At least one have to know that what signal a circuit having in and also give at its output. So oscilloscope and this kind of display device is of great importance. There are a lot of model of its found in market. Tektronix is a worldwide brand for oscilloscope. this company provide both analog and digital oscilloscope.
Other company doing so are Agilent, Kenwood, Instec, Uni-T.
Some company also provide hendheld type of oscilloscope with multimeter.
These machines are divided also according to measurable frequency limit so as 20 MHz, 40MHz, 60MHz, 100MHz and another parameter is sample rate for it such as 10000 sample/s, 20000 sample/s, 30000sample/s even some are
about 1M sample/s so far.
Oscilloscope are of different capability such as 2-channel, 4-channel or even with 6-8 channel. All channel work with same time and show real time graph on crt or lcd display.
If you need to design a filter circuit then you must need a oscilloscope.
