Meachom Bridge Oscillator

Meachom Bridge Oscillator with circuit

The Meacham bridge oscillator, illustrated in figure 1460, provides the greatest frequency stability of any vacuum-tube oscillator yet devised, but the region of maximum frequency stability is limited to the lower frequencies because of the increased effect of the 8cray circuit capacitances when the frequency becomes greater than a few hundred kilocycles per second. The oscillator is of the crystal-stabilized type employing tuned circuits. At frequencies above 1000 kv the effect of the stray capacitance is sufficient to reduce the stability to a point where little is to be gained by the use of the Meacham

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Crystal Oscillators

Oscillators with Crystals Having Two Sets of Electrodes

The original crystal oscillator devised by Dr. Nicolson, as well as a number of the earlier crystal oscillators tested by Dr. Cady, employed crystals with, effectively, two pairs of electrodes. The basic circuit is shown in figure 1-156. The re quired phase inversion of, the amplifier output voltage is provided by the crystal unit operating at a mode for which the polarities of the plate and grid terminals with respect to ground are 180 degrees out of phase. The circuit shown operates the crystal unit very

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Miller Oscillator with Cricuit

1428. The Miller oscillator is the crystal equivalent of a Hartley oscillator in which no mutual inductance exists between the plate-to cathode and gridt o.cithode inductances. ( See figure 1-140.) The Miller oscillator has an average frequency deviation of approximately 1.5 times that of the Pierce circuit. The plate circuit must appear inductive in order that the correct phase shift will be produced in E, the plate r-f voltage, to compensate for the

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COLPITTS OSCILLATOR

The type of oscillator we are discussing in this tutorial largely depends on a resonant network that consists of 2 capacitors (series capacitance is equal C in total) and an inductor (L) connected in parallel with them. This L-C network resonates at a frequency, f = 1/2 π(LC)^-1/2. The op amp is wired in the circuit as an inverting amplifier with a gain of about 30. Its non-inverting (1) input is kept at half the supply voltage (1V/2) by the two 22 kΩ resistors that are acting as a potential divider. The LC network is placed in the +ve feedback loop of the op amp. At the resonant frequency level the output coming from the op amp makes the network to resonate. The tapped point between the capacitors exists at 1V/2, but the part of the oscillating signal across C2 is fed to the inverting amplifier. It is then amplified and maintains the network oscillating strongly.

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Oscillator

An oscillator is a circuit that produces a repetitive waveform on its output with only the dc supply voltage as an input.A repetitive input signal is not required.The output voltage can be either sinusoidal or nonsinusoidal depending on the type of oscillator.

The basic concept of an oscillator is illustrated in figure(a).Essentially an oscillator converts electrical energy in the form of dc to electrical energy in the form of ac.A basics sinusoidal oscillator consists of an amplifier for gain( either discrete transistor or op-amp ) and a positive feedback circuit that produces phase shift and provides attenuation,as shown in figure(b).

The basic oscillator concept showing three common types of output waveforms sine wave,square wave and sawtooth.

Oscillator Principles:

Positive Feedback:

Positive feedback is characterized by the condition wherein a portion of the output voltage of an amplifier is fed back to the input with no net phase shift,resulting in a reinforcement of the output signal.This basic idea is illustrated in figure.As you can see the in phase feedback voltage Vf is amplified to produce the output voltage which in turn produces the feedback voltage.That is,a loop is created in which the signal sustains itself and a continous sinusoidal output is produced.This phenomenon is called oscillation.

Conditions for Oscillation:

Two conditions are required for a sustained state of oscillation.

1.The phase shift around the feedback loop must be zero degree.

2.The voltage gain Acl,around the closed feedback loop (loop gain) must equal 1 (unity).

The voltage gain around the closed feedback loop(Acl) ia the product of the amplifier gain (Av) and the attenuation (B) of the feedback circuit.

Acl=Av.b

For example,if the amplifier has a gain of 100,the feedback circuit must have a attenuation of 0.01 to make the loop gain equal to 1( that is Av.B=100*0.01=1).These conditions of oscillation are illustrated in figure.

Start-Up Conditions:

So far,you have seen what it takes for an oscillator to produce a continous sine wave output.Now let's examine the requirements for the oscillation to start when the dc supply voltage is turned on.As you know,the unity-gain condition must be met for oscillation to be sustained.For oscillation to begin,the voltage gain around the positive feesback loop must be greater than 1 ,so that the amplitude of the output can be build up to a desired level.The gain must then decrease to 1 so that the output stays at the desired level

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