HI John...
The Friis equation extends out with a term for every stage of gain or loss
in the system. So adding the pad at the end of the chain adds its noise
factor, minus 1, and then divides it by all of the gain ahead of it. Thus
the contribution usually becomes very small. That's why you seldom see Friis
written with more than 2, or sometimes three terms.
Tom Holmes, N8ZM
Tipp City, OH
EM79
> -----Original Message-----
> From: VHFcontesting [mailto:vhfcontesting-bounces@contesting.com] On
Behalf
> Of John Santillo
> Sent: Wednesday, October 24, 2012 8:16 AM
> To: 'George Sintchak'; 'VHF Contesting Reflector'
> 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
> >
> >
> >
> > _______________________________________________
> > VHFcontesting mailing list
> > VHFcontesting@contesting.com
> > http://lists.contesting.com/mailman/listinfo/vhfcontesting
>
> _______________________________________________
> VHFcontesting mailing list
> VHFcontesting@contesting.com
> http://lists.contesting.com/mailman/listinfo/vhfcontesting
>
> _______________________________________________
> VHFcontesting mailing list
> VHFcontesting@contesting.com
> http://lists.contesting.com/mailman/listinfo/vhfcontesting
_______________________________________________
VHFcontesting mailing list
VHFcontesting@contesting.com
http://lists.contesting.com/mailman/listinfo/vhfcontesting
|