Mixer system

[0001] FIG. 1 illustrates a traditional clock and data recovery system. To generate a signal CLK having a phase and frequency which matches the phase and data rate of DATA, a serial non-return-to-zero (NRZ) data stream, a traditional clock and data recovery system combines in a closed-loop system data phase detector 11, charge pump 12, loop filter 13, and voltage-controlled oscillator (VCO) 14. Within the NRZ data stream, successive 1 logic levels and 0 logic levels are transmitted sequentially, with each logic level held for a period of time called a bit-time. In this system, the data phase detector determines if the falling edge of CLK leads or lags transitions in the NRZ data stream. Data phase detector output signal UP is asserted for one bit-time if the falling edge of CLK lags a data transition; DOWN is asserted for one bit-time if the falling edge of CLK leads a data transition. The UP and DOWN signals are converted to a current by the charge pump and this current is filtered by the loop filter. The filter outputs a voltage which controls the phase and frequency of the VCO. Negative feedback ensures that the feedback loop's steady-state operating point is such that the falling edge of CLK is in phase alignment with transitions in the NRZ data stream, and this ensures that the rising edge of CLK samples the center of each bit-time. The loop filter provides for the addition of a zero in the system's open-loop transfer function, facilitating stability. Additional circuits and methods are used in some embodiments to help ensure that the VCO will oscillate at a frequency equal to the serial data rate, and never at an integer multiple or fraction of the serial data rate. Those skilled in the art will be familiar with these circuits and methods.

[0002] A challenge in the design of a system of this type is the design of the VCO and minimizing both its self-generated phase noise and its power-supply-induced phase noise. The feedback nature of the closed-loop system can attenuate this phase noise, but only for phase noise frequencies below the system bandwidth, as set by the loop filter. At frequencies above the system bandwidth, VCO phase noise must be minimized. High-Q LC oscillators, high-power (though low-Q) ring-oscillators, and careful power supply filtering and regulating all can help minimize VCO phase noise but do not eliminate it.

[0003] The function of a mixer system is to perform a weighted sum, or mixing, of two or more input signals, each having a frequency, phase, and amplitude, so as to generate an output signal. In one embodiment, the input signals have the same frequency and amplitude, and the phases of the input signals are distributed between 0 and 360 degrees. Each input signal's influence on the output signal is in proportion to a corresponding weight set by a corresponding mixer control voltage. In a preferred embodiment, the mixer control voltage is generated by storing a charge on a capacitor, and is increased or decreased by increasing or decreasing the charge. Changes in the mixer control voltages are further controlled by the combined action of digital state control input signals and the mixer control voltages by means of interconnected mixer control subcircuits. When combined with a data phase detector, the mixer system can be and used within a clock and data recovery system to generate a clock which is phase- and frequency-locked to a serial data stream.

[0004] FIG. 1 is a schematic diagram of a traditional system for clock and data recovery of an NRZ data stream.

[0005] FIG. 2 is a schematic diagram of a modified clock and data recovery system with a mixer system which replaces the charge pump, loop filter, and VCO in the traditional clock and data recovery system of FIG. 1. The mixer system includes a multi-phase signal generator, a mixer control circuit, a mixer, an output buffer, and a duty-cycle control circuit.

[0006] FIG. 3 is a schematic diagram of a mixer control subcircuit.

[0007] FIG. 4 is a schematic diagram of a mixer control circuit.

[0008] FIG. 5 is an illustration of mixer control circuit operation.

[0009] FIG. 6 is a schematic diagram of a mixer.

[0010] FIG. 7 is a schematic diagram of an auto-reset circuit.

[0011] FIG. 8 is a schematic diagram of an output buffer.

[0012] FIG. 9 is a schematic diagram of a duty-cycle control circuit.

[0013] The invention described herein is a new mixer system which includes electronic components which replace the charge pump, loop filter, and VCO in the traditional clock and data recovery system of FIG. 1.

[0014] FIG. 2 illustrates the new mixer system, with these aforementioned components replaced by multi-phase signal generator 22, mixer control circuit 23, mixer 24, output buffer 25, and duty-cycle control circuit 26. As in the traditional clock and data recovery system, data phase detector 11 determines if the recovered clock CLK leads or lags transitions in the NRZ data stream. Data phase detector output signal ADVANCE is asserted for one bit-time if the falling edge of CLK lags a data transition; DELAY is asserted for one bit-time if the falling edge of CLK leads a data transition. The frequency of input reference clock signal REFCLK is substantially (but not necessarily exactly) equal to the data rate of the NRZ data stream. Stated another way, the period of REFCLK is substantially (but not necessarily exactly) equal to a bit-time of the NRZ data stream.

[0015] Multi-phase signal generator 22 generates multi-phase output signals CLKn (n1,2,3,4) each having a frequency equal to the frequency of input signal REFCLK. The phases of the output signals are distributed between 0 and 360 degrees: defining CLK1 to have a phase of 0 degrees, the relative phase of CLK2 is substantially equal to 90 degrees, the relative phase of CLK3 is substantially equal to 180 degrees, and the relative phase of CLK3 is substantially equal to 270 degrees. Further, the multi-phase signal generator outputs two control currents, BIAS1 and BIAS2, in substantial proportion to the frequency of input signal REFCLK. Control current BIAS1 is a current source used by the mixer control circuit to set mixer bandwidth, and control current BIAS2 is a current sink used by the duty-cycle control circuit to set a current drive strength at its output. The multi-phase signal generator can be constructed from a delay-locked loop, for example. The literature contains numerous examples of circuit topologies which make use of current-controlled delay cells to do this. Additional circuitry generates control currents BIAS1 and BIAS2, each having a magnitude in substantially constant proportion to delay-cell current. Those skilled in the art will readily recognize methods to accomplish this.

[0016] Mixer control circuit 23 controls the mixing of signals from the multi-phase signal generator by means of mixer control voltages BIASNn and BIASPn (n1,2,3,4). Whenever ADVANCE is briefly asserted, the state of the mixer control circuit will shift so as to cause the mixer to generate a mixer output signal MCLK of a slightly earlier phase. Whenever DELAY is briefly asserted, the state of the mixer control circuit will shift so as to cause the mixer to generate a mixer output signal MCLK of a slightly later phase. This is a continuous, analog operation: whenever ADVANCE (or DELAY) is asserted, the mixer control voltages steadily change such that the phase of the MCLK is steadily advanced (or delayed), relative to the phase of the mixer input signals. The data phase detector will assert either ADVANCE or DELAY for one bit-time whenever a data transition occurs. In a preferred embodiment, the phase shift observed in MCLK when either ADVANCE or DELAY is asserted for one bit-time is a small fraction (0.1 or smaller) of the bit-time.

[0017] Mixer 24 has an operating frequency and mixes multi-phase input signals CLKn (n1,2,3,4) so as to generate mixer output signal MCLK whose frequency is the mixer operating frequency and is substantially (but not necessarily exactly) equal to the frequency of the multi-phase input signals. To buffer mixer output signal MCLK, buffer 25 is used, and duty-cycle control circuit 26 ensures that the duty cycle of CLK is substantially equal to 50%.

[0018] Negative feedback ensures that CLK will have a substantially constant phase relationship relative to the transitions in the serial NRZ data stream. Specifically, the falling edge of CLK will be in phase with the transitions, and the rising edge of CLK will be in phase with the center of each bit-time. The process of mixing the multi-phase input signals involves converting each input signal to a current, scaling each current by multiplying it by a weight, (as set by control voltage pairs BIASNn and BIASPn; n1,2,3,4), and summing the scaled currents onto MCLK. The mixer control circuit uses control current BIAS1 from the multi-phase signal generator to scale each weight in equal proportion to the mixer operating frequency, thereby affecting the total current drive onto (and, therefore, the bandwidth of) mixer output terminal MCLK. Best mixer operation is achieved when the bandwidth of MCLK is approximately equal to or slightly higher than the mixer operating frequency.

[0019] The mixer control subcircuit, illustrated in FIG. 3, stores and manipulates charge on integration capacitor M8 according to its state control inputs. Inputs BIASNLEAD and BIASNLAG are analog state control input terminals, BIASN and BIASP are mixer control output terminals, BIAS1 is an analog terminal, and inputs ADJUST, /ADJUST, ADVANCE, and DELAY are digital state control input terminals. For proper circuit operation, whenever ADVANCE or DELAY is asserted high, ADJUST is asserted high and /ADJUST is asserted low. When neither ADVANCE nor DELAY is asserted high, ADJUST is de-asserted low and /ADJUST is de-asserted high.

[0020] When DELAY is asserted high (and ADJUST is asserted high and /ADJUST is asserted low) for a period of time, M2 conducts current from the integration capacitor to VSS, with the magnitude of this current in accordance with the BIASNLEAD voltage; M1 conducts a current onto the integration capacitor from VDD, with the magnitude of this current in accordance with the BIAS1 voltage; and M6 conducts a current from the integration capacitor to VSS, with the magnitude of this current in accordance with the voltage on BIASN. These three currents are summed and integrated by integration capacitor M8, with an integration time equal to the time DELAY is asserted high.

[0021] Similar operation results when ADVANCE is asserted high (and ADJUST is asserted high and /ADJUST is asserted low) for a period of time. In this case, however, rather than M2 in accordance with the BIASNLEAD voltage, M3 conducts a current from the integration capacitor to VSS, with the magnitude of this current in accordance with the BIASNLAG voltage.

[0022] Transistor M4 and diode-connected transistor M5 are used to generate a ground-referenced BIASN voltage from the vdd-referenced BIASP voltage. This voltage on the BIASN terminal controls currents through M6 and M7.

[0023] An understanding of the operation of the mixer control subcircuit is obtained through consideration of the operation of the mixer control circuit, comprising four interconnected mixer control subcircuits U1, U2, U3, and U4, and diode-connected p-type FET M1, as illustrated in FIG. 4. Here, the ADVANCE input terminals of the four subcircuits are connected together, to an input terminal of OR gate U5, and to input terminal ADVANCE; the DELAY input terminals of the four subcircuits are connected together, to an input terminal of OR gate U5, and to input terminal DELAY; the ADJUST input terminals of the four subcircuits are connected together, to the output terminal of OR gate U5, and to the input terminal of inverter U5; the /ADJUST input terminals of the four subcircuits are connected together and to the output terminal of inverter U5; and the BIAS1 terminals of the four subcircuits are connected together, to input terminal BIAS1, and to the gate and drain terminals of diode-connected transistor M1.

[0024] The BIASN terminals of the mixer control subcircuits are interconnected in the following manner: the BIASN terminal of mixer control subcircuit U1 is coupled to the BIASNLAG terminal of mixer control subcircuit U2 and to the BIASNLEAD terminal of mixer control subcircuit U4, the BIASN terminal of mixer control subcircuit U2 is coupled to the BIASNLAG terminal of mixer control subcircuit U3 and to the BIASNLEAD terminal of mixer control subcircuit U1, the BIASN terminal of mixer control subcircuit U3 is coupled to the BIASNLAG terminal of mixer control subcircuit U4 and to the BIASNLEAD terminal of mixer control subcircuit U2, the BIASN terminal of mixer control subcircuit U4 is coupled to the BIASNLAG terminal of mixer control subcircuit U1 and to the BIASNLEAD terminal of mixer control subcircuit U3. Additionally, the BIASN and BIASP terminals of mixer control subcircuit U1 are coupled to output terminals BIASN1 and BIAP1, respectively; the BIASN and BIASP terminals of mixer control subcircuit U2 are coupled to output terminals BIASN2 and BIAP2, respectively, the BIASN and BIASP terminals of mixer control subcircuit U3 are coupled to output terminals BIASN3 and BIAP3, respectively, and the BIASN and BIASP terminals of mixer control subcircuit U4 are coupled to output terminals BIASN4 and BLASP4, respectively.

[0025] FIG. 5 illustrates operation of the mixer control circuit of FIG. 4. In FIGS. 5a, 5b, 5c, and 5d, the dark data points illustrate initial exemplary state voltages of the mixer control circuit with ADVANCE and DELAY input terminals held low and control current BIAS1 sourced into the BIAS1 input terminal.

[0026] The light data points of FIG. 5a illustrate new exemplary state voltages following a positive pulse of duration equal to one bit-time applied to the ADVANCE input terminal, where the BIASP2 mixer control voltage has decreased and the BIASP3 and BIASP4 mixer control voltages have increased. These shifts in mixer control voltages result in a corresponding advance in phase of MCLK. The light data points of FIG. 5b illustrate new state voltages following a positive pulse of duration equal to one bit-time applied the DELAY input terminal, where the BIASP2 and BIASP3 mixer control voltages have increased and the BIASP4 mixer control voltage has decreased. These shifts in mixer control voltages result in a corresponding delay in phase of the MCLK. An analog feedback loop comprising M1, M4, M5 and M7 of each mixer control subcircuit acts to hold substantially constant the sum of the BIASPn (n1,2,3,4) voltages while still allowing the individual BIASPn (n1,2,3,4) voltages to change in response to control signals applied to the ADVANCE and DELAY terminals.

[0027] The light data points of FIG. 5c illustrate new state voltages following an increase in the magnitude of control current BIAS1 due to an increase in the mixer system operating frequency. Coupled with a plurality of assertions of the ADVANCE and/or DELAY and /ADJUST input terminals, this current increase then gives rise to an increase in the mixer control voltages BIASP2, and BIASP3, and BIASP4. These shifts in mixer control voltages have two important effects: one is a corresponding increase in available current drive and bandwidth of MCLK; a second is a corresponding increase in available current drive at mixer control terminals BIASPn (n1,2,3,4). This increase in available current drive at BIASPn is substantially proportional to the increase in the mixer system operating frequency and a subsequent decrease in ADVANCE and DELAY pulse widths. A net result is that a mixer phase shift (measured in degrees) resulting from a single detected transition in the serial NRZ data stream is substantially independent of mixer system operating frequency. FIG. 5d is similar to FIG. 5c, but illustrates the effect of a decrease in magnitude of control current BIAS1.

[0028] The mixer control circuit is coupled to the mixer, illustrated in FIG. 6. The mixer is comprised of four pairs of dual-gate transistors M1N and M1P, M2N and M2P, M3N and M3P, and M4N and M4P. Each pair of dual-gate transistors forms a switched voltage-controlled current source and sink controlled by 1) a signal from the multi-phase signal generator to alternately enable the current source and the current sink, and 2) one pair of mixer control signals (of BIASNn and BIASPn; n1,2,3,4) from the mixer control circuit to control the current sink and current source magnitudes, respectively. Depending on the mixer control voltages, a select subset of mixer input signals CLKn (n1,2,3,4) influences mixer output signal MCLK to a greater degree than the others. In a preferred embodiment, the mixer control voltages are controlled by the mixer control circuit in such a way that a subset of the input signals substantially influence MCLK, and by adjusting the mixer control voltages, the phase of MCLK is adjusted to any value between 0 and 360 degrees, relative to the phase of one of the mixer input signals.

[0029] Mixer bandwidth, as set by the available current drive from the four pairs of dual-gate transistors in FIG. 6 and the total capacitance at mixer output terminal MCLK, is indirectly set by control current BIAS1 from the multi-phase signal generator. In a preferred embodiment, the mixer bandwidth is approximately equal to or slightly higher than its operating frequency. Also in a preferred embodiment, the magnitude of control current BIAS1 is set by the multi-phase signal generator, comprised of a delay-locked loop, for example, operating at the same frequency as the mixer. Those skilled in the art will recognize how to generate control current BIAS1 in proportion to the current used in a current-controlled delay-cell element which is in turn used within the delay-locked loop or phase-locked loop used to generate the mixer input signals. At higher frequencies, control current BIAS1 is larger than at lower frequencies. This results in a higher peak mixer control voltage (i.e., a lower peak BIASPn relative to VDD, and a higher peak BIASNn relative to VSS; n1,2,3,4) due to feedback action of M3 coupled to the BIAS1 terminal within each of the four mixer control subcircuits. The higher peak mixer control voltage results in increased current drive on MCLK, thereby increasing mixer bandwidth, as desired.

[0030] Note that the mixer illustrated here is a single-ended mixer and is exemplary. Other mixer topologies are possible, including a differential mixer. In preferred embodiments, the general topology used in the multi-phase signal generator is the same as that used in the mixer. For example, if the mixer is single-ended, a single-ended multi-phase signal generator is preferable. If the mixer is differential, a differential multi-phase signal generator is preferable.

[0031] Three additional components complete the mixer. These are an auto-reset circuit, an output buffer, and a duty-cycle control circuit.

[0032] The benefit of the auto-reset circuit arises from the fact that there exists an invalid state from which the mixer control circuit is potentially unable to recover. This invalid state exists when all four BIASPn (n1,2,3,4) mixer control voltages are at or near VDD. The auto-reset circuit in FIG. 7 detects this condition when none of transistors M2, M3, M4, and M5 are on, and M1 pulls /RESET low. M1 is a long-channel, low-current, n-type FET. Through the action of a logic inverter formed by M6 and M7, this forces RESET high, turning M8 on, and pulling BIASP1 low to VSS. This then turns M2 on, pulling /RESET high, which then results in RESET going low. The detected invalid state is thus successfully cleared, and the auto-reset circuit has no further effect on the mixer control circuit.

[0033] The output buffer in FIG. 8 comprises two cascaded inverters, and is used to provide a second mixer output CLK with a stronger current drive, as compared to the current drive at first mixer output MCLK. The output buffer consists of a first inverter, comprising transistors M1 and M2, whose input is coupled to MCLK, and whose output is coupled to a second inverter, comprising transistors M3 and M4. The output of the second inverter is coupled to CLK.

[0034] The duty-cycle control circuit is shown in FIG. 9. Duty cycle is defined here as the time the periodic signal CLK is a logic high divided by the period of CLK. This is a negative feedback circuit, and by monitoring the duty cycle of CLK and controlling the duty cycle of MCLK, the duty cycle of CLK is forced to be substantially equal to 50%. Coupled to CLK are the source of M6 and the source of M7. M6 and M7 are biased by transistors M1, M2, M3, M4, and M5 so that when M6 and M7 are each on 50% of the time (due to a 50% duty cycle CLK signal), the net current from these transistors onto duty-cycle control node DCC is substantially zero. Biasing of the duty-cycle control circuit is accomplished by means of a control current at input terminal BIAS2. In a preferred embodiment, this control current is a current source which is proportional to the mixer system operating frequency. To further understand the negative feedback mechanism which controls CLK duty cycle, consider the following example:

[0035] First assume that the duty cycle of CLK is substantially less than 50%. This results in M6 being on less than 50% of the time, and M7 being on greater than 50% of the time. As a result, the net current from M6 and M7 will tend to reduce the voltage on DCC. Through the action of transistors M1, M1, M12, and M13, this drop in voltage on DCC will result in an increase in the duty cycle of MCLK and, therefore, in the duty cycle of CLK. With appropriate device size choices (e.g., p-type FETs M1, M2, M3, M6, M12, and M13 of equal size, and also n-type FETs M4, M5, M7, M10, M11 of equal size), the circuit will stabilize when the duty cycle of CLK is substantially equal to 50%. Loop stability is enhanced with M8 and M9 acting as loop filter capacitors.

[0036] The claims herein describe a mixer system comprising a 4-input mixer coupled to a 4-phase signal generator and a mixer control circuit, and the mixer control circuit comprises four interconnected mixer control subcircuits. In some applications, it may be desirable to mix a different number of input signals. Those skilled in the art will recognize that to mix a larger or smaller number of input signals, the mixer is readily scaled, and the number of interconnected mixer control subcircuits can be increased or decreased to match the change in the number of input signals.

[0037] In the claims, first and second control current sources are described. The magnitude of the current generated by each of these current sources is further described as being in proportion to the mixer input frequency. Those skilled in the art will recognize how to use a delay-locked loop to generate these currents.

[0038] Gate connections to any of the dual-gate transistors in the figures may be reversed with no loss in functionality, though minor changes in circuit behavior may result.

[0039] Except to the extent specified in the following claims, the circuit configurations and device sizes shown herein are provided as examples only. Those skilled in the art will recognize that desired and proper circuit operation can be achieved with other circuit configurations, device sizes, and/or combinations of device sizes.

[0040] The mixer can be implemented with discreet components, with semiconductor devices embedded in an integrated circuit such as an application specific integrated circuit (ASIC), or with a combination of both. Individual signals or devices can be active high or low, and corresponding circuitry can be converted or complemented to suit any particular convention. The term coupled used in the claims includes various types of connections or couplings and includes a direct connection or a connection through one or more intermediate components.