Relaxation CCO for PLL-based constant tuning of GM-C filters

[0002] 1. Field of the Invention

[0003] The invention relates to relaxation oscillators for use in PLL circuits and in particular to current controlled oscillators without start-up or amplitude control problems used in PLLs for automatic time-constant or bandwidth tuning of g_m-C filters.

[0004] 2. Description of the Related Art

[0005] The design of a two-integrator based sinusoidal quadrature oscillator, which is widely used in phase locked loop (PLL) bandwidth tuning of g_m-C filters (where g_m-C stands for transconductance-Capacitor), is associated with amplitude control/start-up problems. See Khen Sang Tan and Paul R. Gray, “Fully Integrated Analog Filters Using Bipolar-JFET Technology”, IEEE Journal of Solid-State Circuits, vol. SC-2, pp. 814-821, December 1978, and John M. Khoury, “Design of a 15 MHz CMOS Continuous-Time Filter with On-Chip Tuning”, IEEE Journal of Solid-State Circuits vol. SC-26, no. 12, pp. 1988-1997, December 1991.

[0006] The above-mentioned PLL-based bandwidth-tuning scheme is shown in FIG. 1 and the two-integrator based quadrature sinusoidal current controlled oscillator (CCO) based on the traditional concept is shown in FIG. 2. The PLL-based bandwidth-tuning scheme 10 of FIG. 1 is comprised of phase detector (PD) 12, a charge pump (CP) 14, a loop filter (LF) 16, a voltage to current converter (VIC) 18, and a current controlled oscillator (CCO) 20. Block 12 receives as input the reference frequency f_REFand from block 20 the feedback input f₀. Block 12 feeds block 14 which feeds block 16. The output of block 16 is a voltage which get converted by block 18 to the current I_tunc, Signal I_tuncfeeds other blocks for tuning (not shown) and block 20. FIG. 2 is a more detailed diagram of CCO 20 which shows transconductance amplifiers 22 and 24 coupled in series, each with a transconductance of g_m. The output OUT 2 of 22 is coupled to the negative input IN4 of 24. The output OUT4 of 24 feeds the positive input IN2 of 22. The negative input of 22 and the positive input of 24 are grounded. Blocks 22 and 24 receive equal signals I_tune28 and 29, respectively. The outputs OUT2 and OUT4 are coupled via equal capacitors 26 and 27, respectively, to ground. This type of sinusoidal oscillators suffers from amplitude control and startup problems

[0007] U.S. Patents which have some bearing on the present invention are:

[0008] U.S. Pat. No. 6,201,450 (Shakiba et al) describes a relaxation oscillator which provides a very wide linear range of frequency variation versus control voltage (or current). U.S. Pat. No. 6,111,467 (Luo) discloses a time constant tuning circuit which uses a frequency of a clock to tune the circuit time constant. U.S. Pat. No. 6,084,465 (Dasgupta) describes a time constant tuning circuit in which a reference clock frequency is used to adjust the g_mof a transconductor. U.S. Pat. No. 6,060,957 (Kodmja et al.) teaches a relaxation oscillator with low phase noise particularly applicable to PLLs. U.S. Pat. No. 6,020,792 (Nolan et al.) describes a precision relaxation oscillator with temperature compensation to produce a stable clock frequency. U.S. Pat. No. 5,497,127 presents a voltage controlled oscillator with a wide range of frequencies and which includes a relaxation oscillator. U.S. Pat. No. 5,489,878 (Gilbert) discloses an oscillator which includes two gm/C stages. U.S. Pat. No. 5,418,502 (Ma et al.) teaches the use of an R-C relaxation oscillator where two comparators control the charge/discharge of the capacitor via an SCR. U.S. Pat. No. 5,093,634 (Khoury) shows using a triple input, triple output linear transconductance amplifier as an oscillator by feeding back the output of the amplifier to the input. U.S. Pat. No. 5,070,311 (Nicolai) describes using current sources, selected by a register, to control the charge/discharge of a capacitor and thereby adjusting the frequency of an oscillator. U.S. Pat. No. 4,977,381 (Main) presents a relaxation oscillator where the direction of the current to the capacitor in the oscillator is alternated, based on the state of a bistable circuit. U.S. Pat. No. 4,963,840 (Thommen) is similar to U.S. Pat. No. 4,977,381 above but in addition reduces power consumption. U.S. Pat. No. 4,725,993 (Owen et al.) discloses a low duty cycle relaxation oscillator which periodically gates an ultrasonic frequency relaxation oscillator. U.S. Pat. No. 4,377,790 (Zobel et al.) teaches a relaxation oscillator where a capacitor is charged and discharged between an upper and lower voltage level based on a signal from a comparator. U.S. Pat. No. 4,535,305 (Matsuo et al.) describes a transmission gate relaxation oscillator using a comparator which compares the charge/discharge voltage with a reference voltage. The invention is different from all of the above cited U.S. Patents.

[0009] The invention below describes several equivalent relaxation CCOs that overcome startup/amplitude control problems of sinusoidal CCOs, are simple to design and totally compatible with modern PLL circuits.

[0010] It is an object of at least one embodiment of the present invention to provide a circuit and a method for a relaxation current controlled oscillator (CCO) for phase locked loop (PLL)-based time constant tuning of g_m-C filters.

[0011] It is another object of the present invention to provide a CCO which does not have amplitude control problems.

[0012] It is yet another object of the present invention to provide a CCO which does not have start-up problems.

[0013] It is still another object of the present invention to provide a CCO which is simple to design and is totally compatible with modem PLL circuits.

[0014] It is a further object of the present invention is to provide a CCO where there is great flexibility in choosing the reference frequency f_REFor the oscillation frequency f₀.

[0015] It is yet a further object of the present invention is to provide a CCO with a truly 50% duty cycle output waveform.

[0016] It is still a further object of the present invention is to provide a CCO with a non overlapping output waveform.

[0017] These and many other objects have been achieved by forming a test integrator out of a transconductance amplifier and a capacitor. The output of the integrator is fed to comparators which in turn feed a bistable circuit such as a SR flip flop. The output Q and its inverse QB of the bistable circuit controls either the polarity of the input signals to the transconductance amplifier or the polarity of the input signals to the comparators (when the transconductance amplifier has a differential output). Switches, controlled by the bistable circuit, in turn control the polarity of the input signals. The feedback path created by the transconductance amplifier, comparators, flip-flops, and switches causes continuous oscillation to take place. A DC current input adjusts the g_mof the transconductance amplifier allowing the oscillation frequency of the CCO to be adjusted by varying the DC current input. There is great flexibility in choosing the reference frequency f_REFor the oscillation frequency f₀by the proper choice of a resistive divider.

[0018] These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.

[0019] FIG. 1 is a block diagram of a time-constant Tuning PLL for g_m-C filters of the prior art.

[0020] FIG. 2 is a block diagram of a quadrature sinusoidal oscillator of the prior art.

[0021] FIG. 3ais a schematic of a first preferred embodiment of a CCO for a time-constant tuning PLL of the present invention.

[0022] FIG. 3bis a graph of the input and output waveforms of FIG. 3a.

[0023] FIG. 4ais a schematic of a second preferred embodiment of a CCO for a time-constant tuning PLL of the present invention.

[0024] FIG. 4bis a graph of the input and output waveforms of FIG. 4a.

[0025] FIG. 5ais a schematic of a third preferred embodiment of a CCO for a time-constant tuning PLL of the present invention.

[0026] FIG. 5bis a graph of the input and output waveforms of FIG. 5a.

[0027] FIG. 6ais a schematic of a fourth preferred embodiment of a CCO for a time-constant tuning PLL of the present invention.

[0028] FIG. 6bis a graph of the input and output waveforms of FIG. 6a.

[0029] FIG. 7 is a graph of the tuning characteristics of the typical CCO of the preferred embodiment of the present invention.

[0030] FIG. 8 is a block diagram of the preferred method of the invention.

[0031] FIG. 3ashows the schematic of the first preferred embodiment of the inventive relaxation CCO 30. A test integrator is formed out of the test transconductarnce amplifier 32 (of transconductance gm) and a capacitor 34 (of capacitance C). The output Veap of the integrator is fed to two comparators 36aand 36b. The outputs of the comparators feed a SR latch 38 with outputs Q and QB. SR latch 38 is not further described here since it is a basic component of digital circuitry and well known. The latch controls the polarity of the input DC voltage (across R_B) to the transconductance amplifier. A switching network comprising switching means S1, S2, S3, and S4 is coupled across resistor R_Bof a resistor string and the plus and minus inputs of transconductance amplifier 32. The resistor string itself is a series network of resistors with values R_A, R_C, R_B, R_C, and R_Acoupled between voltage supply V_DDand its return side (typically ground GND). V_Hand V_Lare nodes along the resistor string which couple to the plus and minus inputs of comparators 36aand 36b, respectively. The output Veap of transconductance amplifier 32 couples to the minus and plus input of comparators 36aand 36h,respectively, and to capacitor 34. In addition, current I_tuneis applied to transconductance amplifier 32 of FIG. 3aas well as all transconductance amplifiers 32 of FIGS. 4a, 5a(including transconductance amplifier 52), and 6a.

[0032] If Q is high and QB low (latch set), switching means S₁and S₄are open and S₂and S₃are closed. If QB is high and Q low (latch reset), switching means S₁and S₄are closed with S₂and S₃open. The latter state (when the latch is reset) applies the small voltage across R_Bat the input to the integrator and as a consequence, its output Vcap rises linearly with time. This continues till Vcap reaches V_Hand the upper comparator trips setting the latch (Q is high and QB low). Now S₁, S₄opens and S₂, S₃closes. This also applies the small voltage across R_Bto the input of the integrator, but in the opposite direction. As a result; Vcap now goes down linearly with time till it reaches V_L. At this point, the lower comparator trips and resets the latch. Therefore the above two sequences will repeat again and again causing continuous oscillation to take place. FIG. 3bdisplays the waveshapes at nodes Vcap (Curve 31), Q, and QB (Curves 32 and 33, respectively). Note that the graphs of FIGS. 3b, 4b, 5b, and 6brepresent voltages in the vertical axis.

[0033] Below is the calculation to find the oscillation frequency f₀. $V_L=\frac{R_A}{\sum R}{V}_{\mathrm{DD}},V_H=\frac{R_A+2R_C+R_B}{\sum R}{V}_{\mathrm{DD}},$  where ΣR=2R_A+R_C+R_H  (1)

[0034] $\begin{array}{cc}{I}_O=g_m\frac{R_B}{\sum R}{V}_{\mathrm{DD}}& \left(2\right)\\ T_1=T_2=\frac{C\left(V_H-V_L\right)}{I_O}=\left(1+\frac{2R_C}{R_B}\right)\frac{C}{g_m}& \left(3\right)\\ f_O=\frac{1}{T_1+T_2}=\frac{1}{2\left(1+\frac{2R_C}{R_B}\right)}\frac{g_m}{C}& \left(4\right)\end{array}$

[0035] From (4) $f_0\cong \frac{1}{2\pi }\frac{g_m}{C},$

[0036] if $\begin{array}{cc}\frac{R_C}{R_B}=1.07& \left(5\right)\end{array}$

[0037] Also, $f_0=\frac{1}{2}\frac{g_m}{C},$  if R_C=0  (6)

[0038] Equation (5) shows that the circuit can be adjusted to have a frequency very nearly equal to that of the traditional two-integrator sinusoidal oscillator.

[0039] I_tuneis the DC current input to adjust the g_mof the transconductance amplifier. This means the oscillation frequency f₀of the CCO can be adjusted by varying I_tune. When this CCO in used in the PLL shown in FIG. 1, we have: $\begin{array}{cc}{f}_{\mathrm{REF}}=f_0=\frac{1}{2\left(1+\frac{2R_C}{R_B}\right)}\frac{g_m}{C}& \left(7\right)\end{array}$

[0040] from (7) we have: $\begin{array}{cc}\frac{g_m}{C}=2\left(1+\frac{2R_C}{R_B}\right)f_{\mathrm{REF}}& \left(8\right)\end{array}$

[0041] Since R_C/R_Bis a constant, the g_m/C of the test integrator is determined directly by f_REF. If all the integrators in a g_m-C filter are made identical to the test integrator and if I_tunefrom the PLL is used to control each transconductor (using current mirroring), then the bandwidth of the filter (determined by g_m/C) is also determined by f_REFand can be tuned by it. Equation (8) shows that there is a great flexibility in choosing f_REFor f₀due to the ratio R_C/R_B. In the case of the traditional sinusoidal oscillator, the constant multiplier of f_REFin (8) would be fixed at 2π.

[0042] The transconductance amplifier and the integrator formed by it are referred to as ‘test’ integrators because they are identical to those used in the main filter for which time-constant or bandwidth tuning one wants to do. The problem is to find out the ‘g_m’ of the ones in the main filter. Since this cannot be done directly without affecting the filter performance, an additional integrator, identical to the one in the filter, is used as a ‘test’ element and its ‘g_m’ is found out instead and corrected with a PLL using the inventive circuit.

[0043] FIG. 4ashows a second preferred embodiment of a CCO in the form of CCO 40. Here differential outputs A (+) and B(−) are used for the transconductor 32. However, again only one timing capacitor 34 is used. This implementation uses output switching instead of input switching of FIG. 3a. This helps in getting rid of any delay through the transconductor and, therefore, allows higher frequency of operation. It is to be noted that non-overlapping output waveforms are generated, as shown in FIG. 4bCurves 47 and 48, to avoid discharging of the timing capacitor 34 during switching. Curves 47 and 48 are enlarged sections of Curves 45 and 46, respectively, to more clearly demonstrate non-overlapping. FIG. 4balso shows the waveforms for outputs A and B (Curves 41 and 42, respectively), the waveform at Vcap and node C (Curves 43 and 44, respectively), and output waveforms for φ2 and φ1 (Curves 45 and 46, respectively). The resistor string of FIG. 4ais the same as that for FIG. 3aincluding connections to transconductor 32, comparators 36aand 36b, V_DDand ground. A switching network comprising switching means S1-4 is coupled between outputs A and B and nodes Vcap and C. Vcap in turn is coupled to the positive and negative inputs of comparators 36aand 36b, respectively, and via timing capacitor 34 (of capacitance C) to ground. Node C is coupled via resistors R_Dto both V_DDand ground. Note also that capacitors Cd are connected at the NOR gates of SR latch 38 to help generate the non-overlap. R_Dfor FIG. 4ais defined as:

[0044] R_D=R_A+R_C+R_B/2

[0045] Note that the suffix for S1 to S4 indicates during which phase the switching means is active, such that: S1-φ2 and S3-φ2 are on when output φ2 of SR latch 38 is high, and that S2-φ1 and S4-φ1 are on when output φ1 is high.

[0046] FIG. 5ashows a third preferred embodiment of a CCO in the form of CCO 50. This implementation uses two differential output transconductors 32 and 52. This implementation also employs output switching. The resistor string of FIG. 5ais the same as that for FIG. 3aincluding connections to transconductor 32, comparators 36aand 36b, V_DDand ground. A switching network comprising switching means S1 to S4 is coupled between outputs A and B of transconductor 32 and node Vcap and outputs C and D of transconductor 52. More specifically, A is coupled via S4-φ1 to C and via S1-φ2 to Vcap, B is coupled via S3-φ2 to D and via S2-φ1 to Vcap. As in FIG. 4, Vcap is coupled to the positive and negative inputs of comparators 36aand 36b, respectively, and via timing capacitor 34 (of capacitance C) to ground. Outputs C and D are coupled to nodes V_L′ and V_H′, respectively. Nodes V_L′ and V_H′ are part of a second resistive network comprising two resistor strings in parallel having resistors of value R_Aand R_Din series between V_DDand ground, respectively, and, similarly, resistors R_I)and R_Ain series between V_DDand ground, respectively. R_Dis defined as: R_D=R_A+R_B+2R_C. Note that the voltages at nodes V_Hand V_H′ are identical, so are the voltages at nodes V_Land V_L′.

[0047] Current from one output of the transconductor (32) charges the timing capacitor 34 of capacitance C and the other transconductor 52 helps to maintain continuity of the current from the other output of transconductor 32. The advantage of this implementation is that transconductor 32 outputs A and B do not see voltage jumps when the switching means connect them to the timing capacitor, unlike in the second embodiment of FIG. 4. This enables higher frequency of operation. Here also, non-overlapping output waveforms are generated. As in FIG. 4a, capacitances Cd are added to SR latch 38, which help to generate the non-overlapping waveform.

[0048] Note that the suffix for S1 to S4 indicates during which phase the switching means is active, such that: S1-φ2 and S3-φ2 are on when output φ2 of SR latch 38 is high, and that S2-φ1 and S4-φ1 are on when output φ1 is high.

[0049] FIG. 5bshows the waveforms for Vcap (Curve 51), A (Curve 52), B (Curve 53), C (Curve 54), D (Curve 55), φ2 (Curve 56), and φ1 (Curve 57). Curves 58 and 59 are enlarged sections of Curves 56 and 57 to more clearly demonstrate non-overlapping.

[0050] FIG. 6ashows a fourth preferred embodiment of a CCO in the form of CCO 60. This is a fully differential implementation employing both input and output switching and four timing capacitors 64a-deach of capacitance C. Each timing capacitor is charged to the appropriate reference voltage and applied to the output of transconductor 32 for integration. The integration of a pair of capacitors is conducted while charging of the other pair is carried out. In this case also a non-overlapping output waveform is generated. However, the output frequency is double of the previous embodiments. Being fully differential, this circuit provides a truly 50% duty cycle output waveform as errors due to minor mismatches in the differential transistors balance off.

[0051] Still referring to FIG. 6a, the circuit is explained in more detail. Resistive strings R_A−R_E−R_Aand R_D−R_B−R_Dare both coupled between V_DDand a reference potential (typically ground as shown in FIG. 6a). The node between R_Aand R_Eis labelled V_H, and the node between R_Eand R_Ais labelled V_L. In the identical switching arrangement as that of FIG. 3afor switching means S1 to S4, the high side of R_Bcouples via S1-φ2 and S2-φ1 to the positive and negative input of transconductor 32, respectively. The low side of R_Bcouples via S3-φ1 and S4-φ2 to the positive and negative input of transconductor 32, respectively. The plus and minus output of transconductor 32 couples to the plus and minus input of dual-output comparator 66, respectively. The plus and minus outputs of comparator 66 feed SR latch 38. Coupled between nodes V_Hand V_Lare in series switching means S5-φ2, S6-φ1, S7-φ2, S8-φ1. The junction between S6-φ1 and S7-φ2 is node Vcap. A capacitor 64ais coupled between ground and the junction of S5-φ2 and S6-φ1, and capacitor 64bis coupled between ground and the junction of S7-φ2 and S8-φ. Node Vcap connects to the plus output of transconductor 32. Similarly, coupled between nodes V_Hand V_Lare in series switching means S9-φ1, S10-φ2, S11-φ1, S12-φ2. The junction between S10-φ2 and S11-φ1 is node Vcap′ and connects to the negative output of transconductor 32. Capacitor 64cis coupled between ground and the junction of S9-φ1 and S10-φ2, and capacitor 64dis coupled between ground and the junction of S11-φ1 and S10-φ2. Node Vcap′ connects to the minus output of transconductor 32. Resistor R_Dis defined as: R_D=R_A+R_C, and resistor R_Eis defined as: R_E=R_B+2R_C.

[0052] Note that the suffix for S1 to S12 indicates during which phase the switching means is active, e.g., S1-φ2, S4-φ2 and S5-φ2 are on when output φ2 of SR latch 38 is high, and S2-φ1 S3-φ1, 6-φ1 are on when output φ1 is high.

[0053] FIG. 6bshows the waveforms for Vcap (Curve 61), Vcap′(Curve 62), φ2 (Curve 63), and φ1 (Curve 64). ). Curves 65 and 66 are enlarged sections of Curves 63 and 64 to more clearly demonstrate non-overlapping, caused by capacitors Cd.

[0054] The following table shows the comparison of the four embodiments:

[0055] Simulation Results:

[0056] Simulation results show that the circuit functions as expected. The calculated (with formula) and simulation frequency are very close. The curve of FIG. 7 shows a typical tuning characteristic of the CCO relating the bias current (I_tune) to the CCO frequency (f₀), valid for any of the above described preferred embodiments.

[0057] The method of providing a relaxation current controlled oscillator is shown in FIG. 8 and comprises the following steps:

[0058] Block 1: forming an integrator from a test transconductance amplifier and capacitive means;

[0059] Block 2: coupling the output of the integrator to comparator means;

[0060] Block 3: applying the output of the comparator means to a bistable circuit;

[0061] Block 4: applying a DC tuning current to adjust the transconductance g_mof the transconductance amplifier;

[0062] Block 5: controlling switching means via the bistable circuit to alternate the state of the bistable circuit thus producing continuous oscillations.

[0063] In summary the advantages of the present invention include:

[0064] (1) A relaxation oscillator which is compatible with PLL-based tuning of g_m-C filters.

[0065] (2) No start-up/amplitude control problems have to be overcome.

[0066] (3) There is more flexibility in deciding f_REFor f₀.

[0067] While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.