Editing Wien bridge oscillator (section)

===Meacham's bridge stabilized oscillator===
[[File:Meachams bridge oscillator schematic.png|thumb|300px|Simplified schematic of a Meacham's bridge oscillator published in Bell System Technical Journal, Oct 1938. Unmarked capacitors have enough capacitance to be considered short circuits at signal frequency. Unmarked resistors and inductor are considered to be appropriate values for biasing and loading the vacuum tube. Node labels in this figure are not present in the publication.]]

<!-- The information in this section comes from Meacham's paper -->
<!-- Meacham claims that his circuit is new, has very high frequency stability and very pure sinusoidal output -->
<!-- Meacham does not claim that his circuit is better than previous circuits.  Although probably true, it would be Own Research to assert it. -->
<!-- This is own opinion: Meacham's circuit would not make a useful wide frequency range lab instrument because it uses tuned transformers. -->
Larned Meacham disclosed the bridge oscillator circuit shown to the right in 1938. The circuit was described as having very high frequency stability and very pure sinusoidal output.<ref name="Meacham 1938"/>  Instead of using tube overloading to control the amplitude, Meacham proposed a circuit that set the loop gain to unity while the amplifier is in its linear region. Meacham's circuit included a quartz crystal oscillator and a lamp in a [[Wheatstone bridge]].

In Meacham's circuit, the frequency determining components are in the negative feed back branch of the bridge and the gain controlling elements are in the positive feed back branch.  The crystal, Z<sub>4</sub>, operates in series resonance.  As such it minimizes the negative feedback at resonance.  The particular crystal exhibited a real resistance of 114 ohms at resonance.  At frequencies below resonance, the crystal is capacitive and the ''gain'' of the negative feedback branch has a negative phase shift.  At frequencies above resonance, the crystal is inductive and the ''gain'' of the negative feedback branch has a positive phase shift. The phase shift goes through zero at the resonant frequency.  As the lamp heats up, it decreases the positive feedback.  The Q of the crystal in Meacham's circuit is given as 104,000.  At any frequency different from the resonant frequency by more than a small multiple of the bandwidth of the crystal, the negative feedback branch dominates the loop gain and there can be no self-sustaining oscillation except within the narrow bandwidth of the crystal.

In 1944 (after Hewlett's design), [[James Kilton Clapp|J. K. Clapp]] modified Meacham's circuit to use a vacuum tube phase inverter instead of a transformer to drive the bridge.<ref>{{harvnb|Clapp|1944a}}</ref><ref>{{harvnb|Clapp|1944b}}</ref> A modified Meacham oscillator uses Clapp's phase inverter but substitutes a diode limiter for the tungsten lamp.<ref>{{harvnb|Matthys|1992|pp=53–57}}</ref>

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