Post subject: clarification on the reference article
- LNA's using 3db QH Posted: Thu Jul 06, 2006 9:51 am
Joined: Wed Jul 06, 2005 5:54 pm
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.
Post subject: LNAPosted: Thu Jul 06,
2006 3:27 pm
Joined: Fri Feb 17, 2006 6:25
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.
Post subject: Posted: Thu Jul 06,
2006 5:17 pm
Joined: Wed Jul 06, 2005
thanks for clarifying.