The Phasatrol

Radio Record, Volume II, Issue 7, 31 August 1928, Page 27

The Phasatrol

"THE Phasatrol is a device for the prevention of oscillation in tuned radio-frequency amplifiers. It is easy to adjust, and is said to have a more constant gain on all wave-lengths than other neutralising systems. The Phasatrol unit is a variable high resistance combined with fixed condenser of suitable value, their purpose being to shift the phase of the alternating voltages in the plate circuits of the R.F. amplifying tubes, so that over a certain waveband, tube oscillation and excessive regeneration are eliminated. When the Phasatrol is connected in the plate circuit of an R.F. valve as shown in the diagram, the fixed condenser of the Phasatrol changes the time factor of the feed-back impulses, so that instead of returning to meet the signal impulses "‘in phase" and building them up into undesired oscillation, they travel through the grid-to-plate capacity of the valve and arrive on the grid just after the signal oscillations have gone. As a result there is no puilding-up action to cause unwanted oscillation. In theory the phase dif-

ference is never absolute, but the slight regeneration that takes place is negligible. The function of the variable resistance is principally to feed the direct B current to the plate of the valve. This resistance is non-inductive and non-capacitative and has no effect on the phase: displacement. The unit is adjusted to balance the receiyer by turning the set-screw provided slowly backwards in anti-clock direction until maximum signal strength without oscillation is obtained on low wave-lengths. Once the correct adjustment has been obtained, no further attention is required. The Phasatrol. is used very successfully for neutralising Browning-Drake R.I. circuits.