Encyclopedia of Science

[seth.roberts]

I am currently attempting an experiment to determine Transmission losses of different panels underwater. I am using a single transducer at 224kHz and firing pulses at a spherical target. I can see the target through the recieved pulses on my ossciloscope but I can't seem to calculate a sensible value for the transmission loss. Is there anyone who has conducted tests on panels who might be able to help?

[CynthiaA]

The trace

In its simplest mode, the oscilloscope repeatedly draws a horizontal line called the trace across the middle of the screen from left to right. One of the controls, the timebase control, sets the speed at which the line is drawn, and is calibrated in seconds per division. If the input voltage departs from zero, the trace is deflected either upwards or downwards. Another control, the vertical control, sets the scale of the vertical deflection, and is calibrated in volts per division. The resulting trace is a graph of voltage against time (the present plotted at a varying position, the less recent past to the left, the most recent past to the right).

If the input signal is periodic, then a nearly stable trace can be obtained just by setting the timebase to match the frequency of the input signal. For example, if the input signal is a 50 Hz sine wave, then its period is 20 ms, so the timebase should be adjusted so that the time between successive horizontal sweeps is 20 ms. This mode is called continual sweep. Unfortunately, an oscilloscope's timebase is not perfectly accurate, and the frequency of the input signal is not perfectly stable, so the trace will drift across the screen making measurements difficult.


Trigger

To provide a more stable trace, modern oscilloscopes have a function called the trigger. When using triggering, the scope will pause each time the sweep reaches the extreme right side of the screen. The scope then waits for a specified event before drawing the next trace. The trigger event is usually the input waveform reaching some user-specified threshold voltage in the specified direction (going positive or going negative).

The effect is to resynchronise the timebase to the input signal, preventing horizontal drift of the trace. In this way, triggering allows the display of periodic signals such as sine waves and square waves. Trigger circuits also allow the display of nonperiodic signals such as single pulses or pulses that don't recur at a fixed rate.

Types of trigger include:

external trigger, a pulse from an external source connected to a dedicated input on the scope.
edge trigger, an edge-detector that generates a pulse when the input signal crosses a specified threshold voltage in a specified direction.
video trigger, a circuit that extracts synchronising pulses from video formats such as PAL and NTSC and triggers the timebase on every line, a specified line, every field, or every frame. This circuit is typically found in a waveform monitor device.
delayed trigger, which waits a specified time after an edge trigger before starting the sweep. No trigger circuit acts instantaneously, so there is always a certain delay, but a trigger delay circuit extends this delay to a known and adjustable interval. In this way, the operator can examine a particular pulse in a long train of pulses.

[sciencehelp00]

i need help on what 3 main coral reefs are post to me or message if you know. Please

[CynthiaA]

Coral reefs are classified into 3 main types:

Fringing Reefs are continuous with the shore and are exposed at low tide. They are coral platforms.

Barrier Reefs can surround an island with a lagoon. They are separated from shore by a wide, deep lagoon.

Atoll Reefs contain passages to the sea and surround lagoons with no center island.

The Great Barrier Reef off of NE Australia is the largest known complex of coral reefs. It is 10 miles to 90 miles (16-145 km) wide and about 1,250 miles (2010 km) long. It is separated from the shore by a lagoon 10 miles to 150 miles (16 - 240 km) wide.