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ATF-38143-TR2 Datasheet(PDF) 9 Page - Agilent(Hewlett-Packard) |
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ATF-38143-TR2 Datasheet(HTML) 9 Page - Agilent(Hewlett-Packard) |
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9 / 14 page  9 Noise Parameter Applications Information Fmin values at 2 GHz and higher are based on measurements while the Fmins below 2 GHz have been extrapolated. The Fmin values are based on a set of 16 noise figure measurements 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 pre- sented with an impedance matching network that trans- forms the source impedance, typically 50 Ω, to an impedance represented by the reflection coefficient Γ o. The designer must design a matching network that will present Γ 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 Γ o. If the reflec- tion coefficient of the matching network is other than Γ o, then the noise figure of the device will be greater than Fmin based on the following equation. NF = Fmin + 4 Rn | Γ s – Γo | 2 Zo (|1 + Γ o| 2)(1 – Γ s| 2) Where Rn/Zo is the normalized noise resistance, Γ o is the opti- mum reflection coefficient required to produce Fmin and Γs 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 associated printed circuit board loss. Γ o is typically fairly low at higher frequencies and increases as frequency is lowered. Larger gate width devices will typically have a lower Γ o as compared to narrower gate width devices. Typically for FETs, the higher Γ o usually infers that an impedance much higher than 50 Ω is required for the device to produce Fmin. 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 imped- ance requires very hi-Q compo- nents 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 muilti-layer 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. A discussion concerning calculated and measured circuit losses and their effect on amplifier noise figure is covered in Agilent Application 1085. 88759/05-6.PM6.5J 2001.04.26, 9:18 AM Page 9 Adobe PageMaker 6.5J/PPC |
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