t4225b ATMEL Corporation, t4225b Datasheet - Page 5

• Manufacturer Part Number: t4225b
• Description: Low-cost Time-code Receiver
• Manufacturer: ATMEL Corporation

Rev. A3, 03-Dec-98
Please note:
The signals and voltages at the Pins REC, INT, FLA,
FLB, Q1A, Q1B, Q2A and Q2B cannot be measured by
standard measurement equipment due to very high inter-
nal impedances. For the same reason, the PCB should be
protected against surface humidity.
Design Hints for the Ferrite Antenna
The bar antenna is a very critical device of the complete
clock receiver. Observing some basic RF design rules
helps to avoid possible problems. The IC requires a reso-
nant resistance of 50 k
by a variation of the L/C-relation in the antenna circuit.
It is not easy to measure such high resistances in the RF
region. A more convenient way is to distinguish between
the different bandwidths of the antenna circuit and to cal-
culate the resonant resistance afterwards.
Thus, the first step in designing the antenna circuit is to
measure the bandwidth. Figure 16 shows an example for
the test circuit. The RF signal is coupled into the bar
antenna by inductive means, e.g., a wire loop. It can be
measured by a simple oscilloscope using the 10:1 probe.
The input capacitance of the probe, typically about 10 pF,
should be taken into consideration. By varying the
frequency of the signal generator, the resonant frequency
can be determined.
At the point where the voltage of the RF signal at the
probe drops by 3 dB, the two frequencies can then be
measured. The difference between these two frequencies
is called the bandwidth BW
value of the capacitor C
it is easy to compute the resonant resistance according to
the following formula:
R
where
R
BW
C
(in Farad).
res
res
res
A
wire loop
+
is the resonant resistance,
is the value of the capacitor in the antenna circuit
is the measured bandwidth (in Hz)
2
RF signal
77.5 kHz
generator
p
1
BW
C
res
W
A
Figure 15.
res
to 200 k
in the antenna circuit is known,
A
C
of the antenna circuit. As the
res
w
W
10 M
Probe
10 : 1
. This can be achieved
W
Scope
94 7907 e
If high inductance values and low capacitor values are
used, the additional parasitic capacitances of the coil
(
tor should be no problem if a high Q type is used. The
Q value of the coil differs more or less from the DC
resistance of the wire. Skin effects can be observed but do
not dominate.
Therefore, it should not be a problem to achieve the
recommended values of the resonant resistance. The use
of thicker wire increases the Q value and accordingly
reduces bandwidth. This is advantageous in order to
improve reception in noisy areas. On the other hand,
temperature compensation of the resonant frequency
might become a problem if the bandwidth of the antenna
circuit is low compared to the temperature variation of the
resonant frequency. Of course, the Q value can also be
reduced by a parallel resistor.
Temperature compensation of the resonant frequency is
a must if the clock is used at different temperatures.
Please ask your supplier of bar antenna material and of
capacitors for specified values of the temperature
coefficient.
Furthermore, some critical parasitics have to be
considered. These are shortened loops (e.g., in the ground
line of the PCB board) close to the antenna and undesired
loops in the antenna circuit. Shortened loops decrease the
Q value of the circuit. They have the same effect like
conducting plates close to the antenna. To avoid
undesired loops in the antenna circuit, it is recommended
to mount the capacitor C
antenna coil or to use a twisted wire for the antenna-coil
connection. This twisted line is also necessary to reduce
feedback of noise from the microprocessor to the IC
input. Long connection lines must be shielded.
A final adjustment of the time-code receiver can be
carried out by pushing the coil along the bar antenna. The
maximum of the integrator output voltage V
INT indicates the resonant point. But attention: The load
current should not exceed 1 nA, that means an input
resistance
Therefore, a special DVM or an isolation amplifier is
necessary.
v
20 pF) must be considered. The Q value of the capaci-
w
1 G
W
of the measuring device is required.
res
as close as possible to the
T4225B
INT
5 (16)
at Pin