|
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
Top 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.
Top 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
|