PLL multibit

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A PLL multibit or multibit PLL is a phase-locked loop (PLL) which achieves improved performance compared to a unibit PLL by using more bits. Unibit PLLs use only the most significant bit (MSB) of each counter's output bus to measure the phase, while multibit PLLs use more bits. [1] PLLs are an essential component in telecommunications.

Contents

Multibit PLLs achieve improved efficiency and performance: better utilization of the frequency spectrum, to serve more users at a higher quality of service (QoS), reduced RF transmit power, and reduced power consumption in cellular phones and other wireless devices.

Concepts

A phase-locked loop is an electronic component or system comprising a closed loop for controlling the phase of an oscillator while comparing it with the phase of an input or reference signal. An indirect frequency synthesizer uses a PLL. In an all-digital PLL, a voltage-controlled oscillator (VCO) is controlled using a digital, rather than analog, control signal. The phase detector gives a signal proportional to the phase difference between two signals; in a PLL, one signal is the reference, and the other is the output of the controlled oscillator (or a divider driven by the oscillator).

In a unibit phase-locked loop, the phase is measured using only one bit of the reference and output counters, the most significant bit (MSB). In a multibit phase-locked loop, the phase is measured using more than one bit of the reference and output counters, usually including the most significant bit.

Unibit PLL

In unibit PLLs, the output frequency is defined by the input frequency and the modulo count of the two counters. In each counter, only the most significant bit (MSB) is used. The other output lines of the counters are ignored; this is wasted information.

PLL structure and performance

A PLL includes a phase detector, filter and oscillator connected in a closed loop, so the oscillator frequency follows (equals) the input frequency. Although the average output frequency equals the input frequency, the oscillator's frequency fluctuates or vibrates about that average value. The closed loop operates to correct such frequency deviations; higher performance PLL reduces these fluctuations to lower values, however these deviations can never be stopped. See Control theory. Phase noise, spurious emission, and jitter are results of the above phenomena.

PLL synthesizer characteristics

Frequency settling time is the time it takes the PLL to hop to another frequency. Frequency hopping is used in GSM, and still more in modern systems. In CDMA, frequency hopping achieves better performance than phase coding.

Fine frequency resolution is the capability of a PLL to generate closely spaced frequencies. For example, a cellular network may require a mobile phone to set its frequency at any of a plurality of values, spaced 30 kHz or 10 kHz.

The performance envelope of a PLL defines the interrelation between the above essential criteria of performance - for example improving the frequency resolution will result in a slower PLL and higher phase noise, etc.

The PLL Multibit expands the performance envelope of the PLL - it enables to achieve faster settling time together with fine frequency resolution and with lower phase noise.

Effects of unibit

As one progresses from the MSB toward the least significant bit (LSB), the frequency increases. For a binary counter, each next bit is at twice the frequency of the previous one. For modulo counters, the relationship is more complicated.

Only the MSB of the two counters are at the same frequency. The other bits in one counter have different frequencies from those in the other counter.

All the bits at the output of one counter, together, represent a digital bus. Thus, in a PLL frequency synthesizer there are two buses, one for the reference counter, the other for the output (or VCO) counter. In a uni-bit PLL, of the two digital buses, only one bit (line) of each is used. All the rest of the information is lost.

Complexity of PLL design

PLL design is an interdisciplinary task, difficult even for experts in PLLs. This - for the Unibit PLL, which is simpler than the Multibit PLL. The design should take into account: [2] [3] [4] [5] [6] [7]

Multibit PLL

Principle of operation

The above PLL uses more of the bits in the two counters. There is a difficult problem, of comparing signals at different frequencies, in two digital buses which count to a different final value.

Improved performance is possible by using the faster bits of the counters, taking into account the additional available information.

The operation of the PLL is further disrupted by overflow in the counters. This effect is only relevant in multibit PLLs; for Unibit PLL, there is only the one-bit signal MSB, therefore no overflow is possible.

Implementation

The additional degree of freedom in Multibit PLLs allows to adapt each PLL to specific requirements. This can be effectively implemented with programmable logic devices (PLD), for example those manufactured by Altera Corp. [7] Altera provides both digital components and advanced design tools for using and programming the components.

Early multibit PLLs used a microprocessor, a microcontroller or DSP to close the loop in a smart implementation. [8] [9] [10] [11] [12] [13] [14]

Benefits

A multibit PLL offers fine frequency resolution and fast frequency hopping, together with lower phase noise and lower power consumption. It thus enhances the overall performance envelope of the PLL.

The loop bandwidth can be optimized for phase noise performance and/or frequency settling speed; it depends less on the frequency resolution.

Improving the PLL performance can make better use of the frequency spectrum and reduce transmit power. And indeed, PLL performance is being constantly improved. [1]

Related Research Articles

Electronic oscillator Type of electronic circuit

An electronic oscillator is an electronic circuit that produces a periodic, oscillating electronic signal, often a sine wave or a square wave or a triangle wave. Oscillators convert direct current (DC) from a power supply to an alternating current (AC) signal. They are widely used in many electronic devices ranging from simplest clock generators to digital instruments and complex computers and peripherals etc. Common examples of signals generated by oscillators include signals broadcast by radio and television transmitters, clock signals that regulate computers and quartz clocks, and the sounds produced by electronic beepers and video games.

A Costas loop is a phase-locked loop (PLL) based circuit which is used for carrier frequency recovery from suppressed-carrier modulation signals and phase modulation signals. It was invented by John P. Costas at General Electric in the 1950s. Its invention was described as having had "a profound effect on modern digital communications". The primary application of Costas loops is in wireless receivers. Its advantage over other PLL-based detectors is that at small deviations the Costas loop error voltage is as compared to . This translates to double the sensitivity and also makes the Costas loop uniquely suited for tracking Doppler-shifted carriers especially in OFDM and GPS receivers.

Phase-locked loop Control system in which an oscillator relies on a filter to calibrate its input

A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is related to the phase of an input signal. There are several different types; the simplest is an electronic circuit consisting of a variable frequency oscillator and a phase detector in a feedback loop. The oscillator generates a periodic signal, and the phase detector compares the phase of that signal with the phase of the input periodic signal, adjusting the oscillator to keep the phases matched.

Analog synthesizer Synthesizer that uses analog circuits

An analogsynthesizer is a synthesizer that uses analog circuits and analog signals to generate sound electronically.

A numerically-controlled oscillator (NCO) is a digital signal generator which creates a synchronous, discrete-time, discrete-valued representation of a waveform, usually sinusoidal. NCOs are often used in conjunction with a digital-to-analog converter (DAC) at the output to create a direct digital synthesizer (DDS).

A digitally controlled oscillator or DCO is used in synthesizers, microcontrollers, and software-defined radios. The name is analogous with "voltage-controlled oscillator." DCOs were designed to overcome the tuning stability limitations of early VCO designs.

Phase detector Electrical circuit detecting phase difference

A phase detector or phase comparator is a frequency mixer, analog multiplier or logic circuit that generates a signal which represents the difference in phase between two signal inputs.

A variable frequency oscillator (VFO) in electronics is an oscillator whose frequency can be tuned over some range. It is a necessary component in any tunable radio transmitter or receiver that works by the superheterodyne principle, and controls the frequency to which the apparatus is tuned.

Voltage-controlled oscillator Electronic oscillator controlled by a voltage input

A voltage-controlled oscillator (VCO) is an electronic oscillator whose oscillation frequency is controlled by a voltage input. The applied input voltage determines the instantaneous oscillation frequency. Consequently, a VCO can be used for frequency modulation (FM) or phase modulation (PM) by applying a modulating signal to the control input. A VCO is also an integral part of a phase-locked loop. VCOs are used in synthesizers to generate a waveform whose pitch can be adjusted by a voltage determined by a musical keyboard or other input.

Tuner (radio)

A tuner is a subsystem that receives radio frequency (RF) transmissions and converts the selected carrier frequency and its associated bandwidth into a fixed frequency that is suitable for further processing, usually because a lower frequency is used on the output. Broadcast FM/AM transmissions usually feed this intermediate frequency (IF) directly into a demodulator that converts the radio signal into audio-frequency signals that can be fed into an amplifier to drive a loudspeaker.

Direct digital synthesis

Direct digital synthesis (DDS) is a method employed by frequency synthesizers used for creating arbitrary waveforms from a single, fixed-frequency reference clock. DDS is used in applications such as signal generation, local oscillators in communication systems, function generators, mixers, modulators, sound synthesizers and as part of a digital phase-locked loop.

In electronics, a frequency multiplier is an electronic circuit that generates an output signal and that output frequency is a harmonic (multiple) of its input frequency. Frequency multipliers consist of a nonlinear circuit that distorts the input signal and consequently generates harmonics of the input signal. A subsequent bandpass filter selects the desired harmonic frequency and removes the unwanted fundamental and other harmonics from the output.

Delta-sigma modulation is a method for encoding analog signals into digital signals as found in an analog-to-digital converter (ADC). It is also used to convert high bit-count, low-frequency digital signals into lower bit-count, higher-frequency digital signals as part of the process to convert digital signals into analog as part of a digital-to-analog converter (DAC).

Delay-locked loop

In electronics, a delay-locked loop (DLL) is a digital circuit similar to a phase-locked loop (PLL), with the main difference being the absence of an internal voltage-controlled oscillator, replaced by a delay line.

Detector (radio)

In radio, a detector is a device or circuit that extracts information from a modulated radio frequency current or voltage. The term dates from the first three decades of radio (1888-1918). Unlike modern radio stations which transmit sound on an uninterrupted carrier wave, early radio stations transmitted information by radiotelegraphy. The transmitter was switched on and off to produce long or short periods of radio waves, spelling out text messages in Morse code. Therefore, early radio receivers had only to distinguish between the presence or absence of a radio signal. The device that performed this function in the receiver circuit was called a detector. A variety of different detector devices, such as the coherer, electrolytic detector, magnetic detector and the crystal detector, were used during the wireless telegraphy era until superseded by vacuum tube technology.

A frequency divider, also called a clock divider or scaler or prescaler, is a circuit that takes an input signal of a frequency, , and generates an output signal of a frequency:

A frequency synthesizer is an electronic circuit that generates a range of frequencies from a single reference frequency. Frequency synthesizers are used in many modern devices such as radio receivers, televisions, mobile telephones, radiotelephones, walkie-talkies, CB radios, cable television converter boxes, satellite receivers, and GPS systems. A frequency synthesizer may use the techniques of frequency multiplication, frequency division, direct digital synthesis, frequency mixing, and phase-locked loops to generate its frequencies. The stability and accuracy of the frequency synthesizer's output are related to the stability and accuracy of its reference frequency input. Consequently, synthesizers use stable and accurate reference frequencies, such as those provided by a crystal oscillator.

Korg Mono/Poly Analog synthesizer, manufactured by Korg from 1981 to 1984

The Korg Mono/Poly (MP-4) is a 44 key "mono-polyphonic" analog synthesizer manufactured by Korg from 1981 to 1984. This keyboard is the sister synthesizer to the Korg Polysix. It has four highly stable voltage-controlled oscillators (VCOs), a 4-pole, self-oscillating low pass filter (LPF), wide modulation capabilities and pseudo-polyphony (paraphony).

Two independent clocks, once synchronized, will walk away from one another without limit. To have them display the same time it would be necessary to re-synchronize them at regular intervals. The period between synchronizations is referred to as holdover and performance under holdover relies on the quality of the reference oscillator, the PLL design, and the correction mechanisms employed.

The terms hold-in range, pull-in range, and lock-in range are widely used by engineers for the concepts of frequency deviation ranges within which phase-locked loop-based circuits can achieve lock under various additional conditions.

References

  1. 1 2 Marc Zuta, "A new PLL with fast settling time and low phase noise". Microwave Journal, June 1998, pp. 94–108.
  2. ↑ Floyd M. Gardner, Phaselock Techniques, Second edition. ISBN   0-471-04294-3
  3. ↑ Vadim Manassewitsch: Frequency Synthesizers, Theory and Design. Second edition. ISBN   0-471-07917-0
  4. ↑ Bar-Giora Goldberg, Digital Techniques in Frequency Synthesis
  5. ↑ William C. Lindsey, Marvin K. Simon, Telecommunication Systems Engineering
  6. ↑ Marvin Frerking, Crystal Oscillator Design and Temperature Compensation
  7. 1 2 "Device Data Book". Altera Corporation.
  8. ↑ U.S. Patent No. 4450518 ITT Industries, Inc. A closed loop control system for adjusting an oscillator frequency using a microprocessor and DAC
  9. ↑ U.S. Patent No. 4503401 Allied Corporation A PLL with a microprocessor controlling a VCO for extending the frequency range of the PLLO
  10. ↑ U.S. Patent No. 4646030 Tektronix, Inc. Oscillator is frequency and phase locked. Uses microprocessor and DAC, DAC output to programmable delay circuit
  11. ↑ U.S. Patent No. 5053723 U.S. Philips Corp. PLL with microprocessor controlling a VCO through a switching network and PDM
  12. ↑ U.S. Patent No. 5182528 Zuta Marc, Computer controls oscillator through both a coarse digital bus and a fine analog control using a DAC
  13. ↑ U.S. Patent No. 5363419 Advanced Micro Devices, Inc. VCO is controlled using analog signal derived from counter and DAC, and a coarse loop
  14. ↑ U.S. Patent No. 5448763 Motorola Inc. PLL synthesizer, processor determines the channel spacing. PLL has faster lock time and lower noise
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