U2270B Datasheet, PDF (5/13 Pages) TEMIC Semiconductors – Read / Write Base Station IC

U2270B
Oscillator (Osc)
The frequency of the on-chip oscillator is controlled by a
current fed into the R_F input. An integrated compensation
circuit ensures a widly temperature and supply voltage in-
dependent frequency which is selected by a fixed resistor
W ^{between RF (pin 15) and VS (pin 14). For 125 kHz a resis-}
tor value of 110 k is defined. For other frequencies, use
the following formula:
+ W R_f
14375
f₀ [kHz]
–
5
k
This input can be used to adjust the frequency close to the
resonance of the antenna. For more details refer to the ap-
plicatons and the application note ANT019.
V_CC
2 kW
R_f
R_F
9695
Figure 8. Equivalent circuit of Pin R_F
Filter (LPF)
The fully-integrated low-pass filter (4th order butter-
worth) removes the remaining carrier signal and
high-frequency disturbancies after demodulation. The
upper cut-off frequency of the LPF depends on the se-
lected oscillator frequency. The typ. value is fosc/18. That
means that data rates up to fosc/25 are possible if Bi-phase
or Manchester encoding is used.
A high-pass characteristic results from the capacitive
coupling at the input Pin 4, as shown in figure 9. The input
voltage swing is limited to 2 V_pp. For frequency response
W ^{calculation, the impedances of the signal source and LPF}
input (typ. 220 k ) have to be considered. The recom-
mended values of the input capacitor for selected data
rates are shown in the chapter “Applications”.
Note:
After switching on the carrier, the dc voltage of
the coupling capacitor changes rapidly. When
the antenna voltage is stable, the LPF needs
approximately 2 ms to recover full sensitivity.
V_Bias + 0.4 V
R_S
10 kW
~~
C_IN
210 kW
V_Bias – 0.4 V
V_Bias
12601
Figure 9. Equivalent circuit of Pin Input
Amplifier (AMP)
The differential amplifier has a fixed gain, typically 30.
The HIPASS pin is used for dc decoupling. The lower
cut–off frequency of the decoupling circuit can be
calculated as follows:
+ _p f_cut
2
1
C_HP R_i
W ^{The value of the internal resistor Ri can be assumed to be}
2.5 k .
Recommended values of C_HP for selected data rates can
be found in the chapter “Applications”.
R
+
–
R
LPF
V_Ref
Schmitt
trigger
R
R
R_i
HIPASS
C_HP
12578
Figure 10. Equivalent circuit of pin HIPASS
TELEFUNKEN Semiconductors
Rev. A3, 13-Dec-96
5 (13)