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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. )...
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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 the medium. 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.
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