One minor problem with this example is that the equation works in linear
power, not dB. Convert the dB values back to their ratios to get the right
result.
A 10 dB pad is .1 in linear terms of its gain, but 10 in terms of its
noise
factor. The output S/N is 1/10th of the input S/N in linear terms. A 7 dB
Noise Figure is a Noise Factor (the linear form) of 5. If the amp has a
gain
of 20 dB, that is 100 in linear terms.
So if the pad is first in line, then Friis predicts that Ft= 10 + (5-1)/.1
=
50 or just a hair over a 17 dB NF.
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.
Working the Friis equation in dB might come close to the right answer in
some cases, but the error could lead to some bad conclusions.
Regarding the description of this as a power amplifier, one might wonder
why
noise figure is important. Remember that the definition of noise figure is
the reduction in signal to noise ratio from input to output of a device.
An
amplifier takes the incoming signal and accompanying noise and amplifies
it
all by the same amount. A perfect amplifier adds no noise, thus the output
has the same S/N. But a real amp adds some noise of its own to the input
noise, and so deteriorates the S/N. Noise figure is the dB change
(decrease)
in S/N at the output.
If a power amp (think high power linear foot warmer) is adding noise to
its
output, that is energy that will also be transmitted and potentially cause
interference. It also wastes some of the amps power on useless noise and
reduces efficiency. And then even if the added noise from the atmosphere
is
insignificant, the S/N that the signal carries with it is now the best you
can see at the receiver.
Tom Holmes, N8ZM
Tipp City, OH
EM79
-----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
>
>
>
> _______________________________________________
> VHFcontesting mailing list
> VHFcontesting@contesting.com
> http://lists.contesting.com/mailman/listinfo/vhfcontesting
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