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DEMO-ATF-5X1M4E Datasheet(PDF) 10 Page - Broadcom Corporation. |
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DEMO-ATF-5X1M4E Datasheet(HTML) 10 Page - Broadcom Corporation. |
10 / 16 page 10 S and Noise Parameter Measurements The position of the reference planes used for the measurement of both S and Noise Parameter measurements is shown in Figure 20. 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 alu‑ minum substrate. Both source leads are connected directly to ground via a 0.010" thickness metal rib which provides a very low inductance path to ground for both source leads. The inductance associated with the ad‑ dition of printed circuit board plated through holes and source bypass capacitors must be added to the computer circuit simulation to prop‑ erly model the effect of grounding the source leads in a typical amplifier design. Gate Pin 2 Source Pin 3 Drain Pin 4 Source Pin 1 Reference Plane Microstrip Transmission Lines Sx Figure 20. Noise Parameter Applications Information 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 presented with an impedance matching network that transforms the source impedance, typically 50Ω, to an impedance repre‑ sented 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 com‑ pleted 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 reflection 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 = F min + 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 optimum reflec‑ tion coefficient required to produce Fmin and Γ s is the reflection coeffi‑ cient of the source impedance actu‑ ally 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 fre‑ quency 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 50Ω is required for the device to produce Fmin. At VHF frequencies and even lower L Band frequencies, the required imped‑ ance can be in the vicinity of several thousand ohms. Matching to such a high impedance requires very hi‑Q components in order to minimize cir‑ cuit losses. As an example at 900 MHz, when airwwound 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 muiltilayer molded in‑ ductors 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 ampli‑ fier noise figure of nearly 0.65 dB. SMT Assembly The package can be soldered us‑ ing either lead‑bearing or lead‑free alloys (higher peak temperatures). Reliable assembly of surface mount components is a complex process that involves many material, process, and equipment factors, including: method of heating (e.g. IR or vapor phase reflow, wave soldering, etc) circuit board material, conductor thickness and pattern, type of solder alloy, and the thermal conductivity and thermal mass of components. Components with a low mass, such as the Minipak 1412 package, will reach solder reflow temperatures faster than those with a greater mass. The recommended leaded solder time‑temperature profile is shown in Figure 21.This profile is representative of an IR reflow type of surface mount assembly process. After ramping up from room temperature, the circuit board with components attached to it (held in place with solder paste) passes through one or more preheat zones.The preheat zones increase the temperature of the board and compo‑ nents to prevent thermal shock and begin evaporating solvents from the solder paste. The reflow zone briefly elevates the temperature sufficiently to produce a reflow of the solder. The rates of change of temperature for the ramp‑up and cool‑down zones are chosen to be low enough to not cause deformation of board or dam‑ age to components due to thermal shock. The maximum temperature in the reflow zone (Tmax) should not exceed 235°C for leaded solder. These parameters are typical for a surface mount assembly process for the ATF‑541M4. As a general guide‑ line, the circuit board and compo‑ nents should only be exposed to the minimum temperatures and times the necessary to achieve a uniform reflow of solder. The recommended lead‑free reflow profile is shown in Figure 22. |
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