The T (and below Π) configurations are most commonly used as they provide bidirectional matching. See the dB/20 term in the power of 10 term for computing the voltage ratio K from dB, above. Though, we need the voltage (or current) ratio K to find the resistor values from equations. The amount of attenuation is customarily specified in dB (decibels). Multiplying by 75/50 would convert table values to match a 75 Ω source and load.įormulas for T-section attenuator resistors, given K, the voltage attenuation ratio, and ZI = ZO = 50 Ω. Multiply all R values by the ratio (600/50) to correct for 600 Ω matching. Telephone utility and other audio work often requires matching to 600 Ω. The table in the figure below lists resistor values for the T and Π attenuators to match a 50 Ω source/ load, as is the usual requirement in radio frequency work. This impedance is a constant (50 Ω) with respect to attenuation– impedance does not change when attenuation is changed. The Z-(arrows) pointing toward the attenuator indicates that the impedance seen looking into the attenuator with a load Z on the opposite end is Z, Z=50 Ω for our case. ![]() The Z-(arrows) pointing away from the attenuator in the figure below indicate this. The T and Π attenuators must be connected to a Z source and Z load impedance. Find the attenuation in dB.ĭB = 10 log10(PI / PO) = 10 log10 (10 /1) = 10 log10 (10) = 10 (1) = 10 dBĮxample: Find the voltage attenuation ratio (K= (VI / VO)) for a 10 dB attenuator.ĭB = 10= 20 log10(VI / VO) 10/20 = log10(VI / VO) 1010/20 = 10log10(VI / VO) 3.16 = (VI / VO) = AP(ratio)Įxample: Power into an attenuator is 100 milliwatts, the power out is 1 milliwatt. Examples using the Decibel EquationsĮxample: Power into an attenuator is 10 Watts, the power out is 1 Watt. That is, the source and load resistance need to be equal. Once again, the voltage ratio form of equation is only applicable where the two corresponding resistors are equal. We will use the latter form, since we need the voltage ratio. The two most often used forms of the decibel equation are:ĭB = 10 log10(PI / PO) or dB = 20 log10(VI / VO) The power change in decibels in terms of power ratio is:Īssuming that the load RI at PI is the same as the load resistor RO at PO(RI = RO), the decibels may be derived from the voltage ratio (VI / VO) or current ratio (II / IO): A gain of -3 dB is the same as an attenuation of +3 dB, corresponding to half the original power level. An attenuation of 3dB corresponds to cutting power in half, while a gain of 3 db corresponds to a doubling of the power level. For example, a 10 dB attenuator followed by a 6 dB attenuator provides 16dB of attenuation overall.Ĭhanging sound levels are perceptible roughly proportional to the logarithm of the power ratio (PI / PO).Ī change of 1 dB in sound level is barely perceptible to a listener, while 2 db is readily perceptible. Power ratios expressed as decibels are additive. The voltage ratio must be derived from the attenuation in decibels. Voltage ratios, as used in the design of attenuators are often expressed in terms of decibels. Multiple attenuator sections may be cascaded when even weaker signals are needed as in the figure below. T section and Π section attenuators are common forms.Ĭommon configurations are the T and Π networks shown in the figure above. However, the formulation is more complex. Not considered in this section, unequal source and load impedances may be matched by an attenuator section. ![]() ![]() In this section we will only consider the special, and most common, case where the source and load impedances are equal. In addition, it must match both the source impedance Z I and the load impedance Z O, while providing a specified amount of attenuation. In the case of a stand-alone attenuator, it must be placed in series between the signal source and the load by breaking open the signal path as shown in the figure above. An attenuator section can also provide isolation between a source and a troublesome load.Ĭonstant impedance attenuator is matched to source impedance ZI and load impedance ZO. It could provide a fixed or adjustable amount of attenuation. (figure below) The attenuator could be built into the signal generator, or be a stand-alone device. Attenuators weaken or attenuate the high level output of a signal generator, for example, to provide a lower level signal for something like the antenna input of a sensitive radio receiver. It is convenient to discuss them along with decibels.
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