Digital signed phase-to-frequency converter for very high frequency phase locked loops

Abstract: A phase-to-frequency converter for use in a very high frequency phase locked loop receives an input signal indicating plus/minus phase errors and an enable signal. The input signal is converted into a count by a counting circuit (21). An upper part of the count signal is used to generate a 3-phase sawtooth digital pattern. A lower part of the count is converted by a lower-bit pulse density modulation circuit (28) to generate a signal indicating the binary weight of the lower part of the count. The output of the lower-bit pulse density modulation circuit is applied, along with the 3-phase digital pattern, to three higher-bit phase density modulation circuits (36,38,40) The output of the higher-bit phase density modulation circuits is the digital output of the phase-to-frequency converter and is converted from a digital to an analog signal by RC filters (35,37,39).

Description: This invention relates to the field of high frequency phase locked loops and, in particular, to digital or quasi-digital phase locked loops. Many attempts have been made to fabricate as integrated circuits such loops for operation at very high frequency such as at more than 100MHz. Although some high frequency phase locked loops have been successfully fabricated as integrated circuits, traditional analog phase locked loops are difficult to fabricate as integrated circuits because a phase locked loop normally requires many analog circuit elements and interactive resistor-capacitor ('RC') time-constants. Due to process variations, these analog integrated circuit phase locked loops tend to have a great deal of variation in their performance specifications. Additionally, conventional digital phase locked loops require a clock frequency many times higher than the phase locked loops' operating frequency in order to achieve acceptable phase resolutions. This requirement for a high frequency clock, which may need to operate at frequencies higher than 1 GHz, has limited the use of digital phase locked loops in very high speed applications. One aspect of the invention concerns a quasi-digital high frequency phase locked loop which comprises a phase detector, a signed phase-to-frequency converter, a three-phase ring oscillator and a frequency- controlled oscillator (FCO). The use of the FCO and the phase-to-frequency converter allows the use of a clock frequency which is no higher than the operating frequency of the phase locked loop to achieve acceptable phase resolution. The technique requires a signed phase-to-frequency converter to convert the phase error information generated by the phase detector to multiphase analog waveforms to drive the FCO. The output frequency of the phase-to-frequency converter determines the capture or locking range of the phase locked loop. The phase error direction, either positive or negative, is represented by the phase relationship, either leading or lagging, of the multi-phase outputs from the phase-to-frequency converter, which the FCO interprets as either an increase in the operating frequency or a decrease in the operating frequency. Known methods to create a digital multi-phase phase-to-frequency converter use complicated state machines to perform the necessary signal conversion and waveform synthesis, followed by digital-to-analog converters and anti-aliasing filters. This approach is not cost-effective and requires the fabrication of many precision analog circuits, which cannot be done with sufficient consistency. A digital phase-to-frequency converter of simpler design would be a significant advance on known digital phase-to-frequency converters, especially where high frequency operation is needed. Summary of the Invention In a preferred embodiment, the present invention comprises a phase-to-frequency converter of very high frequency phase locked loop operation. As input, the phase-to-frequency converter receives a phase signal, indicating leading or lagging phase, and an enable signal. In response to these digital inputs, an up/down counting circuit may convert the phase information to a frequency quantity. The most significant bits ('MSB's) of the up/down counting circuit may be applied to a 3-phase waveform generator. The least significant bits ('LSB's) may be applied to a four-bit pulse density modulation (PDM) circuit, which generates a frequency modulated output signal, whose frequency is dependent upon the signal received from the up/down circuit. As the least significant bits of each of the 3-phase waves are identical, only one least significant bit pulse density modulation circuit is needed for the three waves. After the up/down counting signal has been transformed into a 3-phase waveform and the least significant bits have been processed by the least significant bit pulse density modulation, each of the three 3-phase waveforms may be simultaneously processed, along with the carry output from the least significant bit pulse density modulation circuits. The digital output from the most significant bit pulse density modulation circuits may be buffered and then processed by three RC filters converting the digital output to analog signals. Detailed Description Figure 1 is a block diagram of a phase locked loop containing the present invention; Figure 2 is a block diagram of a preferred embodiment of the present invention; Figure 3 illustrates the up-down counting circuit output in response to the phase error and enable inputs; Figure 4 illustrates a 3-phase triangular wave and its phase relationships; Figure 5 shows how the least significant bits of a 3-phase sawtooth waveform are shared in common; Figure 6 shows how the 3-phase sawtooth waveform is generated using most significant bit processing techniques; Figure 7 is a block diagram of a 4-bit pulse density modulator; Figure 8 illustrates how pulse density modulation converts a sawtooth wave into a triangular wave; Figure 9 provides a second illustration of how the sawtooth wave is converted into a triangular wave; Figure 10 is a schematic diagram of the preferred embodiment of the present invention. Figure 1 of the drawings illustrates a quasi-digital high frequency phase locked loop (PLL) 10 which comprises in this embodiment a phase detector 12, a signed phase-to-frequency (P-to-F) converter 14, a three-phase ring oscillator 16 and a frequency- controlled oscillator (FCO). The use of the FCO and the phase-to-frequency converter allows the use of a clock frequency which is no higher than the operating frequency of the phase locked loop to achieve acceptable phase resolution. The output frequency of the phase-to-frequency converter determines the capture or locking range of the phase locked loop. The phase error direction, either positive or negative, is represented by the phase relationship, either leading or lagging, of the multi-phase outputs from the P-to F converter 14; these outputs are interpreted by the FCO as either an increase in the operating frequency or a decrease in the operating frequency. Figure 2 is a simplified block diagram of a preferred converter of the present invention, implemented to provide a 3-phase output. Phase-to-frequency converter 14 receives two signals, an up/down signal and an enable signal. The leading/lagging phase quantities are represented by the up/down signal, and the enable signal provides a "hold" signal. Up/down counter 21 receives the phase error signal and the enable. During periods when the phase error is positive and the enable signal is active, counter 21 increments its output. During periods when the phase error is negative and enable is active, counter 21 decrements its output. When the enable signal is inactive, counter 21's output remains constant at its previous level. The output of counter 21 is the cumulative current value of the decrements, increments, and holds at any given point in time. Up/down counter 21 produces a 7-bit output signal. The three MSBs generated by 3-bit MSB up/down counter 24 are transmitted to 3-phase waveform generator 25. The mathematical calculations necessary to transform the 3-bit input into a 3-phase waveform are described in detail below. The phase relationship of the 3-phase waveforms, either leading or lagging, represents the positive and negative frequencies, relative to the normal frequency of operation. After the 3-phase waveforms are generated by generator 25, the 3-phase waveforms are transmitted to 3 2-bit PDM modulators 36, 38, and 40. Previously the 4 LSBs generated by 4-bit LSB up/down counter 22 were transmitted to a 4-bit PDM circuit 28. PDM circuit 28 comprises 4-bit adder 29 and 5-bit register 31. Each of PDM circuits 36, 38, and 40 also comprises an adder and register (not illustrated) coupled in a similar fashion to PDM circuit 28. PDM is performed in this invention by recursively adding the input of the PDM adder to the output of the PDM register. For example, in circuit 28, the 4-bit input on adder 29 is added to the 4-bit LSB output of register 31. The output of each iteration appears on the carry-out output of register 31. In the preferred embodiment, the carry-out output of PDM modulater 28 is transmitted to the carry input of each of PDM circuits 36, 38 and 40. The adders' final carry output reflects the binary weighting of the digital code received by the PDM circuits. The final outputs from PDM circuits 36, 38 and 40 are transmitted to buffer 33 and thence to RC filters 35, 37, and 39, which transform the digital signal to an analog one. As shown in Fig. 2, phase error information from a digital phase detector is conveyed to P-to-F converter 14 through two logic inputs labelled UD_PI and EN_PI. EN_PI acts as a flag, indicating the presence of a phase error. The two states of UD_PI represent the direction of the phase error, either +1 or -1. In response to the sign of the phase error (UD_PI) and the phase error flag (EN_PI), up/down counter 21 accumulates positive or negative counts, each count representing a unit error. As shown in Fig. 2, up/down counter 21 is made up of 4-bit LSB counter 22 whose output is the 4-bit LO_CNT and 3-bit MSB counter 24, whose 3-bit output is HI_CNT. UD_PI controls the counting direction and EN_PI enables the counter. As shown in Fig. 3, the output of up/down counter 21 is a sawtooth waveform with unit steps. A waveform synthesizer 25 (Fig. 2) is used to convert the output of up/down counter 21 to a group of multi-phase triangular waveforms for the FCO. The phase relationship among the multi-phase outputs (leading or lagging) is interpreted as positive or negative frequencies by the FCO. This is illustrated in Fig. 4, where a 3-Phase triangular wave is drawn and the phase relationship between the waves is noted. Instead of triangular waves, the 3-Phase waveforms generated in the present invention take the form of a sawtooth digital pattern. This allows the multi-phase waveform generators to share circuitry with both counter 21 and the PDM circuits. The 'triangular' waveform used herein can be obtained by a simple PDM inversion, once the sawtooth pattern has been generated. Multi-phase sawtooth patterns have the unique advantage that they share the same counting direction among the different phases. As shown in Fig. 5, for a group of n-phase signals, one cycle of the sawtooth pattern is divided into n slots, denoted in the figure as the number of clock pulses. Each pattern can be described using the same LSBs but with different offsets for the MSBs. As a result, the same 4-bit LO_CNT output generated by counter 21 can be used to generate the common LSB PDM circuit output for each of the multi-phase waveforms. By contrast, the opposite counting directions used in multi-phase triangular patterns make,the sharing of the PDM circuits, even for the LSBs, impractical. To generate a group of N-phase sawtooth patterns, each cycle of the sawtooth digital pattern is divided into n slots defined by an m-bit MSB (m is the smallest integer such that 2 m =>n). Incrementing the MSB by one represents a precise 360°/n phase shift, or 360°/2n for the case of a triangular pattern. By repeating this process n times, n-phase patterns can be obtained. Two sawtooth cycles are required to construct one cycle having a triangular output pattern. Counter 21 thus must generate m+1 bits to describe the multi-phase triangular patterns completely. For a 3-phase sawtooth pattern, a two bit MSB designated MSK_HI_CNT is needed to divide one cycle into three slots (n=3, m=2). The MSBs generated by counter 21 (HI_CNT) are thus 3 bits wide (2+1). HI_CNT is used to generate the sign bits for the 3-phase patterns in a manner that will be described hereafter. The counting sequence for HI_CNT is "0-1-2-4-5-6" when counting up and "6-5-4-2-1-0" when counting down. MSK_HI_CNT is derived from HI_CNT in the manner shown in equation (1) below: MSK_HI_CNT = HI_CNT AND 3 Adding one to MSK_HI_CNT results in a precise 60° phase shift (360°/{2x3}) for the triangular patterns. If the waveform is inverted, as for the case of Phase 2, it represents a 120° phase shift. A Modulo function (base 3) acts on the sum to keep the 2-bit MSBs (PHSE 1, PHSE2, and PHSE3) of the 3-Phase patterns within the valid states of 0, 1, and 2. Equations 2 through 7 below describe the 2-bit MSB processing. A pictorial representation of the process is shown in Fig. 6. PHSE1 = MOD 3 {MSK_HI_CNT + 0} PHSE2 = MOD 3 {MSK_HI_CNT + 1} PHSE3 = MOD 3 {MSK_HI_CNT + 2} CNT 1 = PHSE 1 x 16 + LO_CNT CNT 2 = PHSE 2 x 16 + LO_CNT CNT 3 = PHSE 3 x 16 + LO_CNT In equations 2 through 7, LO_CNT is the 4-bit shared LSB, PHSE1, PHSE2 and PHSE3 are the 2-bit MSBs with unit weighting of 16 (LO_CNT=4 bits), CNT 1, CNT 2 and CNT 3 are the 3-phase sawtooth codes, and MOD 3 (x) is the modulus (base 3) of x. A reset input (T-CLR2) is provided to assure consistent output sequence upon power up during testing. PDM is used extensively in the present invention to provide digital to analog conversions. The use of PDM eliminates the need for many analog circuits having precision components such as resistors or capacitors. Also, the digital nature of the PDM circuit is highly immune to system noise. A block diagram of the 4-bit-PDM circuit 28 used herein, comprising 4-bit adder 29, 5-bit register 31 and RC filter 35, is shown in Fig. 7. The RC filter after the PDM circuit is used for anti-aliasing purposes. The RC low-pass filter at the carry output removes the high-frequency clock and its harmonics, recovering the base band signal of the input digital pattern, thereby converting the carry information input to an analog voltage output. PDM as used herein is a recursive accumulating process for the input digital code, accumulation occurring at a rate defined by the system clock ('CLK'). The frequency of the adder's carry output, after being integrated by an RC filter, reflects the binary weighting of the received digital code. Each of the 3-Phase sawtooth patterns illustrated in Fig. 6 can be described by a 2-bit MSB (derived from the two LSBs of HI_CNT) and a 4-bit common LSB (LO_CNT). As shown in Fig. 2, the MSB PDM converters for each phase are 2-bits wide-and share a common carry output from 4-bit LO CNT PDM circuit 28. A total of 10 PDM circuits (3x2+4) are thus needed, as compared with 18 (3x6) if triangular patterns were used. This method thus saves the use of 8-bit adders and their associated registers. The frequency of occurrence of the PDM's carry output, after being integrated by an RC filter, reflects the binary weighting of the received digital code. When the carry output is active-high, the analog output voltage is proportional to the binary weighting of the input digital code. When the carry is active-low, the analog voltage becomes inversely proportional to the binary weighting of the code. A logic inversion of the PDM's carry bit thus results in an analog voltage inversion. By alternating the inversion of the PDM's carry output over two consecutive cycles of the sawtooth pattern, one cycle of triangular waveform is generated. The sign-bits of the multi-phase patterns define the timings for PDM inversions. HI_CNT of counter 21 is used by sign-bit generator 26 to generate the sign-bits. Fig. 8 illustrates how a sawtooth digital pattern is converted to a triangular wave using PDM inversion technique. For the case of 3-phase sawtooth patterns using 2-bit MSB and 4-bit common LSB, the triangular outputs can reach a maximum frequency (ƒ max ) of CLK/(16x3x2) with a horizontal resolution of 96 steps. Fig. 9 is a further example of the conversion of sawtooth digital patterns to triangular waves for both cases of count-up and count-down of counter 21. Fig. 10 is a schematic diagram of a preferred embodiment of P-to-F converter 14 as used in a 250 MHz PLL for a 125 M-bit/sec Fiber Distributed Data Interface (FDDI) system. The PLL frequency is two times the transmission bit rate. The 3-bit up/down counter for generating HI_CNT of P-to-F counter 21 is implemented with a programmable logic device ('PLD') 41. Binary Up-Down counter 45 generates 4-bit output signal LO_CNT for use by adder 47 and register 49. The 3-Phase MSB waveform generator, sign-bit generators and the three 2-bit MSB PDM circuits are implemented with another PLD 43. The clock (16A) is a 15.625 MHz clock derived from a 250 MHz crystal oscillator (250,MHz/16) (not shown). The maximum frequency of the 3-Phase outputs is 162.76 KHz (EN_PI = 1), thus providing a maximum possible locking range of ± 651 ppm (250 MHz ± 162.76 KHz), with a 42 picosecond (1/250MHz x 96) average phase resolution for the PLL. The PDM circuits are clocked by input C32A at 31.25 MHz (2 x C16A) to provide wide tolerances for the cut off frequencies of the RC low-pass filters 51, 53, and 55. (162.76 KHz < ƒ -3dB <<31.25 MHz). ƒ -3dB of the three RCs are normally much higher than the pole-and-zero frequencies of the PLL's loop filter and should not affect the PLL's loop characteristics. For silicon implementation, the three RC's are matched to ensure similar signal delays among the 3-phase signals. The 3-Phase PDM carry outputs PDM1, PDM2, and PDM3 are buffered by XOR gates 52, 54 and 56 to ensure that system switching noise will not affect the signal quality of the synthesized outputs. For debugging purposes, read/write capability is implemented for accessing the contents of the 7 bit P-to-F converter 14 by using data buffer 61 and Int_IO_BUS 0 to 7. Circuit initialization is not required as the PLD are coded with traps for illegal states. One possible modification to the present invention is to use 6-phase instead of 3-phase signals for the P-to-F converter. A simple logic inversion of PDM1, PDM2, and PDM3 can be used to drive the three additional 3 Phase signals. Six RC-low pass filters (instead of three) are needed.

Claims: A phase-to-frequency converter for converting a phase signal indicating positive and negative phase errors into a plurality of frequency signals comprising: counting means (21) for counting the positive and negative phase errors, the counting means generating a multi-bit parallel digital signal indicating a cumulative current value of the phase error; digital pattern generator means (25) coupled to the counting means for generating a first multi-phase digital pattern from the multi-bit parallel digital signal; first modulation means (28) coupled to the counting means, the first modulation means converting the multi-bit parallel digital signal to a first serial digital signal; and second modulation means (36,38,40) coupled to the first modulation means and the digital pattern generator means, the second modulation means combining the first serial digital signal and the first multi-phase digital pattern to form a second multi-phase digital pattern. A converter according to claim 1 wherein the first modulation means (28) further comprises an adding means coupled to the counting means; and a register means coupled to the adding means and the second modulation means, the register means storing a result from the adding means and transmitting to the adding means a predefined number of least significant bits of the result for iterative addition by the adding means and transmitting to the second modulation means a highest order bit in the result for use as the first serial digital signal. A converter according to claim 1 or claim 2 wherein the digital pattern generator means generates a three-phase sawtooth digital pattern. A converter according to any foregoing claim wherein the second modulation means (36,38,40) comprises three two-bit modulation means, each modulation means further comprising a 2-bit adder and a 3-bit register, a carry-in input on the 2-bit adder receiving the first serial digital signal, a first 2-bit operand input on the 2-bit adder receiving a lower 2-bit output from the 3-bit register, a second 2-bit operand input on the 2-bit adder receiving one of the multi-phase digital patterns, a highest order bit in the 3-bit register providing one of the second multi-phase digital patterns. A converter according to any foreging claim further comprising output means (35,37,39) coupled to the second modulation means for converting the second multi-phase digital pattern to an analog multi-phase output signal. A converter according to claim 5 wherein the output means comprises a plurality of resistor-capacitor filters, one for each phase of the second multi-phase digital pattern. A method of converting a leading and lagging phase error signal, which is in digital form and indicates by positive and negative steps a magnitude of phase error, to a multi-phase frequency-modulated signal, comprising counting the said steps to generate a cumulative multi-bit signal; converting a group of least significant bits of the multi-bit signal to a serial signal; converting a group of most significant bits of the multi-bit signal to a first multi-phase digital pattern; and combining the first multi-phase digital pattern with the said serial signal to form a second multi-phase digital pattern adapted to define a frequency modulated multi-phase signal. A method according to claim 7 wherein the conversion of the group of least significant bits to a serial signal is performed by overflow addition, the serial output being constituted by carry digits.

Inventors: CHIN TSUN-KIT, WONG HEE, CHIN, TSUN-KIT, WONG, HEE
Applicants: NAT SEMICONDUCTOR CORP, NATIONAL SEMICONDUCTOR CORPORATION
Application year: 1992
Main classification: H03L7
IPC codes: H03L7