Some Applications of Pulses

Author: J.B. Hoag

A saw-toothed wave-form may be produced by sending pulses into the input of the circuit of Fig. 30 H.

Fig. 30 H. Pulses are converted into saw-toothed waves with this circuit

Tube 1 is normally biased to cutoff. The positive pulses e₁ charge C quickly through R₁. After the brief charging, C empties slowly through R. For a reasonably linear sweep, the time constant RC must be great in comparison with the time between pulses. The output current i can be used in the magnetic deflecting coils of a cathode-ray tube for television purposes. The flyback time is t₁ and the scanning time is t in the figure.

In the pulse method of Breit and Tuve, for measuring the height of the ionosphere, short pulses are radiated from a transmitter. A nearby receiver picks up the direct pulse almost immediately, and then the echo pulse returning from the ionosphere at a later time. The time delay between the two pulses is measured with a string or cathode-ray oscillograph and is a direct measure of the " virtual " height of the ionosphere above the earth. Similarly, pulses from an airplane, reflected from the ground, can be used to measure its altitude. Indeed, the distance out to any object capable of reflecting the pulses can conceivably be measured in this fashion.

A direct measurement of the velocity of radio waves over short paths has been made¹ by sending out pulses from a fixed station. These were received at a portable station and re-radiated back to the first station. The time delay represented that taken by the radio wave in traveling twice the distance to the mobile station plus the delay time in the electrical circuits. The latter was eliminated by taking observations with the portable unit at two different known distances. The velocity of radio waves was found to be 2.985 · 10¹⁰ cms. per sec, which is in essential agreement with that of light.