CME6005 Datasheet, PDF (5/16 Pages) List of Unclassifed Manufacturers – Single and dual band receiver IC

Preliminary Data Sheet CME6005 C-MAX
7 Design Hints for the Ferrite Antenna
7.1 Dimensioning of antenna circuit for different clock/watch applications
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 resonant resistance of 40 kΩ to 100 kΩ. This can
be achieved by a variation of the L/C-relation in the antenna circuit. In order to achieve this resonant
resistance, we recommend to use antenna capacitors of a value between 2.2nF and 6.8nF. The optimum
value of the capacitor has to be specified respecting the concrete application needs and different boundary
conditions(ferrite material, type of antenna wire, available space for antenna coil).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 calculate the resonant resistance afterwards.
Thus, the first step in designing the antenna circuit is to measure the bandwidth. Figure 12 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 time signal
generator, the resonant frequency can be determined.
Time signal
generator
Scope
Probe
10:1
Wire loop
C_res
Figure 12.
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:
1
R_res=2 _{x π X BWA X Cres}
Where
R_res is the resonant resistance,
BW_A is the measured bandwidth
C_res is the value of the capacitor in the antenna circuit (Farad).
If high inductance values and low capacitor values are used, the additional parasitic capacitance of the coil
must be considered. The Q value of the capacitor 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.
SPEC No.
CME6005
Revision
A.7
State
07.12.04
C-MAX printed
07.12.2004
Version
English
Page
5 of 16