John,
Yes, the NF will increase but only by a very small amount, as Tom has
previously shown...
<<<Snip<<<
If the amp is first (and having described it as a power amp, this might
raise some eyebrows), then:
Ft = 5 + (10-1)/100 = 5 + .09, or just over 7 db.
Ft = 10 log 5.09 = 7.067 dB, so there is a small increase of 0.067 dB.
Not easy to measure the difference, you certainly will not hear the
difference!
YMMV
What's driving this question for you?
73's, George
----- Original Message -----
From: "John Santillo" <u1004467@warwick.net>
To: "'George Sintchak'" <wa2vnv@optonline.net>; "'VHF Contesting Reflector'"
<vhfcontesting@contesting.com>
Sent: Wednesday, October 24, 2012 8:16 AM
Subject: RE: [VHFcontesting] PA Question
George et al, let me rephrase.
Let's say I have a radio with a spec'd output Noise Figure of 7dB. Now I
add a 10dB pad to the output of the radio. Using the Friis equation does
the Noise Figure increase? I understand having to linearize the dB
values.
TNX,
N2HMM
-----Original Message-----
From: VHFcontesting [mailto:vhfcontesting-bounces@contesting.com] On
Behalf
Of George Sintchak
Sent: Wednesday, October 24, 2012 12:15 AM
To: 'VHF Contesting Reflector'
Subject: Re: [VHFcontesting] PA Question
Where is the pad added - input or output?
You don't say what the gain of the amplifier is.
Case 1. If you put the 10dB pad on the input of an amplifier, the Noise
Figure becomes 17 dB, (10+7) and the overall gain is reduced by 10 dB.
If you put the pad on the output of the preamplifier, it also reduces the
gain of the preamp by 10 dB, and the Noise Figure will also be reduced
(somewhat) depending on the gain of the amplifier.
Case 2. With the pad on the output, let's assume the amplifier gain is 20
dB.
The Ft = 7 + (10-1)/20 or 7+ 0.45 = 7.45 dB NF
The Friss equation is applied for (usually) amplifiers in series, showing
that the overall NF is dominated by the 1st stage NF (and gain) as long as
it has resonable (high) gain. You need to be more specific about what you
are asking.
73's, George, WA2VNV
HF+50-1296, SOLP, FN30kv
----- Original Message -----
From: "John Santillo" <u1004467@warwick.net>
To: "'VHF Contesting Reflector'" <vhfcontesting@contesting.com>
Sent: Tuesday, October 23, 2012 9:39 PM
Subject: [VHFcontesting] PA Question
Hello,
I have a PA Noise Figure Question.
If a power amplifier has a Noise Figure of say 7dB and a series pad of
10dB
is added does the Friss Equation hold true for calculating the increased
Noise Figure? For example, using the equation below I calculated a Noise
Figure of just over 19dB with the pad inline. If this is the case, can
the
carrier coming out of the PA be overwhelmed by the noise?
Is the 7dB Noise Figure really the noise out of an amplifier with the
input
terminated above kTB?
Ft = F1 + ( F2-1) / G1
Thanks in advance,
73,
John
N2HMM
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