How to measure filter effectiveness

(1) Insertion loss

The noise reduction effect of a filter is represented by insertion loss. As shown in Fig. 3, a filter was inserted into a circuit connecting a 50 ohms signal source with a load, and then the change in voltage on the load side (B to C) was measured. This is normally expressed in dB.

(2) dB

dB (decibel) is a unit used to express a ratio. As shown in Table 2, the ability to reduce noise to 1/10th is expressed as 20dB, while the ability to reduce noise to 1/100th is expressed as 40dB. Each time noise drops to 1/10th, the dB increases by 20 (for insertion loss).
Using dB is very convenient as it can express a large ratio through addition. The result of measuring noise is also normally expressed in dB. For example, 40dBµV represents a voltage where 1µV is 0 dB, and so it means 100µV.

(3) Numbers to keep in mind

When noise doubles, the dB changes by 6dB. When noise increases tenfold, the dB changes by 20dB. These two numbers are used very often, so it can be useful to keep them in mind.
Note however that these numbers are used when representing voltage. For power, a tenfold increase results in a change of 10dB. This is because power has a relationship proportional to voltage squared (P=V2/R).

(4) S-parameter substitution

When a filter can be regarded as a linear circuit, insertion loss can be substituted by the absolute value of the S-parameter transmission coefficient (S21 or S12) measured on a 50 ohms system. This is because a 50 ohms measurement system is connected before and after the filter, for both measurements. Whenever possible, we use the value measured with the S-parameter as the insertion loss.

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