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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.

Confusion over terminology[edit]

Tuned Oscillator Circuits Tuned Oscillators use a parallel LC resonant circuit (LC tank) to provide the oscillations. There are two common types:. Colpitts – The resonant circuit is an inductor and two capacitors.Hartley– The resonant circuit is a tapped inductor or two inductors and one capacitor.

The term 'digitally controlled oscillator' has been used[citation needed] to describe the combination of a voltage-controlled oscillator driven by a control signal from a digital-to-analog converter, and is also sometimes used to describe numerically controlled oscillators.

This article refers specifically to the DCOs used in many synthesizers of the 1980s[why?]. These include the Roland Juno-6, Juno-60, Juno-106, JX-3P, JX-8P, and JX-10, the Elka Synthex, the Korg Poly-61, the Oberheim Matrix-6, some instruments by Akai and Kawai, and the recent Prophet '08 and its successor Rev2 by Dave Smith Instruments.

  1. What is a Tuned Collector Oscillator? The tuned collector oscillator is one kind of transistor LC oscillator where the tank circuit comprises of a capacitor and a transformer, that is connected to the collector terminal of the transistor. The tuned collector oscillator circuit is the simplest & the basic kind of LC oscillators.
  2. Oscillators, the current consumption increases significantly. Tuning-fork crystals typically have a frequency range of 10 kHz to 200 kHz in fundamental mode and a maximum drive level of 1 µW. These parameters make them the first choice for the 32768-Hz ultra-low-power crystal oscillator in MSP430 microcontrollers.

Relation to earlier VCO designs[edit]

Many voltage-controlled oscillators for electronic music are based on a capacitor charging linearly in an op-amp integrator configuration.[1] When the capacitor charge reaches a certain level, a comparator generates a reset pulse, which discharges the capacitor and the cycle begins again. This produces a rising ramp (or sawtooth) waveform, and this type of oscillator core is known as a ramp core.

A common DCO design uses a programmable counter IC such as the 8253 instead of a comparator.

This provides stable digital pitch generation by using the leading edge of a square wave to derive a reset pulse to discharge the capacitor in the oscillator's ramp core.

Historical context[edit]

In the early 1980s, many manufacturers were beginning to produce polyphonic synthesizers. The VCO designs of the time still left something to be desired in terms of tuning stability.[2] Whilst this was an issue for monophonic synthesizers, the limited number of oscillators (typically 3 or fewer) meant that keeping instruments tuned was a manageable task, often performed using dedicated front panel controls. With the advent of polyphony, tuning problems became worse and costs went up, due to the much larger number of oscillators involved (often 16 in an 8-voice instrument like the Yamaha CS-80[3] from 1977 or Roland Jupiter-8[4] from 1981). This created a need for a cheap, reliable, and stable oscillator design. Engineers working on the problem looked to the frequency division technology used in electronic organs of the time and the microprocessors and associated chips that were starting to appear, and developed the DCO.

The DCO was seen at the time as an improvement over the unstable tuning of VCOs. However, it shared the same ramp core, and the same limited range of waveforms. Although sophisticated analogue waveshaping is possible,[5] the greater simplicity and arbitrary waveforms of digital systems like direct digital synthesis led to most later instruments adopting entirely digital oscillator designs.

Operation[edit]

A DCO can be considered as a VCO that is synchronised to an external frequency reference. The reference in this case is the reset pulses. These are produced by a digital counter such as the 8253 chip. The counter acts as a frequency divider, counting pulses from a high frequency master clock (typically several MHz) and toggling the state of its output when the count reaches some predetermined value. The frequency of the counter's output can thus be defined by the number of pulses counted, and this generates a square wave at the required frequency.The leading edge of this square wave is used to derive a reset pulse to discharge the capacitor in the oscillator's ramp core. This ensures that the ramp waveform produced is of the same frequency as the counter output.

Problems with the design[edit]

For a given capacitor charging current, the amplitude of the output waveform will decrease linearly with frequency. In musical terms, this means a waveform an octave higher in pitch is of half the amplitude. In order to produce a constant amplitude over the full range of the oscillator, some compensation scheme must be employed. This is often done by controlling the charging current from the same microprocessor that controls the counter reset value.

Tuned Oscillators Pdf To Doc

See also[edit]

References[edit]

  1. ^Chamberlin, Hal (1985). 'Basic Analog Modules, Voltage-Controlled Oscillator'. Musical Applications of Microprocessors. section 6, p.181.
  2. ^Russ, Martin (2004). 'Early versus modern implementations'. Sound Synthesis and Sampling. section 2.6.1, p.137.
  3. ^Yamaha CS-80, Vintage Synth Explorer
  4. ^Roland Jupiter 8, Vintage Synth Explorer
  5. ^STG Wavefolder, STG Soundlabs website
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Tuned Oscillators Pdf To Document

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OSCILLATORS Questions with Answers :-

1. An oscillator converts …………….

  1. c. power into d.c. power
  2. c. power into a.c. power
  3. mechanical power into a.c. power
  4. none of the above
    Answer : 2

2. In an LC transistor oscillator, the active device is ……………

  1. LC tank circuit
  2. Biasing circuit
  3. Transistor
  4. None of the above
    Answer : 3

3. In an LC circuit, when the capacitor is maximum, the inductor energy is ……….

  1. Minimum
  2. Maximum
  3. Half-way between maximum and minimum
  4. None of the above
    Answer : 1

4. In an LC oscillator, the frequency of oscillator is ……………. L or C.

  1. Proportional to square of
  2. Directly proportional to
  3. Independent of the values of
  4. Inversely proportional to square root of
    Answer : 4

5. An oscillator produces……………. oscillations

  1. Damped
  2. Undamped
  3. Modulated
  4. None of the above
    Answer : 2

6. An oscillator employs ……………… feedbackCountdowndraw attention to a time-sensitive event occurs.

  1. Positive
  2. Negative
  3. Neither positive nor negative
  4. Data insufficient
    Answer : 1

7. An LC oscillator cannot be used to produce ………………. frequencies

  1. High
  2. Audio
  3. Very low
  4. Very high
    Answer : 3

8. Hartley oscillator is commonly used in ………………

  1. Radio receivers
  2. Radio transmitters
  3. TV receivers
  4. None of the above
    Answer : 1

9. In a phase shift oscillator, we use …………. RC sections

  1. Two
  2. Three
  3. Four
  4. None of the above
    Answer : 2

10. In a phase shift oscillator, the frequency determining elements are …………

Tuned Oscillators Pdf To Doc
  1. L and C
  2. R, L and C
  3. R and C
  4. None of the above
    Answer : 3

11. A Wien bridge oscillator uses ……………. feedback

  1. Only positive
  2. Only negative
  3. Both positive and negative
  4. None of the above
    Answer : 3

12. The piezoelectric effect in a crystal is ……………

  1. A voltage developed because of mechanical stress
  2. A change in resistance because of temperature
  3. A change in frequency because of temperature
  4. None of the above
    Answer : 1

13. If the crystal frequency changes with temperature, we say that crystal has …………. temperature coefficient

  1. Positive
  2. Zero
  3. Negative
  4. None of the above
    Answer : 1

14. The crystal oscillator frequency is very stable due to ………………. of the crystal

  1. Rigidity
  2. Vibrations
  3. Low Q
  4. High Q
    Answer : 4

15. The application where one would most likely find a crystal oscillator is …………….

  1. Radio receiver
  2. Radio transmitter
  3. AF sweep generator
  4. None of the above
    Answer : 2

16. An oscillator differs from an amplifier because it ………

  1. Has more gain
  2. Requires no input signal
  3. Requires no d.c. supply
  4. Always has the same input
    Answer : 2

17. One condition for oscillation is ………….

  1. A phase shift around the feedback loop of 180o
  2. A gain around the feedback loop of one-third
  3. A phase shift around the feedback loop of 0o
  4. A gain around the feedback loop of less than 1
    Answer : 3

18. A second condition for oscillations is ……………….

  1. A gain of 1 around the feedback loop
  2. No gain around the feedback loop
  3. The attention of the feedback circuit must be one-third
  4. The feedback circuit must be capacitive
    Answer : 1

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19. In a certain oscillator Av = 50. The attention of the feedback circuit must be …………

  1. 1
  2. 01
  3. 10
  4. 02
    Answer : 4

20. For an oscillator to properly start, the gain around the feedback loop must initially be

  1. 1
  2. Greater than 1
  3. Less than 1
  4. Equal to attenuation of feedback circuit
    Answer : 2

21. In a Wien-bridge oscillator, if the resistances in the positive feedback circuit are decreased, the frequency……….

  1. Remains the same
  2. Decreases
  3. Increases
  4. Insufficient data
    Answer : 3

22. In Colpitt’s oscillator, feedback is obtained …………….

  1. By magnetic induction
  2. By a tickler coil
  3. From the centre of split capacitors
  4. None of the above
    Answer : 3

23. The Q of the crystal is of the order of …………

  1. 100
  2. 1000
  3. 50
  4. More than 10,000
    Answer : 4

24. Quartz crystal is most commonly used in crystal oscillators because ………….

  1. It has superior electrical properties
  2. It is easily available
  3. It is quite inexpensive
  4. None of the above
    Answer : 1

27. …………. is a fixed frequency oscillator

  1. Phase-shift oscillator
  2. Hartely-oscillator
  3. Colpitt’s oscillator
  4. Crystal oscillator
    Answer : 4

28. In an LC oscillator, if the value of L is increased four times, the frequency of oscillations is …………

  1. Increased 2 times
  2. Decreased 4 times
  3. Increased 4 times
  4. Decreased 2 times
    Answer : 4

29. An important limitation of a crystal oscillator is ……………

  1. Its low output
  2. Its high Q
  3. Less availability of quartz crystal
  4. Its high output
    Answer : 1

30. The signal generator generally used in the laboratories is …………. oscillator

  1. Wien-bridge
  2. Hartely
  3. Crystal
  4. Phase shift
    Answer : 1

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