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Attenuator circuits
Component values for resistive pads and attenuators
This section concerns pi-pads, T-pads and L-pads made entirely from resistors and terminated on each port with a purely real resistance.
- All impedance, currents, voltages and two-port parameters will be assumed to be purely real. For practical applications, this assumption is often close enough.
- The pad is designed for a particular load impedance, ZLoad, and a particular source impedance, Zs.
- The impedance seen looking into the input port will be ZS if the output port is terminated by ZLoad.
- The impedance seen looking into the output port will be ZLoad if the input port is terminated by ZS.
Reference figures for attenuator component calculation
الرموز المستخدمة
Passive, resistive pads and attenuators are bidirectional two-ports, but in this section they will be treated as unidirectional.
- ZS = the output impedance of the source.
- ZLoad = the input impedance of the load.
- Zin = the impedance seen looking into the input port when ZLoad is connected to the output port. Zin is a function of the load impedance.
- Zout = the impedance seen looking into the output port when Zs is connected to the input port. Zout is a function of the source impedance.
- Vs = source open circuit or unloaded voltage.
- Vin = voltage applied to the input port by the source.
- Vout = voltage applied to the load by the output port.
- Iin = current entering the input port from the source.
- Iout = current entering the load from the output port.
- Pin = Vin Iin = power entering the input port from the source.
- Pout = Vout Iout = power absorbed by the load from the output port.
- Pdirect = the power that would be absorbed by the load if the load were connected directly to the source.
- Lpad = عشرة log10 (Pin / Pout ) always. And if Zs = ZLoad then Lpad = 20 log10 (Vin / Vout ) also. Note, as defined, Loss ≥ 0 dB
- Linsertion = عشرة log10 (Pdirect / Pout ). And if Zs = ZLoad then Linsertion = Lpad.
- Loss ≡ Lpad. Loss is defined to be Lpad.
Symmetric T pad resistor calculation
- '"`UNIQ--postMath-00000001-QINU`"' see Valkenburg p 11-3
Symmetric pi pad resistor calculation
- '"`UNIQ--postMath-00000002-QINU`"' see Valkenburg p 11-3
L-Pad for impedance matching resistor calculation
If a source and load are both resistive (i.e. Z1 and Z2 have zero or very small imaginary part) then a resistive L-pad can be used to match them to each other. As shown, either side of the L-pad can be the source or load, but the Z1 side must be the side with the higher impedance.
- '"`UNIQ--postMath-00000003-QINU`"'
- '"`UNIQ--postMath-00000004-QINU`"' see Valkenburg p 11-3
Large positive numbers means loss is large. The loss is a monotonic function of the impedance ratio. Higher ratios require higher loss.
Converting T-pad to pi-pad
This is the Y-Δ transform
- '"`UNIQ--postMath-00000005-QINU`"'
Converting pi-pad to T-pad
This is the Δ-Y transform
- '"`UNIQ--postMath-00000006-QINU`"'
Conversion between two-ports and pads
T-pad to impedance parameters
- The impedance parameters for a passive two-port are
- '"`UNIQ--postMath-00000007-QINU`"'
- It is always possible to represent a resistive t-pad as a two-port. The representation is particularly simple using impedance parameters as follows:
- '"`UNIQ--postMath-00000008-QINU`"'
Impedance parameters to T-pad
- The preceding equations are trivially invertible, but if the loss is not enough, some of the t-pad components will have negative resistances.
- '"`UNIQ--postMath-00000009-QINU`"'
Impedance parameters to pi-pad
- These preceding T-pad parameters can be algebraically converted to pi-pad parameters.
- '"`UNIQ--postMath-0000000A-QINU`"'
Pi-pad to admittance parameters
- The admittance parameters for a passive two port are
- '"`UNIQ--postMath-0000000B-QINU`"'
- It is always possible to represent a resistive pi pad as a two-port. The representation is particularly simple using admittance parameters as follows:
- '"`UNIQ--postMath-0000000C-QINU`"'
Admittance parameters to pi-pad
- The preceding equations are trivially invertible, but if the loss is not enough, some of the pi-pad components will have negative resistances.
- '"`UNIQ--postMath-0000000D-QINU`"'
General case, determining impedance parameters from requirements
Because the pad is entirely made from resistors, it must have a certain minimum loss to match source and load if they are not equal.
The minimum loss is given by
'"`UNIQ--postMath-0000000E-QINU`"'
Although a passive matching two-port can have less loss, if it does it will not be convertible to a resistive attenuator pad.
- '"`UNIQ--postMath-0000000F-QINU`"'
Once these parameters have been determined, they can be implemented as a T or pi pad as discussed above.
انظر أيضاً
- RF and microwave variable attenuators
- Optical attenuator
- دائرة رنين
الهامش
- ^ Valkenburg (1998, pp. 11_3)
- ^ Valkenburg (1998, pp. 11_3-11_5)
- ^ Hayt (1981, p. 494)
وصلات خارجية
مشاع الفهم فيه ميديا متعلقة بموضوع Attenuator (electronics). |
- Guitar amp power attenuator FAQ
- Basic attenuator circuits
- Explanation of attenuator types, impedance matching, and very useful calculator
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