2 Watt Fixed Attenuator DC-6 GHz | 50HFFA-XXX-2/6 with N male/female

Using a fixed attenuator to reduce output VSWR of a low noise amplifier (LNA)

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Bad output VSWR of a low noise amplifier (LNA) can be an issue when being connected to devices in a RF assembly.  The amplifier’s high VSWR creates an impedance mismatch that can lead to excessive ripple in the through loss of the RF assembly.  A common solution is to add a fixed attenuator on the output of the amplifier to mask the VSWR.  The block diagram below has a 6dB fixed attenuator separating the amplifier and the 8Way power divider/combiner.  The 6dB attenuator can reduce 2.0:1 VSWR down to 1.18:1 VSWR.  Most reactive power divider/combiners require a VSWR of < 1.5:1 on all ports for the unit to function properly.

Does adding the fixed attenuator affect the Noise Figure of the assembly?

By adding the fixed attenuator on the output of the amplifier, the noise figure (NF) of the entire RF assembly is hardly affected.  Passive devices have a noise figure that is equal to their insertion loss.  The 6dB fixed attenuator (passive device) has a noise figure equal to its through loss of -6dB.  The 8Way power divider (passive device) has a noise figure equal to its insertion loss of -10dB.  Using a cascaded noise figure calculator and the below figures, the noise figure for the entire RF assembly is just 4.2dB.  Not much higher than the amplifier’s noise figure of 4dB.

Using a fixed attenuator with a low noise amplifier

 

 

 

 

 

Avoid adding a fixed attenuator on the input side of an amplifier

If you want to use a fixed attenuator to adjust the overall through loss of your RF assembly, always add the fixed attenuator on the output side of the amplifier.  In the below block diagram, a 6dB fixed attenuator was wrongly added to the input side of the amplifier.  This will have a large impact to the noise figure of the entire assembly.  Adding 6dB of loss before the amplifier will add 6dB to the noise figure to the RF assembly.  Using the cascaded noise figure calculator with the 6dB fixed attenuator as the first item, the noise figure of the entire RF assembly is 10.06dB.  That is drastically higher than the amplifier’s noise figure of 4dB.