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ATF-331M4 Datasheet(PDF) 11 Page - AVAGO TECHNOLOGIES LIMITED |
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ATF-331M4 Datasheet(HTML) 11 Page - AVAGO TECHNOLOGIES LIMITED |
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11 S and Noise Parameter Measurements The position of the reference planes used for the mea- surement of both S and Noise Parameter measurements is shown in Figure 23. The reference plane can be described as being at the center of both the gate and drain pads. S and noise parameters are measured with a 50 ohm microstrip test fixture made with a 0.010” thickness aluminum substrate. Both source pads are connected directly to ground via a 0.010” thickness metal rib which provides a very low inductance path to ground for both source pads. The inductance associated with the addition of printed circuit board plated through holes and source bypass capacitors must be added to the computer circuit simulation to properly model the effect of grounding the source leads in a typical amplifier design. a matching network that will present o to the device with minimal associated circuit losses. The noise figure of the completed amplifier is equal to the noise figure of the device plus the losses of the matching network preceding the device. The noise figure of the device is equal to Fmin only when the device is presented with o. If the reflection coefficient of the matching network is other than o, then the noise figure of the device will be greater than Fmin based on the following equation. NF = Fmin + 4 Rn | s – o |2 Zo (|1 + o|2)(1- |s|2) Where Rn/Zo is the normalized noise resistance, o is the optimum reflection coefficient required to produce Fmin and s is the reflection coefficient of the source impedance actually presented to the device. The losses of the matching networks are non-zero and they will also add to the noise figure of the device creating a higher amplifier noise figure. The losses of the matching networks are related to the Q of the components and asso- ciated printed circuit board loss. o is typically fairly low at higher frequencies and increases as frequency is lowered. Larger gate width devices will typically have a lower o as compared to narrower gate width devices. Typically for FETs, the higher o usually infers that an impedance much higher than 50 isrequiredforthedevicetoproduceFmin. At VHF frequencies and even lower L Band frequencies, the required impedance can be in the vicinity of several thousand ohms. Matching to such a high impedance requires very hi-Q components in order to minimize circuit losses. As an example at 900 MHz, when air wound coils (Q>100)are used for matching networks, the loss can still be up to 0.25 dB which will add directly to the noise figure of the device. Using multilayer molded inductors with Qs in the 30 to 50 range results in additional loss over the air wound coil. Losses as high as 0.5 dB or greater add to the typical 0.15 dB Fmin of the device creating an amplifier noise figure of nearly 0.65 dB. Figure 23. Position of the Reference Planes. Gate Pin 2 Source Pin 3 Drain Pin 4 Source Pin 1 Reference Plane Microstrip Transmission Lines Px Noise Parameter Applications Information The Fmin values are based on a set of 16 noise figure mea- surements made at 16 different impedances using an ATN NP5 test system. From these measurements, a true Fmin is calculated. Fmin represents the true minimum noise figure of the device when the device is presented with an impedance matching network that transforms the source impedance, typically 50 , to an impedance represented by the reflection coefficient o. The designer must design |
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