Copyright: 1996 - 2024
BSEE - KB3UON
RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling
2 MB. Its primary purpose was to provide me with ready access to commonly needed
formulas and reference material while performing my work as an RF system and circuit
design engineer. The World Wide Web (Internet) was largely an unknown entity at
the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps
while tying up your telephone line, and a nice lady's voice announced "You've Got
Mail" when a new message arrived...
All trademarks, copyrights, patents, and other rights of ownership to images
and text used on the RF Cafe website are hereby acknowledged.
My Hobby Website:
clarification on the reference article - LNA's using 3db QH - RF
RF Cafe Forums closed its virtual doors in 2012 mainly due to other social media
platforms dominating public commenting venues. RF Cafe Forums began sometime around
August of 2003 and was quite well-attended for many years. By 2010, Facebook and
Twitter were overwhelmingly dominating online personal interaction, and RF Cafe
Forums activity dropped off precipitously. If the folks at
phpBB would release a version with integrated
sign-in from the major social media platforms, I would resurrect the RF Cafe Forums,
but until then it is probably not worth the effort. Regardless, there are still
lots of great posts in the archive that ware worth looking at.
Below are the old forum threads, including responses to the original posts.
|-- Amateur Radio
Gripes & Humor
-- CAE, CAD, &
Test & Measurement
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
Joined: Fri Feb 17, 2006 6:25 pm
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.
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
Joined: Wed Jul 06, 2005 5:54 pm
thanks for clarifying.