U4221B Datasheet, PDF (4/12 Pages) TEMIC Semiconductors – Radio Controlled Clock Receiver

U4221B
There is some consideration concerning the calculation of
R_res:
in order to achieve high signal voltage:
R_res should be high
in order to achieve low antenna noise voltage:
R_res should be low
R_res < 200 kW:
the input noise voltage of A 1 dominates
R_res > 300 kW:
the antenna noise voltage dominates
That means the resonant resistance should be between
200 kW and 300 kW.
Q of antenna must be high for attenuation of interfering
signals. But the temperature must not influence the
resonance frequency.
Design Hints for the Ferrite Antenna
The bar antenna is the most critical device of the complete
clock receiver. But by observing some basic rf design
knowledge, no problem should arise with this part. The IC
requires a resonance resistance of 200 kW to 300 kW. This
can be achieved by a variation of the L/C-relation in the
antenna circuit. But it is not easy to measure such high
resistances in the RF region. It is much more convenient
to distinguish the bandwidth of the antenna circuit and
afterwards to calculate the resonance resistance.
Thus the first step in designing the antenna circuit is to
measure the bandwidth. Figure 4 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 resonance
frequency can be determined.
RF - Signal
generator
77.5 kHz
Scope
wire loop
C_res
Probe
w10 : 1
10 MW
94 7907 e
Afterwards, the two frequencies where the voltage of the
rf signal at the probe drops 3 dB down can 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 well known,
it is easy to compute the resonance resistance according
to the following formula:
+ _{@ @ @} R_res
2
1
p BW_A C_res
whereas
R_res is the resonance 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
must be considered. It may reach up to about 20 pF. The
Q-value of the capacitor should be no problem if a high
Q-type is used. The Q-value of the coil is more or less dis-
tinguished by the simple DC-resistance of the wire. Skin
effects can be observed but do not dominate.
Therefore it should be no problem to achieve the
recommended values of resonance resistance. The use of
thicker wire increases Q and accordingly reduces
bandwidth. This is advantageous in order to improve
reception in noisy areas. On the other hand, temperature
compensation of the resonance frequency might become
a problem if the bandwidth of the antenna circuit is low
compared to the temperature variation of the resonance
frequency. Of course, Q can also be reduced by a parallel
resistor.
Temperature compensation of the resonance frequency is
a must if the clock is used at different temperatures.
Please ask your dealer of bar antenna material and of
capacitors for specified values of 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 Q
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 capac-
itor C_res as close as possible to the 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.
For the adjustment of the resonance frequency the
capacitance of the probe and the input capacitance of the
IC are to be taken into account. The alignment should be
done in the final environment. The bandwidth is so low
that metal parts close to the antenna influence the
resonance frequency. The adjustment can be done by
pushing the coil along the bar antenna.
4 (12)
TELEFUNKEN Semiconductors
Preliminary Information
Rev. A1, 15-May-96