BACKGROUND OF THE INVENTION

This invention relates to the field of crystal oscillators, and specifically to an arrangement for a fully integrated, adjustable oscillator for use with a crystal, such as an AT-cut crystal. Temperature compensated crystal oscillators are known in the art. These crystal oscillators generally fall into two categories, namely, those having analog compensation techniques and those having digital compensation techniques. However, each of these two categories of crystal oscillators has fallen short of achieving a fully integrated oscillator arrangement for use with a crystal.

In particular, present day temperature compensated crystal oscillators utilize one or more discrete hyper- abrupt junction varactors to set the target frequency (f ₀ ) as well as to correct for variations due to shift in the crystal frequency versus temperature. As is well known in the art, this type of varactor has a linear capacitive reactance versus voltage characteristic and a large tuning ratio, or high voltage sensitivity. The significant drawback to utilizing such a hyper-abrupt varactor is that it cannot be integrated with the rest of the oscillator circuit.

By contrast, an abrupt junction varactor can be integrated onto the same semiconductor device die along with other integrated circuitry, but a device made in this structure suffers from very large "make" tolerance of as much as plus or minus 30%. Moreover, such an integrated abrupt junction varactor also suffers from a low tuning ratio, or voltage sensitivity, tr.at ultimately limits its ability to correct for its own varactor make tolerance as well as for any fixed capacitor make tolerances and supply tolerances, and still provide enough reserve tuning for temperature compensation or aging compensation. Even though the reactance of an integrated varactor is nonlinear versus voltage, this characteristic can be compensated by utilizing an appropriate circuit.

Another problem resulting from attempting to utilize an integrated abrupt junction varactor is that very large variations in RF level can occur due to poor make tolerances. Very high RF levels limit the varactor voltage tuning range since the RF peaks must be kept from forward biasing the varactor diode junction. Very low RF levels can cause oscillations to stop as the temperature varies.

SUMMARY OF THE INVENTION

Briefly described, the present invention contemplates a crystal oscillator circuit arrangement, such as a Pierce oscillator, that utilizes a tuning network which includes at least one integrated varactor (voltage-variable capacitor) as a shunt element for providing at least one type of adjustment of the oscillating signal. More than one type of adjustment can be provided by including a bank of varactors for each of the shunt elements of the tuning network, in which various individual varactors are selected in binary (on- off) fashion to effect digital as well as analog adjustment of the crystal oscillator.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a block diagram of the simplest form of the present invention.

Fig. 2 is a more detailed block diagram depicting further aspects of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to Fig. 1, a block diagram of the simplest form of the present invention is shown at 100 in which a fully integrated oscillator is formed by coupling an inverting amplifier (102) to an external crystal (104) via two blocking capacitors (106,107) to provide an oscillating signal at an output port (108) . In the preferred embodiment, the crystal (104) utilized is an AT-cut crystal, but it is contemplated that other types of crystals can also be utilized.

In addition, at least one integrated varactor (110) is constructed and arranged to couple to the

oscillator arrangement. A second shunt capacitor is also employed which may be either an integrated varactor (112A) or a fixed integrated capacitance (112B, as shown in dotted lines) . As will be seen in a moment, the preferred embodiment contemplates the use of two varactors, but for less stringent tuning requirements, one of the two shunt elements can be a capacitor of fixed value having a moderate tolerance of plus or minus 10%, rather than a varactor. Thus, the crystal and each of the two shunt elements Cl (110) and C2 (112) form a tuning network around the inverting stage of the oscillator. When the oscillator is on frequency, the value of Cl - C2 ■ twice the crystal load capacitance.

Although depicted as a Pierce crystal oscillator in which the crystal operates with a positive value of reactance as an inductor, a Colpitts crystal oscillator configuration can also be utilized.

A control circuit (not shown) can be coupled to the oscillator and can be integrated as part of the oscillator arrangement and utilized for controlling one or more of the integrated varactors. One way to effect a tuning adjustment is by way of terminal #1 (114) , which couples to Cl via resistor (115) . Another way to effect a tuning adjustment is via terminal #2 (116) , which couples to C2 via resistor (117) . Therefore, in this arrangement, the control cicuit is simplified in that it need only control the individual varactors independently of each other, by utilizing only one type of adjustment, namely, controlling an analog (or DC) voltage. However, this topology offers special advantages that will become apparent when considering the next figure.

Turning now to Fig. 2, a more detailed block diagram is shown at 200 in which an amplifier is coupled

to a crystal as a Pierce oscillator that includes a tuning network that offers two types of adjustments of the integrated oscillator arrangement.

In this embodiment, a fully integrated oscillator (202) is formed by coupling an inverting amplifier (102) to an external crystal (104) via two blocking capacitors (106,107), as well as to an included buffer stage (204) followed by a prescaler (206) . The prescaler (206) divides the oscillating signal at node (207) by an integer number to provide a lower frequency output signal at the output (209) . The prescaler output also drives the voltage multiplying charge pump (or DC to DC converter) (210) . The purpose of the voltage multiplying charge pump (210) will be described later in conjunction with the rest of the control circuit integrated on the same chip as part of the oscillator arrangement.

Note also that the oscillator (202) includes a tuning network that couples to the external crystal (104). This tuning network includes a bank of varactors, labelled Cl and C2, respectively, portions of which are selectively connected in parallel as shunt elements on either side of the crystal (104) . In this embodiment, Cl and C2 each include at least one integrated varactor (110, 112A) coupled directly from one side of the crystal to the regulated voltage source line (118) , which is also an RF (or AC) ground for the varactors, similar to the earlier Fig. 1 embodiment.

However, the arrangement in Fig. 2 also includes additional varactors that may be coupled into the circuit utilizing a second type of adjustment, namely digital selection. This topology makes effective use of the structure that results when integrating abrupt junction varactors as part of the overall integrated circuit. That is, in order to realize a switched capacitance in an

N-well CMOS (complimentary metal oxide semiconductor) process, it is necessary that the switch be placed between the crystal and the varactor diode. If it were not so placed, the varactor would not be effectively removed when the switch is open since an excessive process inherent parasitic capacitance would still exist between the varactor diode cathode and the substrate. Similarly, the switch characteristics need to be considered. Each switch (220A through 220B, and 222A through 222B) is an N-channel device and should be made relatively "small ¹¹ to minimize its own parasitic capacitances so as to not desensitize the varactors and yet still have low "on" impedance to minimize RF signal losses (generally described in terms of Q, which is a measure of the energy stored divided by the energy dissipated per cycle of signal) in the circuit. However, their circuit location and desired characteristics require that a relatively high control voltage be used for effective control. This is accomplished by utilizing a voltage multiplying charge pump (210) , since the rest of the oscillator circuit is configured to consume low power and only requires a supply voltage of about 4 volts. Typically, the varactor segment varies from 4pF at zero bias to about 1.6pF at 4V bias, and the switch stray capacitance is on the order of 0.05pF (off) to 0.12pF (on).

As a result, the effective value of varactance in the tuning network (212) can be changed in equal steps, or increments, on each side of the crystal (104) by selectively activating one or more switch pairs (220A and 222A, etc.) Switches (223A-223D) are also N-channel devices and are utilized in the preferred embodiment for test purposes only.

In this way digital selection or parallel connection of one or more of the varactors allows the effective reactance on each side of the crystal (104) to be changed in equal increments, hereinafter known as ganged tuning. Gang tuning the varactors in equal increments via digital adjustment is a key feature of the present invention because it permits the varactors to have a centered DC analog control voltage that keeps them operating at the same RF level to avoid clipping. This digital ganged tuning is effective for initially adjusting the circuit to set the absolute value of the varactors to effectively overcome the varactor make tolerance by switching in different amounts of varactance until the oscillator is on frequency. one will know that the oscillator is on frequency by observing that the value for each of Cl and C2 will equal twice the crystal load capacitance within the crystal make tolerance. Thus, for a typical error of plus or minus 0.5 picofarads, a 20 picofarad crystal exhibits a crystal make tolerance of plus or minus 2.5%. As a result, the absolute uncertainty of varactor capacitance is reduced from a typical plus or minus 30% make tolerance down to plus or minus 2.5%, and no loss of tuning range has been incurred. A second type of tuning can be utilized once this is accomplished. This second type of tuning is analog tuning by means of adjusting the DC voltage level applied to terminal λ or terminal B of the crystal, as shown. These terminals are coupled to the varactors, which are effective in providing aging compensation or automatic frequency control (AFC) at terminal A and temperature compensation (T.C.) at terminal B, respectively, in the circuit topology of Fig. 2. An on-chip circuit for

providing an aging analog control or AFC signal includes a programmable voltage tap (230) that drives an on-chip diode consisting of transistor (232) fed from a current source (234) to provide an approximate incremental change of 0.2 pp in frequency per tap change. When the analog control signal is used to bias an integrated varactance whose temperature characteristics are typically bias dependent, a bias set in the form of a programmable voltage tap needs to be modified by the temperature characteristics of a diode in order to render an integrated varactor"s temperature characteristics independent of the voltage tap selection. In this way, the oscillator temperature compensation, which is performed in the factory at the time of manufacture and compensates for the temperature variation of all components in the oscillator circuit (including the aging adjust varactor) , will not be compromised when the oscillator is later adjusted for aging.

In order to provide for a linear change in frequency per tap change when using an integrated varactor whose reactance varies nonlinearly with voltage, the tap voltage values, which are applied through transistor (223) to node (114) and via resistor (115) to terminal A, must be chosen appropriately. Node 116 couples to terminal B via resistor (117) for T.C. control. Given the known characteristics of an integrated abrupt junction varactor, the present embodiment exhibits ef ective temperature compensation when the aging adjustment is performed at terminal A of crystal 104.

The integrated control circuit is made up of at least the bank of electronic switches (220A through 22OB, and 222A through 222B) , and the level shifters (224) utilized to control one or more switch pairs (220A and

222A, etc.). Typically, the control signals (226) applied to the level shifters take the form of a logical signal that ranges between 0 and 5 volts DC. However, because the present invention contemplates utilizing integrated CMOS devices which occupy a small area and which exhibit a low series "on" impedance, as well as the chosen circuit topology, each of the electronic switches requires a relatively high control voltage in the range of about 13.5 volts via line 227. This high control voltage is necessary to achieve a low "on" resistance in a small switch device which is not coupled directly to ground, but which has an intervening circuit element between it and RF ground.

The level shifters (224) may be configured like a readily available level shifter, such as an MC-14504 available from Motorola. They should rely on an inputted enable signal via line 229 that holds all of the switches (220, 222) "off" during inital oscillator power-up, to ensure successful starting of the oscillator. Otherwise, if the switches were allowed to be "on" during power-up, they could be quite resistive, since the charge pump will not have reached its high value of operating voltage until sometime after the oscillator has started. Hence, the Q of the circuit would be degraded, thereby preventing the oscillator from starting.

Another consideration in the design of the level shifters (224) is that they draw minimal current at the time when they change state and are turned "on" so that a minimum amount of voltage droop occurs on the output of the voltage multiplying charge pump (210) .

The voltage multiplying charge pump, as contemplated, should be powered from a regulated voltage source (via line 213) and clocked from a stable

frequency, preferably at the same frequency as the output of the oscillator itself, or a derivative thereof, no less than the reference output frequency.

In this embodiment, the charge pump (210) is clockedat the reference output frequency provided by the output (209) of the prescaler (206) . The charge pump (210) is configured from a plurality of on-chip diode- connected devices (or other arrangement of integrated devices to achieve a near-ideal diode switching characteristic) and capacitors to achieve a step-up in voltage from approximately 4 volts (supplied via line 118) to about 13.5 volts, via line 227. By so doing, the charge pump (210) is clocked by the prescaler (206) at a lower frequency than the crystal fundamental frequency, so that current consumption is reduced, and so that subharmonic contamination of the output (209) is prevented.

In summary, a fully integrated, adjustable oscillator for use with a crystal has been described that provides at least one type of tuning for a crystal oscillator while operating from a low supply voltage and with low current drain. In this way, a temperature compensated crystal oscillator can be implemented in fully integrated form.