FIG. 1 shows a generic block diagram of a conventional digital temperature-controlled crystal oscillator (DTCXO) of the kind described, for example, in U.S. Pat. No. 4,746,879 to Ma, et al. The oscillator designated generally as 10 is based on a voltage controlled crystal oscillator (VCXO) 11 to which a temperature-dependent voltage control signal is fed in order that the frequency of the signal generated by the VCXO 11 is substantially invariant regardless of fluctuations in temperature. In order to generate the correct compensating temperature-dependent voltage control signal, there is provided a temperature sensor 12 for producing an analog signal representative of the ambient temperature. The analog temperature signal generated by the temperature sensor 12 is converted to a digital equivalent signal by an analog-to-digital (A/D) converter 13 and is fed as an address to a non-volatile memory such as an EEPROM 14 which serves as a look-up table for storing a corresponding temperature-compensating tuning voltage.
The digital temperature compensation value stored in the memory 14 is converted by a digital-to-analog (D/A) converter 15 to an equivalent analog signal and represents the correct analog voltage for the corresponding ambient temperature for generating the desired frequency from the VCXO 11. The various components in the system are responsively coupled to a timing logic and control unit 16 so as to allow access to the look-up table constituted by the non-volatile memory 14, in order to allow the correct temperature-compensating voltage value to be read from the non-volatile memory 14 corresponding to the instantaneous ambient temperature as derived by the temperature sensor 12 and fed to the non-volatile memory 14 via the A/D converter 13.
The operating principles of piezoelectric crystal oscillators which form the basis of DTCXOs are well described in above-referenced U.S. Pat. No. 4,746,879. It is noted in the opening section of this reference that the frequency versus temperature characteristics of piezoelectric crystals are determined primarily by the angles of cut of the crystal plates with respect to the crystallographic axes of quartz. Since there inevitably exist slight differences between the angles of cut in different crystals, the frequency versus temperature characteristics of different crystal oscillators are inevitably slightly different, thus requiring separate, customized temperature-compensation look-up table for each crystal oscillator. This requirement represents a significant manufacturing problem since the data stored in the non-volatile memory 14 must be separately calibrated for each crystal oscillator 10 in turn and is thus not susceptible to large scale mass production.
The patent literature also discloses various approaches for saving memory in look-up tables associated with digital temperature compensated oscillators. For example, U.S. Pat. No. 4,922,212 to Roberts et al. discloses an oscillator temperature compensating circuit using stored and calculated values. Specifically, a calculator calculates the value of a function representing the portion of the oscillator temperature-frequency transfer curve corresponding to the ambient temperature as measured by a suitable temperature sensor. A signal corresponding to the calculated value is then applied to the oscillator as the control voltage. The oscillator temperature-frequency transfer curve is partitioned into a plurality of segments for each of which an offset value equal to the difference between the voltage magnitude at the start of the segment and the desired voltage magnitude, is stored in a look-up table. When a compensating signal value is required, the compensating signal value calculator retrieves from the look-up table the offset values corresponding to the three points defined by the start and end values of the two segments, respectively preceding and succeeding the segment containing the ambient temperature. This allows for the use of standard curve fitting techniques to compute a parabola passing through the three points retrieved from the look-up table, thus permitting computation of the actual compensating signal value for the ambient temperature. Such a technique obviates the need for a very complex function to be determined for the whole frequency-temperature transfer function whilst allowing discrete points of the frequency transfer curve to be stored at sufficiently coarse resolution to provide a significant saving in the required memory for storing the look-up table.
U.S. Pat. No. 5,170,136 to Yamakawa et al. discloses a DTCXO wherein voltage deviations corresponding to differences between the ambient temperature and a plurality of nominal temperatures over the complete working range of the oscillator may be derived in order to determine the correct compensation voltage to be either added to, or subtracted from, the present control voltage in order to produce the required frequency drift.
Likewise, U.S. Pat. No. 5,548,252 to Watanabe et al. discloses a digital control system for reducing the memory capacity in a DTCXO.
What all of these references have in common is the use of interpolational curve fitting techniques to reduce memory capacity. The actual values themselves, albeit fewer in number, which are stored in the look-up table, and which serve as the basis for the calculation of control voltages, must still, in all cases, be determined for each sampled temperature, which itself must be known. This requires accurate temperature stabilization and complex control circuitry as noted above, and complex setup as explained below with particular reference to FIG. 2.
Also worthy of note is U.S. Pat. No. 4,712,078 to Slobodnik, Jr. et al. which discloses a digital compensation circuit for improving the temperature stability of dielectric resonator oscillators. This patent demonstrates yet another use for digital compensation circuitry beyond temperature compensated crystal oscillators. Specifically, Slobodnik, Jr. et al. employ a temperature sensor for indicating a measure of ambient temperature which is correlated with an amount of phase shift necessary to compensate the frequency drift in the dielectric resonator oscillator. The correlation is made using a correction table or correction function which is determined empirically in a calibration process. The necessary phase shift is then supplied by a voltage controlled phase shifter.
FIG. 2 shows schematically a typical producing setup for calibrating the respective look-up tables constituted by the non-volatile memory 14 in a batch of crystal oscillators as shown in FIG. 1. Thus, there is shown a batch 20 of digital temperature-controlled crystal oscillators designated DTCXO.sub.1, DTCXO.sub.2 . . . DTCXO.sub.N, 21, 22 and 23, respectively. Each of the digitally controlled crystal oscillators in the batch 20 is placed in a temperature-controlled oven 24 and is individually coupled via respective buses 25, 26 and 27 to a control logic and processing unit 28. An output 29 of each of the digital temperature-controlled crystal oscillators 21, 22 and 23 is fed to the control logic and processing unit 28 so as to allow testing of the signal frequency generated by the respective oscillator. Such an arrangement is also shown and described in above-referenced U.S. Pat. No. 4,746,879 wherein FIG. 2 shows a block diagram of a computer-controlled test system for multiplexing a frequency counter to up to eight individual oscillators and measuring the output frequency of each one through a calibration and test circuit. The computer test system is provided with a highly accurate temperature sensor which serves as an absolute temperature test reference for test data and provides proper initiation of the calibration sequences. The multiplexers permit simultaneous testing and calibration of a plurality of temperature-controlled oscillators.
Such a process suffers from numerous drawbacks. First, the multiplexing requires complicated and expensive production setup which also requires expensive cabling whilst being limited in the number of crystal oscillators which can be calibrated during a single run. Also, each connection to the DTCXO is effected via a plurality of lines thus further increasing the complexity of each device. Furthermore, normally two runs are required: one for calibration and the second for verification testing. In above-referenced U.S. Pat. No. 4,746,879, the testing and calibration is performed simultaneously but at the expense of an additional multiplexer, thus complicating even further the production setup and increasing still further the cost of cabling. An additional drawback is that there are provided two temperature sensors which must be precisely matched to give the same reading. This requires that both of the temperature sensors be stabilized and thus the heating process is very lengthy and time consuming. Such a problem is inherent in any system wherein the oscillator is calibrated for a plurality of specific known temperatures.
Further, as is also noted in above-referenced U.S. Pat. No. 4,746,879, the stability of the frequency of a crystal oscillator of the kind described is gradually eroded as a result of crystal aging. As the crystal ages, the entire temperature-frequency transfer curve is shifted up or down. As is also noted, the use of crystal oscillators as reference modules in cellular telephones in particular requires them to be extremely compact and further requires a high degree of frequency stability so as to meet FCC requirements. As a result, conventional crystal oscillator-based cellular telephones have to be serviced periodically in order to adjust the oscillator unit to account for any frequency instability consequent to crystal aging. In order to allow proper compensation for crystal aging, one known arrangement allows for the voltage control signal to be fed to the oscillator via a potentiometer so that the offset voltage can be easily adjusted manually. In use, a very accurate frequency meter is fed to the output of the crystal oscillator so as to measure the frequency correctly so that, in the event of any discrepancy between the desired and actual frequency, the potentiometer may be manually adjusted until the two match. This calibration is not amenable to batch operation and must be performed individually in respect of each crystal oscillator and the oscillator must be physically accessible. Moreover, such calibration must be performed at a specific temperature according to the manufacturer's data specification which, in turn, may require the use of a temperature-controlled oven.
Some improvement in the calibration setup is achieved by U.S. Pat. No. 5,392,005 to Bortolini, et al. which allows re-calibration of crystal oscillators to be performed in the field. This may be accomplished by employing an accurate external reference signal to determine when the frequency of the digital temperature-compensated crystal oscillator has drifted from the reference signal in excess of a specification. Any difference between the measured frequency of the oscillator and the external reference signal is then due to aging, requiring a new compensation value for the instantaneous ambient temperature to be calculated using the reference signal. The technique disclosed by Bortolini, et al. is based on the assumption that any frequency drift due to crystal aging is constant throughout the working temperature range of the crystal oscillator. Thus, FIG. 2 of the Bortolini patent shows an original calibration curve and an up-dated calibration curve which is parallel thereto: thus indicating a constant frequency drift which is itself temperature-independent. Bortolini, et al. then simply adjust the values of the temperature compensation voltage magnitude in the look-up table in order to add or subtract a constant value representative of the constant frequency drift. Additionally, the method described requires processing power (CPU) in each unit and, since the measurement process is time-consuming, the temperature must be stabilized throughout the process thus requiring an oven.
In practice, however, this approach is overly simplistic and thus prone to inaccuracy because it takes no account of the so-called "trim effect". Specifically, although it is true that the frequency drift is constant throughout the whole temperature range of the crystal oscillator, the control voltage which is required to achieve this drift is itself not constant throughout the complete range. Non-linearities are particularly likely to occur, for example, at the extremities of the working temperature of the crystal oscillator and Bortolini, et al. make no allowance for such non-linearities. On the other hand, the use of a reference frequency signal appears to be suggested for the first time by Bortolini, et al.: albeit only for field adjustment of crystal oscillators to allow for aging. Thus, there appears to be no suggestion in the prior art to employ a reference signal already during the manufacturing process; nor is there any suggestion as to how this could be used to simplify the calibration, during manufacture, of a batch containing a large number of digital temperature-controlled crystal oscillators.
Solutions to the above-described problem of trim effect are offered in U.S. Pat. Nos. 5,428,319 and 5,081,431. However, the solutions proposed therein involve complex circuitry, processing power and setup procedures during production and periodic aging calibration. Furthermore, the above-mentioned patents offer no simplification of the setup needed to perform periodic aging calibration.