MILITARY, AEROSPACE & DEFENCE
INDUSTRY FOCUS
RMANCE OF LOW NOISE OSPACE MARKET
ENG3 AND ENG4 In both circuit types, the attenuator was placed at the output of the LNA die to improve noise figure. Based on the Friis cascaded noise figure equation, the first stage contributes most to total noise, and introduced noise to the following stages is divided by gain of previous stages. As a result, input noise becomes less significant. In this configuration, the total noise figure is defined as: Where FT
is the total noise figure, FLNA noise figure, FATTN is the LNA is the noise figure of the attenuator,
and GLNA is the gain of the LNA. There is also a possible trade-off of gain reduction due to the loss
in the passive component attenuator following the amplifier stage. In engineering sample Eng3, ribbon bonds were used for die-to-die bonding as well
as RF in/out bonds. In contrast, sample Eng4 LNA and attenuator dies were wire-bonded with double round bond wires. Simulations (using ADS) of the two bond options using ribbon vs. double round revealed only slightly improved input return loss and gain using the ribbon bonds for die-to-die bonding. Therefore, to confirm the simulations, both types were assembled and evaluated.
COMPARING RESULTS FROM DIFFERENT LNA BONDING CONFIGURATIONS ENG3 AND ENG4 VS. ENG2 Having the attenuator at the output improves the output return loss (S22) because the signal reflections are minimised due to impedance matching. This in turn improves matching at the output and consequently results in better output return loss. The expected trade-off of having lower gain was evident for lower frequencies, but in frequencies greater than 22 GHz gain response (S21) was almost equal and even better in one Eng2 sample, which can be justified by unit-to-unit variations.
ENG3 VS. ENG4 When comparing ribbon wire bonds vs. double round bond wire samples, Eng3 had better performance across the frequency range as a result of lesser skin effect and crosstalk in ribbon bonds. Since ribbon bond has a larger surface area compared to its cross section, they have a lower resistance and therefore more power efficiency. Test results showed the ribbon bonded samples had a slightly better gain performance and almost equal or negligibly improved input return loss, and had a significantly better output return loss than double round bonds.
LNA PERFORMANCE PLOTS Figure 1 through Figure 4 illustrate S-parameters for two units of configuration for Eng2, Eng3 and Eng4. This data is obtained by probing the packaged devices. It was determined at the beginning of the evaluation of bonding options for this part that Eng1 would not be considered since simulations revealed that the configuration would prove to be the most unstable. The noise figure test data shown in Figure 5 is measured on an evaluation board.
PREDICTABLE RULES RF engineering, the so-called black magic, is just a series of predictable physics rules. Below is a summary of unlocking this magic for the LNA described in this article: • For LNAs where matching and return loss is a concern due to parasitics, including an attenuator in the package cavity is an excellent method for reducing parasitics and improving return loss. However, the following trade-offs should be considered – Attenuator at input: increases noise figure; Attenuator at output: lowers gain.
• Reducing parasitics by strategically placing the attenuator inside the package also improves S-parameters, which can be used as a measurement of stability using the µ factor, and overall helps in achieving nonconditional stability across the frequency range.
• In super high frequency (3GHz<SHF<30GHz) operations, ribbon bonds have better performance compared to round wires. The trade-off would be complexity of assembly where manufacturability needs to be considered. It is important to note that these results could have been predicted based on
fundamental RF rules and formulas. However, before assembling the different device types, simulations were run for the two die placements and different bondings in ADS and Genesys. The empirical results of the evaluations confirmed the simulations.
Analog Devices
www.analog.com/en Figure 5. Noise figure SEPTEMBER 2021 DESIGN SOLUTIONS 43 Figure 4. Stability comparison Figure 3. Gain—probe data
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