| The project illustrates the construction and design of a wave bubble that contains a wide bandwidth and self-tuning portable RF jammer. |
|
| The objective of the project is to possibly control an inexpensive R/C car using the technology of an iPhone. |
|
| The circuit was built to read and decode the RFID tags with 125 KHz frequency using a single low cost ATtiny2313 ATMEL microcontroller. |
|
|
In this article, we point out some important facts in designing RF application PCB boards. Of course there are many other facts that are not listed below but for the beginners these may be enough for now.
First of all, separate the analog , RF and digital parts of your system and try to design the PCB without mixing them. Likewise separate the RF stages like VCO, amplifier and etc. and don't draw one's line through one another's. 1) Surely use a multilayer PCB. If your PCB design includes only two layer, the top layer should include the power stage, RF signal lines and RF components. Then the bottom layer must be the ground plane.
2) The length of the lines that carry RF/Microwave signals is a very important issue. They should be at most 1/20 length of the wavelength. So there will be no loss. For instance, when we calculate for 433 MHz;
λ (Wavelength) = c (The Speed of Light) / f (Frequency)
λ = 300000000/433000000 = 69,28 cm
Max. Line Length : λ/20 = 3,46 cm If the line must be necessarily longer, then impedance matching with L and C components must be applied at the end of the line.
3) If you use multilayer PCB, draw the short RF lines on the top layer. To reduce the noise, draw the power lines between two ground layers. There must be absolutely a ground layer under the layer that includes the RF signal lines.
4) Draw the RF signal lines quite separately. If they are adjacent to eachother, then crosstalk may occur. (Crosstalk : Undesired transfer of signals between or among two lines such as telephone lines, data lines, or system components. )... |
|
 This is a mobile phone sniffer circuit that can detect the signals being used in the GSM (Global System for Mobile Communication) band at about 900 MHz. Since the signals are digitally encoded, it can detect only the signal activity, not the speech or the message contents. A headphone is used to hear the detected signals. The circuit schematic is given in the .rar archive attachment. There are two separate detector units. Every detector unit consists of a dipole antenna, a choke and a diode. The antenna receives the GSM signals in media. Then a small amount of charge is induced in the choke. The diode demodulates the signal and finishes detecting. The diodes must be schottky diodes or germanium diodes. Since the forward voltage of a silisium diode is high, it won't give a sufficient result in this circuit. LM358 amplifies the received signal. It contains two separate op-amps that are supplied by a common power source. R3 and R7 resistors determine the gain of the amplifiers. When the resistor values are greater than 10M then the noise level increases. If they are small like about 100k, this time it becomes harder to hear the signal. |
|
 Description This is a high output oscillator set at 1Ghz. Designed out of frustration of not having a matched PNP transistor. The circuit will deliver 3v RF with the components shown.
If you were to use two 100mW rated transistors, you would obtain around 300mW out.
Tuned circuit is L1,L2,C2,C3. Values chosen for 1Ghz for test purposes.
Two 2N4427’s will deliver nearly 3 Watts out at VHF frequencies.
Good Heat Sinking should be used. L1+L2 can be moulded chokes.
Note: Q1+Q2 Must be ‘Well Matched’ to prevent either transistor from doing most of the work. Designer & Author: Special thanks to Laszlo Kirschner. |
|
|
|
Page 10 of 12 |
