一切從Concert開始......

xover

5. How Does a Power Line Interference Filter Work?

As passive, bilateral networks, Power line interference filters have complex transfer characteristics, which are extremely dependent upon source and load impedance. The magnitude of this transfer characteristic describes the attenuation performance of the filter. In the power line environment, however, the source and load impedance's are not defined. Therefore the industry has standardized practices to verify filter uniformity through measurement of attenuation with 50ohm resistive source and load terminations. This measurement is defined to be the Insertion Loss (I.L.) of the filter:
I..L. = 10 log * (P(l)(Ref)/P(l))
P(l)(Ref) is the power transferred from the source to the load without the filter;
P(l) is the power transferred when a filter is inserted between the source and load.
The Insertion Loss may also be expressed in terms of voltage or current ratios as shown: I.L. = 20 log *(V(l)(Ref)/V(l)) I.L. = 20 log *(I(l)(Ref)/I(l))
V(l)(Ref) and I(l)(Ref) are measured without a filter ;
V(l) and I(l) are measured with a filter.
It is important to note that Insertion Loss does NOT describe the RFI attenuation provided by a filter in the power line environment. In the power line environment the relative magnitudes of the source and load impedances must be estimated and the appropriate filter configuration selected such that the greatest possible impedance mismatch occurs at each termination. This dependence of filter performance on terminating impedances is the basis for the concept of "mismatching networks".

xover

6. How to Perform Conducted Tests?

Conducted testing requires a quiet RF environment (usually a shielded enclosure) with a line impedance stabilization network, and an RF voltage measurement instrument such as a tuned receiver or spectrum analyzer. The RF ambient of the test environment should be at least 20dB below the desired compliance limit to get accurate results. The line impedance stabilization network (LISN) is required to establish a defined source impedance for the power line input. This is an important part of the test procedure, since this impedance directly affects the measured emission levels. The correct bandwidth for the measurement receiver is a critical test parameter as well.

xover

補充...

Electromagnetic interference (or EMI, also called radio frequency interference or RFI) is a disturbance that affects an electrical circuit due to either electromagnetic conduction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit. The source may be any object, artificial or natural, that carries rapidly changing electrical currents, such as an electrical circuit, the Sun or the Northern Lights.

EMI can be intentionally used for radio jamming, as in some forms of electronic warfare, or can occur unintentionally, as a result of spurious emissions for example through intermodulation products, and the like. It frequently affects the reception of AM radio in urban areas. It can also affect cell phone, FM radio and television reception, although to a lesser extent.

xover

Broadcast transmitters, two-way radio transmitters, paging transmitters, and cable TV are potential sources of RFI and EMI. Other possible sources of interference include a wide variety of devices, such as doorbell transformers, toaster ovens, electric blankets, ultrasonic pest control devices, electric bug zappers, heating pads, and touch controlled lamps. Multiple CRT computer monitors or televisions sitting too close to one another can sometimes cause a "shimmy" effect in each other, due to the electromagnetic nature of their picture tubes, especially when one of their de-gaussing coils is activated.

Electromagnetic interference at 2.4 GHz can be caused by  wireless devices, Bluetooth devices, baby monitors and cordless telephones, video senders, and microwave ovens.

xover

Switching inductive loads, such as electric motors, often cause interference, but it is easily suppressed by connecting a snubber network, a resistor in series with a capacitor, across the switch. Exact values can be optimized for each case, but 100 ohms in series with 100 nanofarads is usually satisfactory.

Switched-mode power supplies can be a source of EMI, but have become less of a problem as design techniques have improved, such as integrated power factor correction.

Most countries have legal requirements that mandate electromagnetic compatibility: electronic and electrical hardware must still work correctly when subjected to certain amounts of EMI, and should not emit EMI which could interfere with other equipment (such as radios).

xover

xover

xover

xover

Filtering AC line - one of the best ideas in audio engineering


This page is NOT for the people who don't know which end of the soldering iron to hold.
Don't kill yourself with electricity and above all - don't blame me. Leave your will at a lawyers office before embarking on the AC messing project.


Lets discuss the AC filtering for a moment.
I am a strong believer in the idea of  filtering RFI from AC in our wall. RFI means radio frequency interference.  But at the same time I am a NON BELIEVER in AC REGENERATORS and all devices claiming to make "perfect AC sinusoid".  There are many cases of products and their makers who claim to offer "improved AC" or perfect AC, and usually they write about light dimmers and appliances introducing "dirty noises" into the AC and this creates noise in our equipment. Absolute majority of this writing is a pure nonsense and it is only meant to SCARE YOU and make you buy the power box for 1000 dollars or so.

xover

Lets talk about FACTS. The AC powered audio devices in fact DON'T USE AC VOLTAGE AT ALL !!!

They are all DC devices and hence they need clean DC not AC. So creating "pure sine" is a most stupid, utterly heretic waste of time.

The AC, after entering our gear, becomes straight away brutally rectified into DC and that is the end of the road for the sine. Then the big electrolytic capacitors accumulate the electrons in a "bottle". These electrons become a source of energy for the active devices in our gear. They are being released slowly as a DC current.
THESE ELECTRONS HAVE NO MEMORY of their "childhood". They do neither remember nor care if they arrived in a form of perfect sine, cosine, square wave, or chaotic ripples. As long as they arrived, their sins are forgiven in the capacitor and they are purified.  So in other words - the hi fi gear is  indifferent to AC shape.