AN OSCILLATOR UNIT WITH IMPROVED FREQUENCY STABILITY

Abstract: The invention relates to an oscillator unit with improved frequency stability, comprising at least three oscillators (1, 2, 3) having the same nominal frequency; and means (14, 15) for selecting one of the oscillators (1, 2, 3) for connection to an output (14a) of the oscillator unit. The oscillator unit further comprises means (4 to 12) for comparing the oscillator frequencies in pairs with each other and for controlling the selection means (14, 15) to replace the oscillator connected to the output of the oscillator unit with another oscillator if the frequency of the oscillator currently connected to said output deviates from the frequencies of the other oscillators at least by a predetermined frequency difference.

Description: An oscillator unit with improved frequency stability The invention relates to an oscillator unit with improved frequency stability. The specifications of the new European digital mobile radio system GSM (Groupe Special Mobile) define that the nominal frequency of the oscillator unit of the base station, that is, the basic fre¬ quency from which the clock and synchronization signals of the base station are derived, must not deviate more than 0.6 Hz from the specified value. Therefore there is a need for an oscillator unit of adequate frequency stability. The object of the invention is to provide an oscillator arrangement with reliable frequency sta¬ bility. This is achieved by means of an oscillator unit according to the invention, which is characterized in that it comprises at least three oscillators having the same nominal frequency; means for selecting one of the oscillators for connection to an output of the oscillator unit; means for comparing the oscillator frequencies in pairs with each other and for controlling the selecting means to replace the oscillator connected to the output of the oscillator unit with another oscillator if the frequency of the oscillator cur¬ rently connected to said output deviates from the frequencies of the other oscillators at least by a predetermined frequency difference. The invention utilizes at least three oscil¬ lators of the same nominal frequency, and one of the oscillators is connected to generate the output fre- quency of the oscillator unit, and the oscillator frequencies are compared in pairs with each other, and it is thereby possible to identify an individual oscillator the frequency of which starts to drift away from the frequency of the other oscillators. If such an unstable oscillator is currently connected to the output of the oscillator unit, it can be replaced with a more stable stand-by oscillator if the devi¬ ation of its output frequency from the output fre¬ quency of the other oscillators exceeds a pre- determined frequency difference. At the same time an alarm can be given to indicate the failure of the oscillator. In this way an oscillator unit of adequate reliability and frequency stability is achieved according to the invention. The invention will now be described in greater detail by means of an illustrating embodiment with reference to the attached figure, which shows a block diagram of an oscillator unit according to the inven¬ tion. The oscillator unit according to the invention comprises at least three oscillators 1, 2 and 3 having the same nominal frequency and comprising out¬ puts la, 2a, 3a, respectively, of which one is switched to an output 14a of the entire oscillator unit by a switching unit 14. The switching unit 14 selects the oscillator to be connected to the output 14a in response to a selection signal 13a from a selection unit 13. The selection unit 13, in turn, causes the switching unit 14 to change the oscillator either on receiving information from a special oscil¬ lator monitoring circuitry about a change in the fre¬ quency of the oscillator connected to the output, or in response to a control signal 15. To monitor the frequencies of the oscillators 1, 2 and 3, their outputs are connected to a special frequency comparison circuitry which compares the output frequencies of the oscillators in pairs with each other and causes the selection unit 13 to re¬ place the oscillator currently connected to the out- put 14a of the oscillator arrangement with another redundant oscillator if the frequency of the current oscillator deviates from the frequencies of the redundant oscillators at least by a predetermined frequency difference. In the preferred embodiment of the invention shown in the figure, the frequency comparison cir¬ cuitry comprises three circuits 4, 5 and 6 performing the logic XOR operation. The input signals of the XOR circuit 4 include the output signals la and 2a of the oscillators 1 and 2; the input signals of the XOR circuit 5 include the output signals 2a and 3a of the oscillators 2 and 3; and the input signals of the XOR circuit 6 include the output signals la and 3a of the oscillators 1 and 3. Each XOR circuit 4, 5 and 6 con- tinuously performs the logic XOR operation on the respective input signal pair, whereby the output signal of the XOR circuit is 0 if the input signals are of the same frequency and of equal phase, and a sequence of pulses if there is a phase difference between the input signals. The width (frequency) of the individual pulses in the sequence of pulses in the output signal thus depends on the amount of the phase difference between the input signals of the XOR circuit. The output signals 4a, 5a, 6a of each XOR cir¬ cuit 4, 5, 6 are connected to a frequency voltage converter 7, 8 and 9, respectively. Each frequency voltage converter 7, 8, 9 converts the sequence of pulses applied to it to a corresponding voltage the level of which is proportional to the frequency dif- ference between the signals being compared, and con¬ tains a threshold level comparator for comparing the voltage level so produced with a predetermined refer¬ ence voltage which corresponds to a predetermined permissible frequency difference between the input signals of the XOR circuit. The logic state of the threshold level comparator and thus the state of the output signal of the entire frequency voltage con¬ verter 7, 8 or 9 changes from 0 to 1 if the converted voltage level exceeds the reference voltage. Thus the state of the output signal of the frequency converter 7 indicates whether the difference between the fre¬ quencies of the oscillators 1 and 2 is less than the predetermined frequency difference. The output signals of the frequency voltage converters 8 and 9 indicate the same matter as concerns the difference between the frequencies of the oscillators 2 and 3, and the oscillators 1 and 3, respectively. When the frequency of one oscillator 1, 2 or 3 deviates from the frequency of the other two oscil¬ lators more than what is permissible, the logic state of two frequency voltage converters is changed from 0 to 1. The unstable oscillator can now be identified by checking the oscillator the output signal of which constituted one of the compared input signals in both converters with output state 1. For example, if the output signals 7a and 9a of the frequency voltage converters 7 and 9 are in the logic state 1, the oscillator 1 can be identified as unstable as it is the only oscillator connected to both the XOR gate 4 and the XOR gate 6. For the above-mentioned oscillator identifica¬ tion, the oscillator unit shown in the figure com¬ prises three circuits 10, 11 and 12 performing the logic OR operation. The output signals 7a, 9a of the converters 7 and 9 are the input signals of the OR circuit 10; the output signals 8a and 9a of the con¬ verters 8 and 9 are the input signals of the OR cir¬ cuit 11; and the output signals 7a and 9a of the con- verters 7 and 9 are the input signals of the OR cir¬ cuit 12. Thus the logic state 1 of the output signal 10a of the OR circuit 10 indicates directly an excessive frequency deviation of the oscillator 1. The output signal 11a of the OR circuit 11 and the output signal 12a of the OR circuit 12 indicate the same matter as concerns the oscillator 2 and the oscillator 3, respectively. The signals 10a, 11a and 12a are applied as control inputs to the selection unit 13. The oscillator unit operates in the following way. Assume that the oscillator 1 is currently con¬ nected by the switching unit 14 to the output 14a of the oscillator unit. Then the frequency of the oscil¬ lator 1 starts to drift away from the frequency of the stand-by oscillators 2 and 3 until the frequency difference exceeds a permissible predetermined value, resulting in that the output signals of the con¬ verters 7 and 9 and thereby the output signal 10a of the OR circuit 10 change their logic state from 0 to 1. The selection unit 13 responds to this by causing the switching unit 14 to connect the stand-by oscil¬ lator 2 to the output 14a in place of the oscillator 1. When the oscillator unit according to the in- vention is applied at the base station of a mobile radio system, the nominal frequency of the oscil¬ lators 1, 2 and 3 is preferably within the range from 10 to 15 MHz and the predetermined permissible fre¬ quency difference is preferably 0.5 Hz. The figure and the description related to it are only intended to illustrate the present inven¬ tion. In its details, the oscillator unit according to the invention may vary within the scope of the attached claims.

Claims: Claims : 1. An oscillator unit with improved frequency stability, c h a r a c t e r i z e d in that it com- prises at least three oscillators (1, 2, 3) having the same nominal frequency; means (14, 15) for selecting one of the oscil¬ lators (1, 2, 3) for connection to an output (14a) of the oscillator unit; means (4 to 12) for comparing the oscillator frequencies in pairs with each other and for con¬ trolling the selecting means (14, 15) to replace the oscillator currently connected to the output of the oscillator unit with another oscillator if the fre¬ quency of the oscillator currently connected to said output deviates from the frequencies of the other oscillators at least by a predetermined frequency difference. 2. An oscillator unit according to claim 1, c h a r a c t e r i z e d in that the oscillator unit comprises a first, a second, and a third oscillator (1, 2, 3); a first (4, 7, 10), a second (5, 8, 11) and a third (6, 9,12) frequency comparison means for comparing the frequencies of the first (1) and the second (2) oscillator, the frequencies of the second (2) and the third (3) oscillator, and the frequencies of the first (1) and the third (3) oscillator, re¬ spectively, to generate a first (10a), a second (10b) and a third (10c) control signal, respectively, if the deviation between the two frequencies compared with each other in each particular case is at least a predetermined frequency difference; and that said selecting means (14, 15), in response to the control signals, replace the oscillator (1, 2, 3) connected to the output (14a) of the oscillator unit with another oscillator if both frequency comparison means to which said oscillator is connected generate a control signal. 3. An oscillator unit according to claim 1 or 2, c h a r a c t e r i z e d in that each frequency comparison means comprises an XOR gate (4, 5, 6) with two inputs, a frequency/voltage converter (7, 8, 9) for converting an output signal (4a, 5a, 6a) of the XOR gate, and a threshold level comparator for com¬ paring the output voltage level of the frequency/ voltage converter with a reference voltage cor- responding to said predetermined frequency dif¬ ference, and for generating a corresponding control signal (7a, 8a, 9a) if the output voltage level of the frequency/voltage converter exceeds this refer¬ ence voltage. 4. An oscillator unit according to any of the preceding claims, c h a r a c t e r i z e d in that said predetermined frequency difference is less than 0.6 Hz. 5. An oscillator unit according to any of the preceding claims, c h a r a c t e r i z e d in that the nominal frequency of the oscillators (1, 2, 3) is in the range from 10 to 15 MHz.

Inventors: JYLHAE RAIMO, JYLHAE, RAIMO
Applicants: TELENOKIA OY
Application year: 1991
Main classification: G06F1
IPC codes: G06F11, H03L1, H03L7