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Shorter Charge-Up (TX) Pulses for Speed Applications
This section is intended to provide information and examples regarding the two most important system
constraints (transponder charge-up level and field strength), so that if any of the parameters in your
system have to be changed, you can work it out using these examples as a basis.
5.2
Shorter Charge-Up (TX) Pulses for Speed Applications
The values in
Table 2-1
regulations, the antenna voltage can be higher for shorter TX pulses. Use
maximum allowed antenna voltage (this level is the temperature limit of the antennas). With
you can calculate the antenna voltage for shorter TX pulses (only valid for countries with RMS FCC
regulations); this voltage must always be lower than that calculated with
There are two constraints:
1. V
= 5 V
cl
2. H
= constant
rms
Constraint 1:
For TX = 50 ms:
= 5 = A × (1 – e
V
cl
where:
R = 302777 Ω
C = 120 nF
A = 6.689
For TX = TX
b
Where
= time chosen
b
TX/RxC
A
= 5/(1 – e
b
With a shorter TX pulse you get a higher coefficient 'Ab'. This means that the peak value of the magnetic
field strength must be Ab/A times higher than in the case of 50 ms charge-up time, to get the same
charge-up voltage.
A = the calculated value for 50 ms.
Constraint 2:
H
is the magnetic field for 50 ms charge-up time:
50ms,peak
For a different TX you have the magnetic field strength multiplication:
To find the correct charge-up time, TX must conform to
is not possible. When you choose a shorter TX pulse, the magnetic field strength can become higher as
shown in
Equation
24
Speed Applications (Detecting Moving Transponders)
are only valid in TX = 50 ms and RX = 40 ms. For countries with RMS and CISPR
TX/RxC
)
)
5-2, but never high enough to charge the transponder to 5 V as in the case with 50 ms.
Equation 4-1
Equation
Equation 5-1
and
Equation
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to calculate the
Equation 4-2
4-1:
(5-1)
(5-2)
5-2. Unfortunately this
SCBU025 – November 1996
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