An oscilloscope is easily the most useful instrument available for testing circuits because it allows you to see the signals at different points in the circuit. The best way of investigating an electronic system is to monitor signals at the input and output of each system block, checking that each block is operating as expected and is correctly linked to the next. With a little practice, we will be able to find and correct faults quickly and accurately.
The screen of a CRO is very similar to a TV, except it is much simpler. We will not go into the similarities except to say that the “picture tube” on a TV and the “screen” on a CRO are both a special type of valve called a Cathode Ray Tube.
It is a vacuum tube with a cathode (negative electrode) at one end that emits electrons and anodes (positive electrodes) to accelerate the electron beam up/down and left/right to hit a phosphor coating at the end of the tube, called the screen.
The electrons are called cathode rays because they are emitted by the cathode and this gives the oscilloscope its full name: Cathode Ray Oscilloscope or CRO.
The main part of the C.R.O. is a highly evacuated glass tube housing parts which generates a beam of electrons, accelerates them, shapes them into a narrow beam, and provides external connections to the sets of plates for changing the direction of the beam.
As well as this tube, there are several electronic circuits required to operate the tube, all within the C.R.O. along with the tube:
intensity and hence the brightness of the screen spots. Don’t run the intensity too high, just bright enough for clear visibility. Always have the spot sweeping left to right or the beam may “burn” a hole in the screen.
(a) X VIA A In this position, an external signal connected to input A is used in place of the internally generated ramp. (This is also known as xy mode.)
(b) .5, 1, 2, 5, etc. Here the internally generated ramp voltage will repeat such that each large (cm) horizontal division corresponds to .5, 1, 2, 5, etc. ms. or µs depending on the multiplier and magnitude settings. (Note also the x1/x5 switch in 14 above.)
A function generator is a device that can produce various patterns of voltage at a variety of frequencies and amplitudes. It is used to test the response of circuits to common input signals. The electrical leads from the device are attached to the ground and signal input terminals of the device under test.
Most function generators allow the user to choose the shape of the output from a small number of options.
The amplitude control on a function generator varies the voltage difference between the high and low voltage of the output signal.
The direct current (DC) offset control on a function generator varies the average voltage of a signal relative to the ground.
The frequency control of a function generator controls the rate at which output signal oscillates.
On some function generators, the frequency control is a combination of different controls.
One set of controls chooses the broad frequency range (order of magnitude) and the other selects the precise frequency.
This allows the function generator to handle the enormous variation in frequency scale needed for signals.
The duty cycle of a signal refers to the ratio of high voltage to low voltage time in a square wave signal.
Analog function generators usually generate a triangle waveform as the basis for all of its other outputs. The triangle is generated by repeatedly charging and discharging a capacitor from a constant current source. This produces a linearly ascending or descending voltage ramp. As the output voltage reaches upper and lower limits, the charging and discharging is reversed using a comparator, producing the linear triangle wave. By varying the current and the size of the capacitor, different frequencies may be obtained.
A 50% duty cycle square wave is easily obtained by noting whether the capacitor is being charged or discharged, which is reflected in the current switching comparator’s output. Most function generators also contain a non-linear diode shaping circuit that can convert the triangle wave into a reasonably accurate sine wave. It does so by rounding off the hard corners of the triangle wave in a process similar to clipping in audio systems.
The type of output connector from the device depends on the frequency range of the generator. A typical function generator can provide frequencies up to 20 MHz and uses a BNC connector, usually requiring a 50 or 75 ohm termination. Specialised RF generators are capable of gigahertz frequencies and typically use N-type output connectors.
Function generators, like most signal generators, may also contain an attenuator, various means of modulating the output waveform, and often the ability to automatically and repetitively “sweep” the frequency of the output waveform (by means of a voltage-controlled oscillator) between two operator-determined limits. This capability makes it very easy to evaluate the frequency response of a given electronic circuit.
Some function generators can also generate white or pink noise.
More advanced function generators use Direct Digital Synthesis (DDS) to generate waveforms. Arbitrary waveform generators use DDS to generate any waveform that can be described by a table of amplitude values.
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