Field of the Invention
The invention relates to a circuit for automatic frequency control (AFC).
In many devices it is necessary to perform an automatic frequency control. The relevant field of use is the electronic tuning of the frequency of oscillator circuits. In this electronic tuning, for example, so-called capacitance diodes are used. In operation, capacitance diodes need a direct voltage of 10 volts to 30 volts which must be constant to a few millivolts. In the case of receivers with battery supply, they can therefore only be used in conjunction with a DC--DC converter. The temperature-dependence of the junction capacitance is a great disadvantage of the capacitance diodes. A diode having the same temperature coefficient is therefore often connected as a temperature-dependent resistor into the feed line of the supply voltage.
In the case of large RF voltages, the curved characteristic of the capacitance diode produces a shift of the mean value of the direct voltage, and thus a change in capacitance. This can lead to distortions. These changes in capacitance can be compensated for through the use of two equal back-to-back connected diodes. The voltage dependence and temperature dependence of the capacitance diodes is effective mainly at high frequencies. Even small changes in capacitance lead to considerable frequency changes at these frequencies. The consequence is a distorted reception of the transmitter tuned in. This is why an automatic frequency control is required, for example, in the case of a diode tuning. Here, the controlled variable is the frequency of the oscillator. Using the automatic frequency control (AFC), the tuning is thus kept stable by controlling the oscillator frequency.
A customary AFT circuit is shown in FIG. 9. In this case, a temperature-compensated voltage-controlled oscillator (VCO) delivers a signal having a particular clock rate to a frequency comparison device 3. This frequency comparison device 3 compares the clock frequency of the temperature-compensated voltage-controlled oscillator (VCO) 9 with a system clock frequency f.sub.ref which is obtained, for example, from a time standard information. The frequency comparison device 3 outputs a digital tracking signal which reproduces or represents the frequency difference between the clock frequency of the VCO 9 and the system clock frequency f.sub.ref, wherein the frequency difference is determined in the frequency comparison device 3. This digital tracking signal is supplied to a digital/analog (D/A) converter 10 which converts it into an analog tracking signal. This analog tracking signal is supplied to the temperature-compensated VCO 9 for controlling its oscillator frequency, which closes the control loop.
As can be seen from FIG. 9, the control loop is thus implemented in an analog manner. This analog implementation, however, results in a number of disadvantages. For example, interference is easily coupled in. There are also problems with respect to the dynamic range needed. This plays a significant role especially in the case of devices in which reduced supply voltages are present such as is the case, for example, in mobile telephones. A further disadvantage is that the constructional expenditure with respect to the D/A converter 10 for tracking the analog tuning voltage is very high since the accuracy of the digital/analog converter 10 is decisive for the precision of the control. Further disadvantages also to be mentioned are that the AFC circuit constructed as shown in FIG. 9 responds only slowly and can not be easily integrated into an integrated circuit due to the analog tracking.
Furthermore, the document Patent Abstracts of Japan, Vol. 15, No. 116 (P-1182), Mar. 20, 1991 and the Japanese patent publication JP 03005966 A (Matsushita Electric Ind Co Ltd) disclose a circuit for an automatic frequency control which permits a simple and precise tracking of an oscillator frequency.