An Oscilloscope (more often called a 'scope), does for electronic equipment what an X-Ray machine does for the human body. It allows you to take a peek into a circuit to see what is happening has it happens, irrespective of the speed with which it is taking place. In fact the heart monitor is just a specialised form of oscilloscope.
The screen of a basic oscilloscope is able to graphically display any regular waveform. A spot of light crosses the screen from left to right (the horizontal X axis) whilst on its way it is offset up or down by voltage (the vertical Y axis). If this takes place fast enough, a regular 'wiggly' line will be produced on the screen, representing a quickly changing voltage on the circuit under test. If the voltage is constant, a straight line results.
In normal use, the circuit under test is always connected to the Y axis and the motion of the spot along the X axis, is generated by what is known as the time base. The time base is triggered to start its 'sweep' across the screen by acertain event or excursion of the measured waveform (the input). It can be set to trigger on a set voltage of the input as it rises or as it falls, once triggered it continues until it reaches the far side of the screen, travelling at a perfectly linear speed. In other words the spot waits on the left until triggered, then travels quickly left to right across the screen whilst writing the waveform. It then flies back to the left, during which the spot is turned off (flyback).
The above triggering delay ensures that the spot will repeatedly start writing and complete writing, at exactly similar points in the waveform under examination. Thus, given a fast enough repetition rate (a fast enough frequency of waveform), an apparently continuous waveform appears on the screen. Any change in the waveform will instantly appear on the 'scopes screen.
The time base is adjustable for not only its point of triggering, but for speed with which it crosses the screen from left to right. The higher the speed or frequency of the waveform on the input, the higher the speed that the time base needs to be set. The upper limit to this writing speed, is one of the specifications of an oscilloscope.
The Y axis input, which connects to the circuit under test, has to be capable of a wide range of adjustments to cope with a very wide range of voltage inputs. For a tiny level of signal a great deal of amplification will be required, to turn this into a usable/displayable level. If the voltage is quite large, an attenuator will reduce this down to an acceptable level for the instrument. The input can be set for AC or DC coupling. AC coupling allows the waveform to float to an average of the input, useful if the waveform has a large DC offset. AC coupling has the disadvantage that at low frequencies the waveform will be distorted and measurements of actual voltage can be done on the actual waveform itself, with no indication of the DC offset.
This part of the 'scope circuit is known as the Y amplifier. The maximum usable frequency that the Y amplifier is able to cope with, is the second important specification of an oscilloscope. Generally the specification of both X and Y sections will be quite similar, not much point to an X specification of 400Mhz, with a Y specification of 10Mhz. The higher the frequency specification, generally the higher will be the cost of such an instrument. Additional features (more below) will add further to the cost.
For waveforms/frequencies which are especially slow, only occur once, or for which there are long periods of inactivity, a 'Storage 'scope' is used. Once triggered by an event or waveform, the waveform briefly described on the screen will be stored, allowing the operator to take as long as he wants to inspect it. If the operator is interested in just one small part of an overall waveform, then a 'scope which would enable this to be seen is known as having a 'delayed time base'. A delay is preset between the triggering point and the start of the sweep across the screen.
Most reasonably modern scopes are switched dual input/dual beam ones, which contain two Y amplifiers. These enable two associated waveforms to be displayed at the same time on the screen. Switched, means that Y input A is displayed, then Y input B is displayed, alternating between the two. This is done so quickly that the operator just sees both displayed on the screen at the same time. For slower frequency waveforms, a 'Chop' mode is available, which switches between the displaying the A then the B waveform as the spot progresses across the screen. The time base triggering section allows the selection of triggering from either the A or B Y input. Prior to the above method of displaying two waveforms, 'scope tubes were designed to produce two spots on the screen, one for each Y input. These were known as true dual beam oscilloscopes.
In addition to the actual instrument, a pair of probes will be required. Most generally useful will be a pair of switch able x1/x10. The x10 attenuates the voltage input to the Y amplifier by a factor of 10. The probe end usually comes complete with a means to adapt it to a variety of testing points on PCB's.
A 'scopes main job, is not one of producing accurately measurable results, but more one of being able to look into a circuit and 'see' what is going on inside it. In this respect alone it can be an invaluable piece of diagnostic test equipment. Frequency (period) can be approximately measured by reference to the time base settings and the calibration points on the screen, voltage can be measured by reference to the settings of the Y amplifier and the on screen calibration points.
For general amateur radio purposes, looking into audio circuits, the IF's of most receivers and the RF output of an HF transceiver, a general purpose 'scope with a specification of 40Mhz will probably do all that is needed. There is not usually a need for a delayed time base or storage, though these extras can be useful.
New scopes of reasonable quality, depending on capability, cost £500 to several thousands of pounds. This is well outside the pockets or needs or most radio amateurs, but with a bit of care good specification modern second hand ones can be had for between £20 and £60.... Presently well below their true second hand value, due to so many being available and many electronics companies closing down. This also applies to many items test equipment, making it a good time to set up your own facilities.