Integrated Circuit Ring Oscillator Having Line Driver With Double Line Feedback

Oscillators working in the frequency range from a few hundred Megahertz up to several Gigahertz are required in many integrated circuits of telecommunications technology. With modem bipolar silicon or MOS semiconductor technologies, such circuits can be integrated on a chip as ring oscillators without external elements (WO 95/06356). The oscillation frequency, however, is greatly dependent on the operating temperature of the chip and on scatter of the semiconductor parameters. The employment of oscillators having less basic precision in, for example, clock recovery circuits therefore requires complicated auxiliary circuits such as reference phase-locked loops (IEEE J. Of Solid-State Circuits, 28 (1993) 12, 1310 . . . 1313) or external components such as quartz crystals (IEEE 1991 Bipolar Circuits and Technology Meeting, 12.4, 293 . . . 296) that make the overall circuit more complex and the structure more expensive. Furthermore, a large tuning range of the oscillator is needed in order to be able to level the initial center frequency error, this leading to increased phase noise (jitter), and also makes it difficult to realize such a circuit at extremely high frequencies (above 5 GHz).

No basic precision that suffices for a clock recovery circuit without a reference phase-locked loop or external balancing can be anticipated even given employment of bond wires as frequency-defining elements of an oscillator (ELECTRONICS LETTERS, 30(1994) 3, 244 . . . 245), due to the manufacturing tolerances in bonding.

It is an object of the invention to provide a way to a less circuit-intensive, high-precision oscillator.

According to the invention, a monolithically integrated ring oscillator is provided with a line driver and a double line circuit that feeds back. This oscillator provides the double line as an integration having an optimally long running or transit time and a line driver having an optimally short delay time on one and the same chip. In a further development of the invention, the running time of the double line is greater than (and optimally long compared to) the delay time of the line driver. The independence of the oscillator frequency from the parameter-conditioned and temperature-conditioned variations of the delay time of the line driver is all the greater the shorter the delay time of the line driver and the longer the running time of the double line compared thereto.

The invention yields the advantage that the expenditure required for pre-tuning, for example with a reference phase-locked loop, given an employment in clock recovery circuits in the case of ordinary ring oscillators constructed only of inverters (as frequency-defining elements), becomes superfluous.

Let it be noted here that it is known (from Radio Fernsehen Elektronik 29(1980) 9, 596-597) that an oscillator can be designed as an inverting amplifier with a delay line storage unit or delay-time register in the feedback wherein the delay time of which must be at least sufficiently long such that the reaction of the inverter output on the leading edge via the feedback does not begin until after the end of the leading edge. The oscillation frequency, not taking the inverter switching time into account, corresponds to the reciprocal value of the doubled delay time of the delay line storage unit and is proportionately reduced by all time events of the inverter. No teaching whatsoever is provided of an integration of a feedback double line having an optimally long running time and of a line driver with an optimally short delay time on one and the same chip.

FIG. 1 is a schematic diagram of a line ring oscillator of the invention;

FIG. 2 is a block circuit diagram of an oscillator circuit of the invention; and

FIG. 3 shows circuit-oriented details of a line driver.

In a scope necessary for an understanding of the invention, FIG. 1 of the drawing schematically shows a line ring oscillator integrated on a chip Ch that has a line driver B and a double line L conducted on the chip that feeds the outputs Q',Q" of the line driver B back to the inputs I',I" thereof in inverted fashion. Such a line ring oscillator can, for example, be realized in Siemens B6HF bipolar technology. The cycle duration of the oscillation that is generated derives as double the sum of the delay time T.sub.B of the line driver B and the running time T.sub.L of the double line L. The running time T.sub.L of the line L is primarily dependent on the very well-reproducible line length and on the hardly divergent dielectric constant of silicon dioxide. The oscillation frequency of the ring oscillator formed in this way is thus hardly dependent at all on temperature and parameter fluctuations. In order to be optimally independent of the parameter-dependent and temperature-dependent delay time T.sub.B of the line driver B, T.sub.B should be optimally small and T.sub.L should be optimally great.

For example, the line driver B can be fed back by an approximately 7 mm long double line for generating a 5 Ghz oscillation, this double line L, formed of 2 .mu.m wide aluminum tracks with a mutual spacing of 2 .mu.m, proceeding (particularly in a third A1 layer) over a metal surface (potentially a first A1 layer) lying at ground potential and has a characteristic impedance of about 50 Ohms. The running time amounts to approximately 70 ps. After a cross-over, the line leads to the input of the line driver B, which has a delay time of about 30 ps in the example being considered.

The signal generated in the oscillator B,L is preferably conducted via an output amplifier A to the output OUT', OUT" of the oscillator circuit, as may be seen from FIG. 2. Line driver B and output amplifier A can be realized in the same way with a driver circuit, as outlined, for example, in FIG. 3.

In the line driver circuit shown in FIG. 3, the levels of the signals fed back via the double line L (in FIGS. 1 and 2) are shifted with the assistance of current source-fed transistors Q1,Q3,Q5 or Q2,Q4,Q6, and are then supplied to a differential amplifier having two emitter-coupled transistors Q17, Q18 whose outputs form the driver outputs. A bias for the current source transistors Q8 through Q14 that can be set at a pin R is generated with the transistors Q15,Q16 and Q7 and the resistors R5, R6 and R7.

Values of resistance are indicated in Ohms for the exemplary embodiment in FIG. 3, whereby the resistors R17, R18 in the line driver preferably have a value of resistance of 100 .OMEGA. and, in the output amplifier, a value of resistance of 50 .OMEGA.. The values of current indicated in FIG. 3 are valid for full drive or modulation of the circuit at the input R.

Given a line ring oscillator formed with such a line driver circuit, the oscillator frequency can fluctuate by 2.5% in the temperature range from 25.degree. through 125.degree. C. and in the scatter range specified by the semiconductor manufacturers. This high precision is critically important for use in clock recovery circuits.

The sensitivity of the oscillator frequency to relative changes of the line driver current amounts to about 0.1, i.e. the oscillation frequency changes by 1% given a 10% change in current. Given use in a phase-locked loop (for example for clock recovery), the oscillator can thus be tuned to a specific reception frequency by appropriate drive or modulation of the line driver B proceeding from its control input R (in FIGS. 2 and 3).

The oscillation frequency can also be influenced in that the line L (in FIGS. 1 and 2) is loaded with controllable capacitances. This can occur, for example, with voltage-dependent junction capacitors, as likewise shown in FIG.3. According to FIG.3, the collector junction capacitance of the transistors QL1 and QL2 loads the outputs Q' and Q" and thereby effects an additional signal delay. When the direct current flowing through these transistors proceeding from the control input q is raised, then the average voltage at the driver outputs Q' and Q" drops due to the additional voltage drop-off at the differential amplifier output resistors R18 and R17. Due to what is thus a lower collector-base voltage, the collector junction capacitance of both the load transistors QL1 and QL2 as well as the power switch transistors Q18 and Q17 increases, and the delay becomes longer. The driver inputs I',I"(in FIGS. and 2) will therefore be preferably capacitatively coupled to the line L (in FIGS. 1 and 2) in order to prevent the base potential of the power switch transistors Q18 and Q17 from becoming too low. Without this having to be shown in detail here, further junction capacitors can also be distributed over the entire course of the line L.

Although various minor changes and modifications might be proposed by those skilled in the art, it will be understood that we wish to include within the claims of the patent warranted hereon all such changes and modifications as reasonably come within our contribution to the art.