Summary:What Exactly is a PLL? PLL stands for 'Phase-Locked Loop' and is basically a closed loop frequency control system, which functioning is based on the phase sensitive detection of phase difference between the input and output signals of the c
What Exactly is a PLL?
PLL stands for 'Phase-Locked Loop' and is basically a closed loop frequency control system, which functioning is based on the phase sensitive detection of phase difference between the input and output signals of the controlled oscillator (CO). You will find no formulas or other complex math within this tutorial. I decided to keep it simple.
The Phase Locked Loop method of frequency synthesis is now the most commonly used method of producing high frequency oscillations in modern communications equipment. There would not be a Radio Amateur or commercial receiver of any worth today that does not employ at least one if not several, phase locked loop systems, to generate stable high frequency oscillations.
PLL circuits are now frequently being used to demodulate FM signals, making obsolete the Foster-Seerly and radio detectors of the early years. Other applications for PLL circuits include AM demodulators, FSK decoders, two-tone decoders and motor speed controls.
The PLL technique has, surprisingly, been around for a long time. In the 1930's the superheterodyne receiver was in its hayday (and it's still going strong today), however attempts were made to simplify the number of tuned stages in the superheterodyne.
But, before we go any further and into any detail, first a little bit of history of the Phase-Locked Loop and prior to that with the superheterodyne.
In the early 1930's, the superheterodyne receiver was king. Edwin Howard Armstrong is widely regarded as one of the foremost contributors to the field of radio-electronics. Among his principal contributions were regenerative feedback circuits, the superheterodyne radio receiver, and a frequency-modulation radio broadcasting system. It superseded the tuned radio frequency receiver TRF also invented by Armstrong in 1918. He was inducted into the National Inventors Hall of Fame in 1980. Armstrong was born on December 18, 1890, in New York City, where he was to spend much of his professional career. He graduated with a degree in electrical engineering from Columbia University in 1913, and observed the phenomenon of regenerative feedback in vacuum-tube circuits while still an undergraduate. At Columbia, he came under the influence of the legendary professor-inventor, Michael I. Pupin, who served as a role model for Armstrong and became an effective promoter of the young inventor. In 1915 Armstrong presented an influential paper on regenerative amplifiers and oscillators to the IRE. Subsequently, regenerative feedback was incorporated into a comprehensive engineering science developed by Harold Black, Harry Nyquist, Hendrik Bode, and others in the period between 1915 and 1940.
Armstrong conceived the superheterodyne radio receiver principle in 1918, while serving in the Army Signal Corps in France. He played a key role in the commercialization of the invention during the early 1920's. The Radio Corporation of America (RCA) used his superheterodyne patent to monopolize the market for this type of receiver until 1930. The superheterodyne eventually extended its domain far beyond commercial broadcast receivers and, for example, proved ideal for microwave radar receivers developed during World War II.
However, because of the number of tuned stages in a superheterodyne, a simpler method was desired. In 1932, a team of British scientists experimented with a method to surpass the superheterodyne. This new type receiver, called the homodyne and later renamed to synchrodyne, first consisted of a local oscillator, a mixer, and an audio amplifier. When the input signal and the local oscillator were mixed at the same phase and frequency, the output was an exact audio representation of the modulated carrier. Initial tests were encouraging, but the synchronous reception after a period of time became difficult due to the slight drift in frequency of the local oscillator. To counteract this frequency drift, the frequency of the local oscillator was compared with the input by a phase detector so that a correction voltage would be generated and fed back to the local oscillator, thus keeping it on frequency. This technique had worked for electronic servo systems, so why wouldn't it work with oscillators? This type of feedback circuit began the evolution of the Phase-Locked Loop.
As a matter of fact, in 1932 a scientist in France by the name of H.de Bellescise, already wrote a subject on the findings of PLL called "La Réception Synchrone", published in Onde Electrique, volume 11. I guess he lacked the funding or did not know how to implement his findings. In either case it is my personal belief that the British scientist team developed further on the findings of Bellescise. No problem, good stuff. That's why papers like Bellescise are there for.
Although the synchronous, or homodyne, receiver was superior to the superheterodyne method, the cost of a phase-locked loop circuit outweighed its advantages. Because of this prohibitive cost the widespread use of this principle did not begin until the development of the monolithic integrated circuit and incorporation of complete phased-lock loop circuits in low-cost IC packages-- then things started to happen.
In the 1940s, the first widespread use of the phase-locked loop was in the synchronization of the horizontal and vertical sweep oscillators in television receivers to the transmitted sync pulses. Such circuits carried the names "Synchro-Lock" and "Synchro-Guide." Since that time, the electronic phase-locked loop principle has been extended to other applications. For example, radio telemetry data from satellites used narrow-band, phase-locked loop receivers to recover low-level signals in the presence of noise. Other applications now include AM and FM demodulators, FSK decoders, motor speed controls, Touch-Tone® decoders, light-coupled analog isolators, Robotics, and Radio Control transmitters and receivers. Nowadays our technology driven society would be at a loss without this technique; our cell phones and satellite tv's would be useless, well, actually they would not exist.
The PLL is a very interesting and useful building block available as a single integrated circuits from several well known manufacturers. It contains a phase detector, amplifier, and VCO, see Fig. 1 and represents a blend of digital and analog techniques all in one package. One of of its many applications and features is tone-decoding.
There has been traditionally some reluctance to use PLL's, partly because of the complexity of discrete PLL circuits and partly because of a feeling that they cannot be counted on to work reliably. With inexpensive and easy-to-use PLL's now widely available everywhere, that first barrier of acceptance has vanished. And with proper design and conservative application, the PLL is as reliable a circuit element as an op-amp or flip-flop.
Fig. 2 shows the classic configuration. The phase detector is a device that compares two input frequencies, generating an output that is a measure of their phase difference (if, for example, they differ in frequency, it gives a periodic output at the difference frequency). If fIN doesn't equal fVCO, the phase-error signal, after being filtered and amplified, causes the VCO frequency to deviate in the direction of fIN . If conditions are right, the VCO will quickly "lock" to fIN maintaining a fixed relationship with the input signal.
At that point the filtered output of the phase detector is a dc signal, and the control input to the VCO is a measure of the input frequency, with obvious applications to tone decoding (used in digital transmission over telephone lines) and FM detection. The VCO output is a locally generated frequency equal to fIN , thus providing a clean replica of fIN, which may itself be noisy. Since the VCO output can be a triangle wave, sine wave, or whatever, this provides a nice method of generating a sine wave, say, locked to a train of pulses. In one of the most common applications of PLLs, a modulo-n counter is hooked between the VCO output and the phase detector, thus generating a multiple of the input reference frequency fIN. This is an ideal method for generating clocking pulses at a multiple of the power-line frequency for integrating A/D converters (dual-slope, charge-balancing), in order to have infinite rejection of interference at the power-line frequency and its harmonics. It also provides the basic technique of frequency synthesizers.
A basic Voltage Controlled Oscillator (VCO) can be seen in Fig. 3. It shows a basic voltage controlled oscillator by which frequency of oscillation is determined by L1, C2, and D2. D2 is a so-called varactor or varicap. Most common diodes will behave as a varicap when reversed biased, but they must be operated below the junction breakdown parameters.
With reverse bias, this diode will act as a capacitor, its depletion zone forming the dielectric properties. Changing the amount of reverse bias within the diode's breakdown limits, will alter the depletion zone width and hence vary the effective capacitance presented by the diode. This in turn changes the frequency resonancy of the oscillator circuit.
But how does this help us? After all, the VCO is not stable. Any slight voltage variation in the circuit will cause a shift in frequency. If there was some way we could combine the flexibility of the VCO with the stability of the crystal oscillator, we would have the ideal frequency synthesis system.
What if we feed the output of a VCO and Crystal Oscillator into a phase detector? What is a Phase Detector? (See Fig. 4). It is similar to a discriminator or ratio detector used in frequency demodulation or it could be a digital device, like an 'Exclusive OR' gate.
If two signals are fed into a phase detector, being equal in phase and frequency, there will be no output from the detector. However, if these signals are not in phase and frequency, the difference is converted to a DC output signal. The greater the frequency/phase difference in the two signals, the larger the output voltage.
Look at Fig. 4. The VCO and Crystal Oscillator outputs are combined with a phase detector and any difference will result in a DC voltage output. Suppose this DC voltage is fed back to the Voltage Control Oscillator in such a way that it drives the output of the VCO towards the Crystal Oscillator frequency--eventually the VCO will LOCK onto the crystal oscillator frequency. This phenomena is referred to as Phase Locked Loop in its most basic form. Only part of the VCO output needs to be sent to the phase detector. The rest can be usable output.
But hold on a minute, the VCO is locked onto the crystal oscillator and is therefore behaving as if it were a fixed frequency oscillator. This gives us the stability of a crystal oscillator, but lost the flexibility we were aiming for. We may just as well use the crystal oscillator alone for all the good this arrangement has done to us. It certainly doesn't appear as if we have accomplished anything at all.
Let's investigate how we can solve this problem. Suppose our crystal frequency was 10 MHz, but we wanted the VCO to operate on 20 MHz. The phase detector will of course detect a frequency difference and pull the VCO down to 10 MHz, but what if we could fool the phase detector into thinking the VCO was really only operating on 10 MHz, when in reality it is operating on 20 MHz. Take a look at Fig. 5. Suppose, for example in Fig. 4 we used a divide-by-four instead of the divide-by-two. Then, at LOCK, the VCO would be oscillating at 40 MHz yet still be as stable as the crystal reference frequency.
There are oscillators that will operate over a large range of frequencies. Variable Frequency Oscillators (VFO) are made to change frequency by changing the value of one of the frequency determining circuits. A VCI is one in which this component is made to change electronically.
Phase Detector: Let's have a look at the basic phase detector. There are actually two basic types, sometimes referred to as Type I, and Type II. The Type I phase detector is designed to be driven by analog signals or digital square-wave signals, whereas the Type II phase detector is driven by digital transitions (edges). They are typified by the most common used 565 (linear Type I) and the CMOS 4046, which contains both Type I and Type II.
The simplest phase detector is the Type I (digital), which is simply an Exclusive-OR gate (see Fig. 5a.). With low-pass filtering, the graph of the output voltage versus phase difference is as shown, for input square-waves of 50% duty-cycle. The Type I (linear) phase detector has similar output-voltage-versus-phase characteristics, although its internal circuitry is actually a "four-quadrant multiplier", also known as a "balanced mixer". Highly linear phase detectors of this type are essential for lock-in detection, which is a fine technique.
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