replacing the serial port on Personal Computers (PC). It
provides high speed flexible interconnectivity between a
PC and its peripherals. It is a low cost solution which
supports data transfer rates up to l2Mbps (USB 1.0) and
480Mbps (USB 2.0). It also provides 5V supply to the
peripheral device. High level of emission was observed in
the power supply wires of the USB while a satellite
receiver (peripheral) was evaluated for its functionality.
On analysis, it was observed that emission from USB can
affect the performance of the peripheral unit depending
on the sensitivity of peripheral device to the power supply
spurious noise. In this paper, the emissions from a typical
PC and methods towards improvement of the emissions
are analysed. Precautions to he taken whenever any
peripheral device is evaluated, for EMI performance, are
discussed. USB, even though a powerful interface, should
he carefully used for a overall performance of the system
in which it is utilized.
1.UNIVERSAL SERIAL BUS
USB is a small port similar to a RJ-45 or RJ-11
connector provided in Personal Computers. Many types
of devices can be connected to the PC using USB. For
example, keyboards, mouse, scanners, finger print
scanners etc. that are usually connected to a serial or
parallel port can be connected to the USB.
The higher data rate of USB 2.0 i.e. 480Mbps opens
up the possibility of connecting next generation, higher
speed and higher resolution printer and scanner devices.
USB transfers signals and power over a four-wire cable
as shown in Figure 1.
The signaling occurs over two wires. There are two
data rates in USB 1.0 itself:
1 The full speed signaling at 12 Mbps rate.
1 A low speed signaling mode at 1.5 Mbps rate
(Fig.1 USB Cable)
Both modes can be supported in the same USB bus
by automatic dynamic mode switching between
transfers. The clock is transmitted, encoded along with
differential data. A SYNC field precedes each packet to
allow the receiver to synchronize their bit recovery
clocks. The cable also carries VBus and GND wires to
deliver power to peripheral devices. Vbus is nominally
+5V at the source. The USB allows cable segments of
variable lengths, up to several meters, by choosing the
appropriate conductor gauge to match the specified IR
drop and other attributes such as device power budget
and cable flexibility.
USB hubs are available and generally support 4 USB
devices. They are self-powered and have a local power
supply that furnishes power to all down stream ports.
Thus USB is going to remain as the communication
port of the PC’s in the coming years; Other than the
regular peripherals, there are some special devices
which function with USB such as satellite receivers for
audio broadcast. Noise problems were noticed during
the functional evaluation of a satellite receiver, which
uses USB bus. The details of the EMI problems and
possible solutions are discussed in this paper.
更新1:
是中文...... 不是台語..... sorry...