Texas Instruments 2000 LF Series, Примечание к применению

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Lit Number: 11-06-21-068
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Which is why the MicroReader, which was designed for applications such as vehicle
immobilizer systems (where the antenna is around the lock barrel) and hotel door
locks, requires low Q antennas.
4.2.1 Determing the Antenna’s Q Value
4.2.1.1 By Measurement
Q values are normally measured using a signal generator and a spectrum analyzer. A
signal is fed into the antenna circuit and the peak amplitude of the resulting output is
detected by a spectrum analyzer.[ƒ
0
] (this should be around 134.2 kHz). The
frequency of the input signal is then raised until a 3 dB drop in the amplitude of the
spectrum analyzer signal is detected [ƒ
2
]. The frequency of the input signal is then
reduced until the signal is at the -3 dB point on the low side [ƒ
1
]. Then using formula
[2] the Q can be determined
Figure 14. Spectrum Analyzer Screen
Q =
ƒ
0
[2]
ƒ
1
- ƒ
2
4.2.1.2 By Calculation
This is method depends on the accurate measure of the series resistance of the
antenna but even when read by an LCR meter will give an adequate approximate
value.
Q =
2
π
ƒL
R
[3]
-3dB
ƒ
0
ƒ
1
ƒ
2