A method and circuitry for automatic control, according to the counter principle, of the frequency of a radio telephone.

0001The present invention relates to a method for automatic control, according to the counter principle and on the basis of the reception frequency, of the frequency of a radio telephone, in which method the reception frequency and the output frequency of a phase-locked loop are mixed in a first mixer, the output frequency and the frequency obtained from a local oscillator are mixed in a second mixer, and the frequency of the phase-locked loop is con­trolled on the basis of the frequency deviation of the output frequency of the second mixer. The invention also relates to a circuitry for the carrying out of this method.

0002Previously, analog methods have been used in which there is formed a control voltage, proportional to the frequency, by means of which the frequency of the local oscillator is controlled. There have also been implemented methods resem­bling the object of the present invention, operating ac­cording to the counter principle; in these, a fixed refer­ence oscillator is used, for example a crystal oscillator of a processor, as is disclosed in, for example, patent application FI 864 909.

0003The disadvantages of the analog methods include the tem­perature creep appearing in them, aging, and the need for calibration, i.e. tuning of the circuitry, at the manufac­turing stage. In known apparatus using a fixed reference oscillator and operating according to the counter princi­ple, at least two separate oscillators are needed, of which the reference oscillator must be very precise and stable. This is costly and requires complicated circuitry.

0004The object of the present invention is to eliminate these problems and to provide an AFC circuitry functioning ac­cording to the counter principle, a circuitry which does not require a precise and stable reference oscillator but in which the frequency of the one and only oscillator is controlled according to the reception frequency on the basis of the result of calculation. To achieve this, the method according to the invention is characterized in that the local oscillator is an oscillator having a controllable frequency, and that this oscillator is controlled on the basis of the said frequency deviation, the reference fre­quency of the phase-locked loop being derived from the frequency of the local oscillator. The characteristics of a circuitry according to the invention are respectively given in Claim 4.

0005Owing to the invention, a stable high-precision reference oscillator is not needed; the basic frequency of the entire telephone can be locked to the reception frequency. Such a digital AFC, operating according to the counter principle, is less sensitive to the aging phenomena than is the tradi­tional analog circuitry based on voltage levels, and it does not require manual calibration at the manufacturing stage.

0006According to a preferred embodiment of the invention, the frequency deviation of one of the mixers is determined by counting the pulses within a certain time interval, the said time interval being determined by counting a certain number of pulses of the local oscillator. When the counting result corresponding to the correct frequency is known, the control voltage of the crystal oscillator is adjusted in the correct direction by program control, on the basis of a calculation result obtained on the basis of some algorithm or measuring results. In this manner the reference fre­quency changes and the measuring procedure is repeated until the desired frequency precision has been obtained. The invention and its other characteristics and advantages are described below in greater detail in the form of an example and with reference to the accompanying drawings, in which <ul id="ul0001" list-style="none"><li>Figure 1 depicts diagrammatically a circuitry according to the invention, and</li><li>Figure 2 depicts in greater detail the AFC block 3 of Fig­ure 1.</li></ul>

0007In Figure 1, reference numerals 1 and 2 indicate two mixers. Numeral 3 indicates the actual AFC circuit which forms the calculation result for the control circuit 4, which for its part controls a crystal oscillator 5 having a controllable frequency. From the crystal oscillator 5 the frequency f to be controlled is fed on the one hand to the mixer 2 and on the other hand to a dividing means 6 (divid­ing number N) as a reference frequency fr for a phase-­locked loop 7. The output frequency k x f of the phase-­locked loop is applied to the mixer 1 in order to be mixed with the incoming reception signal fin.

0008The frequency obtained from the crystal oscillator 5 is further applied via a divider means 8 (dividing number L) to the AFC circuit 3 for the calculation of the predeter­mined time interval, as explained below.

0009The operation of the circuitry is described below with reference to both Figure 1 and Figure 2.

0010A predetermined number of pulses of the crystal oscillator 5 are counted by means of a counter 11 belonging to the AFC circuit, and thereby a certain time interval, or time con­stant, T is determined. The number of pulses of the second intermediate frequency if2 included in the said time inter­val T is counted by means of a counter 12, and according to a certain algorithm a calculation result S is determined, which is applied to the control circuit 4. The control circuit 4 forms, on the basis of the calculation result, a control voltage U for the crystal oscillator 5.

0011The input frequencies 1/L x f and if2 of the counters 11 and 12 are coupled to the counters via switches 9 and 10, which are controlled by a microprocessor 13. The micro­processor also controls the zeroing of the counters 11 and 12.

0012The operation of the circuitry is based on the fact that the frequency deviation of the crystal oscillator 5 is seen directly in the counter 11 (regardless of the divider means 8 preceding the counter), but in the intermediate-frequency signal the deviation is multiplied, if the injection fre­quency k x f is greater than the frequency of the oscil­lator 5. In the case presented, the injection frequency is k x f = M/N x f.

0013In the mixer 1 the absolute value of the deviation remains constant, and so as an absolute value the frequency devia­tion of the first intermediate-frequency signal if1 is equal to the deviation of the injection frequency k x f.

0014In the second mixer 2 the total deviation either increases or decreases, depending on the injection method used in the mixers (injection from above or from below), and on whether the frequency of the oscillator 5 has been divided or mul­tiplied for the second mixing. In the case presented the frequency of the crystal oscillator 5 is applied directly to the second mixer 2.

0015In addition to what has been presented above, the relative proportion of the deviation in the intermediate-frequency if2 is considerably greater than in the injection frequen­ cy, because the intermediate frequency is much lower than the injection frequency (order of magnitude 2,000 - 5,000).

0016In the examples of the figures, the deviation of the oscil­lator is seen approximately 20-fold in the intermediate frequency if2. Thus the effect of the frequency deviation of the oscillator 5 on the time constant T is infinitesi­mal, and the control loop converges greatly on successive counting and control rounds.

0017In the circuitry presented, the reference oscillator 5 does not require separate manual control because of possible frequency changes due to aging; the correction can be car­ried out by changing the basic value of the control voltage U of the crystal oscillator, if the AFC correction is re­peatedly in the same direction, for example 100 times in succession.

0018An additional advantage of the circuitry presented is its ease of integration. In addition, it is possible to use only one crystal oscillator, which gives the basic fre­quency to all the components. The usability of the circuit­ry, of course, depends on the implementation of the radio component and on the frequencies used in it.

0019The counting result of the intermediate-frequency counter can be transferred to the processor in either a serial or a parallel form. It is, of course, possible to add to the circuitry, as options, various counting periods, which are program-selected.