Discussion in 'Wireless & RF Design' started by microelectronix, Jan 3, 2009.

1. ### microelectronix Thread Starter Member

Jan 21, 2008
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Hello all,

How can you generate a high frequency signal using a DC power source (batteries)?

How is this signal generally amplified for small-scale transmitters, such as walkie-talkies? Is it using BJT's? Would the signal just be fed straight to the base?

How is this amplified signal attached to the antenna?

Links to simple schematics would be great, or partial pieces of information (such as how a waveform is generated using a DC source).

Thanks all!

Apr 5, 2008
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3. ### Wendy Moderator

Mar 24, 2008
20,766
2,536
Most of this is basic electronics, although the application can be anything but. Oscillators are the how, and all transmitters use DC. Matter of fact, the AC power has to be converted to DC for it to be remotely usable.

The other half of the equation is antennas. They are as much an electronic component as any transistor or LC circuit. In many cases they are more resonant than the LC circuits that are part of the radio.

4. ### RAH1379 Well-Known Member

Dec 13, 2005
69
1
How can you generate a high frequency signal using a DC power source (batteries)?

first learn how simple single stage amplifiers work, (common emitter,common base, and common collector) then learn about feedback(negative feedback is used in amplifiers while positive feedback is used to make an amplifier oscillate) an amplifier with positive feedback will oscillate at a frequency determined by the resonance of the feedback circuit,that will be passed on to amplifers which either amplify it or multiply the frequency(frequency multiplier stages are also amplifiers with harmonics which can be filtered at their output to use only the harmonic wanted,which can be further amplified and applied to the antenna.frequency multiplier stages are used when the desired radio frequency is higher than the oscillator used and can multiply the oscillators frequency by factors of 2x,3x, or 4x, and several stages of multiplication can be used to get up to the desired frequency of the transmitter.

5. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
What RAH Stated above is a good start. With the information above, you will have an amplified "carrier". Some form of modulation to transmit information over the radio link is needed to make it useful beyond morse code uses. Many modulation techniques (Amplitude, Frequency, etc) are integrated with the RF amplifier stages.

6. ### RAH1379 Well-Known Member

Dec 13, 2005
69
1
It will also help if you get a block diagram of a transmitter, receiver, or whatever system you are trying to learn, and learn the blocks or stages first, then what and how each one does what it does.The most complex systems can be broken down into many stages, which not only helps with understanding them but also helps if you have to troubleshoot problems with the system.

7. ### kakin New Member

Mar 24, 2009
9
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how can you generate a high frequency signal using dc: using an oscillator, most simple radio transmitters use a tank circuit (capacitor and inductor connected in parallel) to generate it, and a transistor to maintain it.

how is this signal amplified: Amplified with transistors (bjt, fet and others) or with opamp.

how is this signal applied to the antenna: connecting the antenna to the oscillator, most likely using a small capacitor to block any dc from interfering with the signal.

schematic with little how-it-works: simple fm transmitter

hope this info is useful :]