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Clarification on the Reference Article - LNA's using 3db QH - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement


balajisu
Post subject: clarification on the reference article - LNA's using 3db QH Posted: Thu Jul 06, 2006 9:51 am

Captain


Joined: Wed Jul 06, 2005 5:54 pm
Posts: 24
Location: Germantown,MD
Hi

Below is the link from the reference section. This paper uses Quadrature Hybrid cascaded with LNA to get better VSWR. I believe the Quadrature Hybrid provides 3dB loss. Assuming the LNA is designed to have a very low NF . Is it true that the net NF of the cascaded system will be closer to the NF/loss provided by the Quadrature Hybrid. So if this is true what is the true significance of this design technique. Any comments in this regard would be appreciated.

www.rfcafe.com/references/articles/ LNA_Design_Using_Quadrature_Hybrid.PDF


Thanks
Balaji


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Joe
Post subject: LNAPosted: Thu Jul 06, 2006 3:27 pm

Captain

Joined: Fri Feb 17, 2006 6:25 pm
Posts: 6
Location: US
The increase in noise figure will be due to the actual resistive loss of the coupler. This could be .1dB, .25dB .5dB, or even 1dB. The actual loss is dependent on the frequency and the type of construction. The 3dB referred to is the result of the split with 1/2 the power going to each output. It is not real loss.

The signal and noise at the input are both split, amplified by the amplifier in each arm, and recombined. The net gain of the system is the gain of one amplifier (they are both the same to maintain balance) less the real loss of the coupler. The noise figure is the noise figure of the amplifier stage degraded by the resistive losses of the input hybrid. This will be tenths of dB's not 3dB.

The advantage of the balanced amplifier is that it is easier to match, partictularly if a wider bandwidth is required. The noise figure will be a few tenths of a dB worse than not using the hybrid. In most cases this is a good tradeoff.


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balajisu
Post subject: Posted: Thu Jul 06, 2006 5:17 pm

Captain


Joined: Wed Jul 06, 2005 5:54 pm
Posts: 24
Location: Germantown,MD
thanks for clarifying.

regards
Balaji







Posted  11/12/2012

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