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
circuit. The oscillator is principally employed with GT-cut crystals in frequency standards, to generate frequencies of 100 kc. In figure 1-160, it can be seen that if the bridge were perfectly balanced there would be no excitation voltage. At the start of oscillations, the ratio of R1 (practically equal to the series-arm R of the crystal unit) to R2 is smaller than the ratio of the R3 to R4. But R4 is a thermistor it is the resistance of a tungsten lamp which sharply increases in value as the temperature rises. (A semiconductor such as carbon, silicon, or german ium can be used, in which case the resistnce will decrease with temperature. The negative tempera ture coefficient of the semiconductor is generally larger than the positive coefficient of tungsten, but the semiconductor thermistor is more expensive and is much more difficult to duplicate because of its great sensitivity to impurities.) As oscillations build up, the current through R4 increases to a point where the heat generated from the power losses raises the the thermistor temperature, and hence the resistance, to a point where the bridge is almost balanced. Equilibrium is reached when the imbalance of the bridge is just sufficient to supp ly heat to the thermistor at the same rate at which it escapes. For maximum amplitude stabili ty, the mbient temperature of R4 should not be permitted to vary over a wide range. Normally, the tungsten lamp will heat to a dull red of approxim ately 600 degrees centigrade. A variation of over 100 degrees in the ambient temperature could have a significant effect on the equilibrium power losses in the thermistor, if extreme precision were des ired. The operating temperature of the lamp is very low compared with the rated temperature, and consequently the lamp can be expected to last indefinitely. The oscillator should be designed and adjusted so that the phase shift occurs entirely in the bridge. That is, the tube should operate into a pure resistance,
circuit. The oscillator is principally employed with GT-cut crystals in frequency standards, to generate frequencies of 100 kc. In figure 1-160, it can be seen that if the bridge were perfectly balanced there would be no excitation voltage. At the start of oscillations, the ratio of R1 (practically equal to the series-arm R of the crystal unit) to R2 is smaller than the ratio of the R3 to R4. But R4 is a thermistor it is the resistance of a tungsten lamp which sharply increases in value as the temperature rises. (A semiconductor such as carbon, silicon, or german ium can be used, in which case the resistnce will decrease with temperature. The negative tempera ture coefficient of the semiconductor is generally larger than the positive coefficient of tungsten, but the semiconductor thermistor is more expensive and is much more difficult to duplicate because of its great sensitivity to impurities.) As oscillations build up, the current through R4 increases to a point where the heat generated from the power losses raises the the thermistor temperature, and hence the resistance, to a point where the bridge is almost balanced. Equilibrium is reached when the imbalance of the bridge is just sufficient to supp ly heat to the thermistor at the same rate at which it escapes. For maximum amplitude stabili ty, the mbient temperature of R4 should not be permitted to vary over a wide range. Normally, the tungsten lamp will heat to a dull red of approxim ately 600 degrees centigrade. A variation of over 100 degrees in the ambient temperature could have a significant effect on the equilibrium power losses in the thermistor, if extreme precision were des ired. The operating temperature of the lamp is very low compared with the rated temperature, and consequently the lamp can be expected to last indefinitely. The oscillator should be designed and adjusted so that the phase shift occurs entirely in the bridge. That is, the tube should operate into a pure resistance,
so that the instant the plate current is maximum the peak transformer voltages
should occur with polarities as indicated in figure 1-160. Tran8forme rs having powdered-i ron,
toroidal cores can provide a coefficient of coupling very close to unity in the
low-frequency range. The following analysis of the frequency stability and the
activity stability of the Meacham oscillator, except for minor deviations and
extensions, has been guided by the postulates and basic considerations as
presented by W. A. Edson.*
0 comments:
Post a Comment