U4223B Datasheet, PDF (5/18 Pages) ATMEL Corporation – Time-Code Receiver with A/D Converter

U4223B
V_clk
mV
100
50
0
4
7 8 11 12 t/ms
Now, the time-code
signal can be read
Falling edge initiates
time-code conversion
Now, the AGC value can be read
Rising edge initiates
AGC signal conversion
Thus, the first step in designing the antenna circuit is to
measure the bandwidth. Figure 17 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 fre-
quency of the signal generator, the resonant frequency
can be determined.
RF signal
generator
77.5 kHz
Scope
Figure 14.
In order to minimize interferences, we recommend a
voltage swing of about 100 mV. A full supply-voltage
swing is possible but reduces the sensitivity.
VCC
CLK
GND
Figure 15.
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 kW to 200 kW. This can be achieved
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.
wire loop
C_res
Probe
10 : 1
w10 MW
Figure 16.
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_A of the antenna circuit. As the
value of the capacitor C_res in the antenna circuit is known,
it is easy to compute the resonant resistance according to
the following formula:
R_res + ₂
1
p BW_A C_res
where
R_res is the resonant resistance,
BW_A is the measured bandwidth (in Hz)
C_res is the value of the capacitor in the antenna circuit
(in Farad).
If high inductance values and low capacitor values are
used, the additional parasitic capacitances of the coil
(v20 pF) must be considered. The Q value of the capa-
citor 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.
Rev. A7, 06-Mar-01
5 (18)