new babies to RFID design want to ask some question

Luo

What frequency range does the directional coupler have to work in. The reason that I ask is that, at microwave frequencies, one can print a pattern on a PC board to implement a direcvtional coupler. Also one can buy MMIC and hybrid coupler components.

Another way to implement a -3db. dircetional coupler is to use a quarter-wavelength of Sage wireline.

There are several books and computer programs that can help you determine the printed circuit line widths and spacings. I haven't had to design one for several decades now, so things may have changed a little.

Andy replies:

Backscatter Reflection --

The incident power hits a modulated reflector. The reflector is varied between a state of high reflection and low reflection at a data rate, often using FSK. The TIRIS system uses 600 Khz and 1200 Khz for example...... The receiver picks up the incident signal as well as the reflected signal. The reflected signal contains sidebands equal to the modulation rate, while the incident signal does not. Typically the receiver is a homodyne receiver, using I and Q detectors,and the output is the original data modulating signal which was applied to the modulated reflector . The data is then processed...

RANGE- For 30 cm, the powere transmitted to the backscatter reflector can be in the milliwatt region. 50 mw should be plenty. This can be calculated using the reflection characterics of the reflector and the RADAR equation. Refer to the article " Data Communications by means of Reflective Transmission" which appeared in Microwaves and Wireless, by C A Sharpe ---- sometime around 1994 or so.... The equations are given there, as well as an explanation of the TIRIS system and the Titile 21 requirements for California toll tags. The power required depends on the change in RCS (radar cross section) of the modulated reflector ( the TAG). With an RCS change of

ANTENNA - Typically a patch arrangement is used at this frequency range. The TIRIS system originally used an array of 4 patches in a line to give the necessary beamwidths. It was manufactured by SEAVEY ENGINEERING in Massachusetts. I am not sure if the antenna presently used has changed, but it was a good, robust design....

DIRECTIONAL COUPLER --- A circulator would be a good choice, but they are expensive. A microstrip or stripline directional coupler is a suitable alternative. They are inexpensive, but not as sensitive. Since your distance is so small, you can afford to lose the sensitivity. If you tap the microstrip at quarter wavelengths with a diode, you can do the I and Q on a single coupler. It is not efficient, but is very cheap, and works great at small distances.

Good luck with this. I have my doubts that a software designer has

sufficient background to create and efficient design for the RF circuitry involved tho. One needs a thorough and wide background to make the approximations and shortcuts needed to create a cheap and dirty transceiver, in my opinnion. Perhaps you can copy an existing desigh. If you can get a schematic of

the TIRIS programmer for the TAG, you would have a good starting point, but you need to get hold of one to see how it is mechanically laid out. At microwave, the layout is much more important than the schematic.....

C A Sharpe (Andy)

Retired Systems Engineer and inventor of the TIRIS 915 tag system for Texas Instruments.

Good point!