Write a note on low level modulation and high level modulation
Unauthorised use of any frequency is generally considered a serious offence, so I discourage anyone from setting up their own radio station for the fun of it. Modulated class C power amplifier Carrier oscillator The carrier oscillator generates the carrier signal, which lies in the RF range.
One thing that is important is to understand that simulators have limitations, and some may be incapable of resolving the end result without adding artefacts that are essentially the result of simulator resolution. This amplification is necessary to drive the audio power amplifier.
Frequency Multiplier The sub-multiple frequency of the carrier signal, generated by the carrier oscillatoris now applied to the frequency multiplier through the buffer amplifier.
An input resistor will generally be necessary to reduce the signal level to a few millivolts at most - an initial value of around 1k is suggested. This limitation isn't specific to the 'perfect' modulator though - it applies for all simulations that involve RF and audio.
Superheterodyne am transmitter block diagram
In this way the audio modulates the voltage supplied to the final amplifier, and the level of the output signal is modulated in line with the audio. There are many ways in which amplitude modulation can be generated and a variety of circuits that can be used. There is also a fixed offset due to the diode bias voltage. It has the advantage that linear amplifiers are not required for the RF amplification stages after AM modulation has been applied. A variety of circuits can be used to generate the amplitude variations needed. The linear amplifiers are used in order to avoid the waveform distortion in AM wave. If you can get them, germanium diodes are still a good choice. Could you build this and would it work? The circuit below shows a simple Schottky detector, with mV of forward bias applied to improve linearity. So if W total power is to be transmitted, the modulating power will be W. One way that a fairly good amplitude modulator can be simulated is by including a sub-circuit of a complete low distortion VCA voltage controlled amplifier , but this is a serious undertaking.
The matching between the two should be perfect at all transmitting frequencies. Thus, by increasing frequency deviations in the FM signal, the noise effect can he reduced.
Low level transmitter block diagram
The AF and RF levels need to be set carefully, using an oscilloscope and ideally a frequency analyser. One thing that is important is to understand that simulators have limitations, and some may be incapable of resolving the end result without adding artefacts that are essentially the result of simulator resolution. This type of detector was one of the very first ever used to detect RF, and although there were other, earlier, detectors they weren't linear and were often insensitive. The disadvantage is as discussed above - a 10kW transmitter needs a 5kW audio amplifier. The frequency multiplier generates higher harmonics of carrier oscillator frequency. Nevertheless many transmitters still require AM modulators and the concepts and theory behind them is still relevant. However the principles are the same for both valve and semiconductor technologies. This is shown in below Figure c. Unfortunately, there does not appear to be an equivalently simple method to produce FM frequency modulation , but many simulators include that facility for 'advanced' signal sources.
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