8/6/2019 Technique of Work

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8/6/2019 Technique of Work

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LCD displays important information about jamming; such as jammed carrier and jamming level, and its options and characteristics are controlled by mentioned PIC.

To change level of jamming and jammed carrier, there are controlling buttonsconnected to PIC 16F877A, some of these buttons are for level of jamming and theothers are for jammed carrier. To change level, we change power of the generatedsignal in the PIC so we could control the jammed area. More ever, to change jammedcarrier we can switch off one generated signal while the other one is on, there are twogenerated signals, so one band will be jammed and other one will not, both bands will

be jammed, or both will not.

Here, the importance of PIC 16F877A is clear in both illustration and control,and success of this PIC is controlled by the code written, so due to its importance, acomplete chapter is written to present the code.

For the operations on the signal, the story begins inside PIC 16F877A,namely, in PORT C, who has a capability of generating PWM (Pulse WithModulation). This PWM vibrate in magnitude between 0 ± 5 volts, and its frequencyis 500 Hz. Thus, we generate the signal which we want, and we can change itscharacteristics to satisfy our requirements.

RF stage is one in which we will modulate the generated signal to the wantedfrequency. It is the most important stage, since its output will interface with thecarriers and thus mobiles will be jammed. This stage consists of: VCO, power amplifiers and antennas,

VCO changes frequency of its output depending on the voltage of its input in a proportional relationship, so by changing its input voltage we can change its outputfrequency. It could be considered as the heart of RF stage, since it performs the major function in RF of modulation. With DC in input, the output of VCO will be aspecified frequency, so in order to cover each of two bands on which carriers areexisting input should not be DC, it could be a triangular waveform or any suitable ACone, which will span a specific frequency range.

The VCOs should cover the bands we need (GSM 925-970 MHz and DCS1785-1900), available in low cost and run at low power consumption. We chose onesof type CVCO55BE whose characteristics satisfies our conditions.

But unfortunately, output of the chosen type of VCO which has a power of 5dBm doesn't satisfy our needs, so we added two power amplifiers to increase its gainto a level enough to be transmitted and reaches needed distances. Because whensignal is transmitted it will face attenuation and noise from many sources, so it shouldhave enough gain to resist against these damping factors in order to reach to distancewhich we want. The gain of our power amplifier is 34 dBm which is suitable andenough to our needs.

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In transmitting, suitable antenna is necessary component, antenna's systemmust be matched to the transmission system so as to have optimal power transfer. Inour project, we used two 1/4 wavelength monopole antennas, with 50 inputimpedance so that the antennas are matched to the system. We used monopoleantenna since the radiation pattern is omni-directional.