The demodulator is an important component of a modulated DC amplifier circuit. It restores the amplified AC voltage to a DC voltage, whose magnitude and polarity correspond to the amplitude and phase of the AC voltage.


Generally speaking, modems can be roughly divided into the following four categories based on their form and installation method:

1. External modem

The external modem is placed outside the chassis and connected to the host through a serial communication port. This modem is convenient, agile, and easy to install, with flashing indicator lights for monitoring the modem's working condition. But external modems require additional power and cables.

2. Built in modem

The built-in modem requires disassembling the chassis and setting up interrupts and COM ports during installation, which is quite cumbersome. This type of modem requires an expansion slot on the motherboard, but does not require additional power or cables, and is cheaper than external modems.

3. PCMCIA plug-in modem

Plug in modems are mainly used for laptops, with a slim and compact size. Combined with mobile phones, mobile office can be easily achieved.