transistor in a class C amplifier configuration. The oscillator frequency can be either fixed by a crystal or made adjustable (VFO) with a capacitor/resistor combination. A 100pF capacitor is used in place of the crystal for VFO operation. Amplitude modulation is accomplished with a second transistor that controls the DC voltage to the output stage. The modulator stage is biased so that half the supply voltage or 6 volts is applied to the output stage with no modulation. ..
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Crossed from: AM radio | Clicks: 3937 | Votes: 0 | Comments: 0 | Rating: 0 | Rank: 0
AM power transmitter
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The circuit for a powerful AM transmitter using ceramic resonator/filter of 3.587 MHz is presented here. Resonators/filters of other frequencies such as 5.5 MHz, 7 MHz and 10.7 MHz may also be used. Use of different frequency filters/resonators will involve corresponding variation in the value of inductor used in the tank circuit of oscillator connected at the collector of transistor T1.
The AF input for modulation is inserted in series with emitter of transistor T1 (and resistor R4) using a transistor radio type audio driver transformer as shown in the circuit. Modulated RF output is developed across the tank circuit which can be tuned to resonance frequency of the filter/resonator with the help of gang condenser C7...
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CW 250mW transmitter
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Here is a simple little HF TX. It may be modified for all the HF bands, but the details given are only for the 7MHz band. The basic transmitter uses two transistors; BC547 (as usual) and will deliver over 250mW of power. CW keying may be achieved by disconnecting the 56 ohm resistor in the emitter of TR2, and connecting it to ground via the key. Increase the value of the 10nf in the emitter of TR2 if you need longer time-constant keying (keying envelope). The optional PA stage will increase the RF output of the HF TX to over 3 watts. TR3 is one of those TO220 plastic transistors found in CB set PAs, I will leave the choice up to you as there are hundreds of them. ..
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Crossed from: Vacuum Tubes - Valves | Clicks: 13653 | Votes: 0 | Comments: 0 | Rating: 0 | Rank: 0
3.5-30 MHz Valve Transmitter
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This transmitter is very stable and will deliver up to seven watts of power with the components and tubes shown. Do not be put-off by valves as they are VERY easy to work with and it seems that there are quite a few valves around. Because no-one wants these obsolete things they may be bought for next to nothing at radio rally's. If you are not too particular what type of valve you want then there are lots of valves available. Above is the circuit of the transmitter including the coil winding information. The oscillator is a simple untuned Pierce oscillator which will oscillate at the fundamental frequency of the crystal. A 25pf variable capacitor between the grid of the first valve and earth will allow the final frequency to be "pulled" a little but I never used this facility...
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27MHz CW transmitter
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The original TX was designed to be only PULSE Modulated for a proportional R/C system, but I later used the same circuit to make an HF bands Amplitude Modulation (AM) transmitter using PA-Base Modulation. It eliminated the need for a modulation transformer; it can be driven with a simple Op-Amp amplifier. Frequency range of the original prototypes were as low as 1MHz and the top frequency was 40MHz. The circuit is the same, just the crystal and coils are changed. The CW output power is about 200mW at 29MHz. It is only 70mW Amplitude modulated, but modulation peaks can rise to 200mW with 80% modulation...
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CW Transmitter 5 Watt
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This is a very simple 5 watt CW TX based upon a TTL logic chip. There is just one "tricky" component and this is Cx. This component should have an impedance of about 10 - 50 ohms at the frequency of interest. If you wish to reduce the transmitter power, increase the value of Cx. It is Cx which causes the square wave from the output transistor to approximate a sine waveform. The value of Cx is the price of simplicity in this TX...
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30 Meters CW transmitter
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This transmitters' intended purpose is for morse-code only in the 30 meter band (10Mhz). It is a low-power QRP type and needs to be connected to your existing tranceiver. The harmonic rejections on the prototype were measured at 40dB on 20Mhz and 50dB on 30Mhz. The transmitter is build as a Colpitts Oscillator with a strong 2N2219(A) transistor. HF-output of the oscillator is 100 to 500 mW, depending on the supply voltage of 5 to 20 Volts. The transmit frequency is stabilized with the 10Mhz crystal. A slight detuning is possible by putting a 150pF trimmer capacitor between C2 and the xtal. ..
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Crossed from: Remote Control | Clicks: 9250 | Votes: 0 | Comments: 0 | Rating: 0 | Rank: 0
Tx/Rx Control
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This circuitry offers some features that are highly desirable when building a QRP (or ANY) transceiver. It also includes a combination of features that many might not always take the time to include in their projects. Considerable effort has been made to assure that in this design, the RIT (RX frequency offset control) voltage and the TX frequency offset control signals are immune to supply voltage variations. IC1 requires a regulated +6 V voltage for proper operation. This is achieved by using the highly regulated VFO power supply rail. The main power supply rail (+9 to +15 V unregulated) powers the relay coil and its transistor circuit. The separate power sources improves the stability...
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CW Transmitter
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This one-valve (ECL82) CW Transmitter occupies a very special place in my amateur radio history. I was first licensed in 1994 but did not operate on air. I committed to only ever operate equipment which I had made myself. I had no time for radio construction and several years passed. In early 2002 I came across a design by Jan Axing, SM5GNN and decided to build it. The transmitter worked first time and tuning was exactly as Jan had said it would be. At the same time, I built a solid state HF receiver using various new and unusual techniques such as a Tayloe switching mixer, Huff & Puff stabilised VFO and polyphase audio phase shift network for unwanted sideband cancellation. My first ever QSO was on 25'th March 2002. Over the next few years I made a number of modifications to the transmitter, and it remained my ONLY transmitter for over three and a half years! During this time I made over 550 QSO's, many of which were long ragchews lasting an hour or two. The little ECL82 valve has served well!..
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Crossed from: PLL Circuit | Clicks: 9157 | Votes: 0 | Comments: 0 | Rating: 0 | Rank: 0
FM PLL transmitter
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This new FM transmitter is very easy to make and doesn't need any RF tuning. First of all ,we have used an integrated VCO: The POS150 from Mini-circuits. This excellent RF circuit covers all the FM Band in a voltage range of 4V to 8V. The Kvco factor is very stable all over the FM band, consequently, we have applied the AF signal directly on the control voltage line coming from the PLL. A 4dB attenuator allows the VCO to drive the MSA1105 RF power stage without exceeding the 1dB gain compression. Finally,a low pass filter ensures a good harmonics rejection.(H3<-60dBc). This new design conducts to new RF performances like: Phase/noise equals to -103dBc/Hz at 10KHz from the carrier.. +17dBm output power level (50mW)...
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HF GENERATOR using a 68HCx11
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The GHF1 is a small generator covering the HF and LF band from 30 Hz to 30 MHz in 6 ranges. The sinusoidal signal obtained can be amplitude modulated (AM) or frequency (FM).
The GHF1 also has a sweep function.
An LCD display 2 x 16 characters allows the frequency, mode and selected for each, the modulation rate, the value of the trip sweep.
GHF1 claims to replace the generators of the past, (often called at that time "heterodyne") with significantly better performance. Unlike these old generators
which were type LC, switched coils and lines for variable capacitor for adjusting the frequency, GHF1 is of the type RC, which allows excellent MAX038 from MAXIM...
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Crossed from: Computer Interface | Clicks: 5295 | Votes: 0 | Comments: 0 | Rating: 0 | Rank: 0
Serial IR transmitter using PIC16F628
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This is a programmable infrared (remote control) transmitter, which can be controlled from a PC serial port. It is capable of sending many remote control formats, including the Philips RC-5 standard. The controller will accept commands on the serial port. Settings are: 19200 bps, 8 bits, no parity, 1 stopbit, no flow control (XON/XOFF or RTS/CTS). Commands consist of hex coded bytes and must be written on the port as ASCII characters separated by space, terminated by ENTER (ASCII char 0d). You can use a terminal emulator program to test out the circuit (for example minicom on linux, NC terminal on DOS, or hyperterminal on windows), but the settings usually won't work at first, so it is recommended that you write a small program to set the parameters and send commands by pressing keys on the keyboard...
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Crossed from: RF Amplifiers | Clicks: 7874 | Votes: 0 | Comments: 0 | Rating: 0 | Rank: 0
50MHz Linear Power Amplifier
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This page investigates a simple PA for 50MHz. This power amplifier will boost upp a low level signal about10-20mW
to over 100mW. The power consumption will yet be quit low. At the bottom of this page I will present a complete transmitter based on a crystal controlled VCO. This PA is based on a common cheap power transistorBFG97. I use this transistor in my previous transmitter projects. This PA work in class C and will deliver about 100mW. The current consumption is about 100-150mA at 9V. Why I have choosen 9V as power supply is because I will in most cases use my transmitter with a common 9V battery. If you use higher voltage you will increase the output power. I don't want to consume all to much energy from the battery, so I have settled with 100mW output power. ..
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Crossed from: FM Transmitter | Clicks: 1951 | Votes: 0 | Comments: 0 | Rating: 0 | Rank: 0
FM PLL controlled VCO
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The main oscillator is printed in blue and is voltage controlled.
In this construction the VCO range is 88 to 108 MHz. As you can see from the blue arrows, some energy goes to an amplifier and some energy goes to the PLL unit. You can also see that the PLL can control the frequency of the VCO. What the PLL do is that it compare the VCO frequency with the reference frequency (which is very stable) and then regulated the VCO voltage to lock the oscillator at desired frequency. The last part that will affect the VCO is the audio input. The amplitude of the audio will make the VCO change in frequnency FM (Frequency Modulation).
I will explain it all in detail under section Hardware and schematic. ..
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50MHz Transmitter alarmer with PIC16F84
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This project is about an IR-detector (Infra Red) combined with a 100mW 50MHz transmitter.
The detector is sensitive to changes in IR-spectrum and is commonly used in burglar alarms.
When the detector sens a person/animal/ghost/angry clown, it will transmitt a warning signal
to a portable receiver and you will get a pre-warning. This project is aimed for typical 007-persons who doesn't want to be surprised by other.
Maybe you have been in some situation where you wanted to know if there was any person
comming or leaving. I will not ask you why!
Maybe you sleep on the second floor and you want to have a pre-warning if any one should be at first floor or if you are out camping and you don't want any "blairwitch" to sneak up on you!
This project is very useful in many situation, I just want you to use this in good purpose.
The total project consist of a 50MHz portable transmitter with an IR-detector sensitive for motion, and a 50MHz portable receiver. In this page I will explain the transmitter part...
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