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Noise Figure Measurement??? - RF Cafe Forums
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Simon
Liu |
Post subject: Noise Figure Measurement???
Posted: Wed Sep 06, 2006 11:41 am
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Captain |
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Joined: Wed Jan 05, 2005
2:20 am Posts: 13 Location: BeiJing,China
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Today, I tried to measure the NF of RF power amplifer
with Aglient Spectrum analyzer and Signal generator.
I know there is three ways to measure NF, 1.
use NF analyzer (I don't have one) 2. Q-factor
(I don't have two noise source) 3. Gain method.
NF = Pn(tot)@output+174dbm-Gain The problem
as following, 1. The Pn(tot)@output is ~-70dbm[pwr
in 0dBm, Gain(RFPout-RFPin):32dB, PA is working(Vcc(3.5v),
Vpc(2.2v, 25%DC)), center frequency 1710MHz]. So
base on the result, the NF of PA should be ~68dB:-(.
it is impossible. My question are 1. Did
I do something wrong during test NF? 2. Did the
formula right for cal NF by Gain method? 3. Do
I need to set the fixed RBW for Spectrum Analyzer?
Is there any relationship between NF and RBW setting
for SpecAnalyzer? 4. Could anyone provide me
some other ways to test it with my available equipment?
as well as any helpful appnote.
Simon Liu
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languer |
Post subject:
Posted: Wed Sep 06, 2006 3:20 pm
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Captain |
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Joined: Sun May 14, 2006
8:53 pm Posts: 20 Location: Earth
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First let me correct one statement:
Quote:
2. Q-factor (I don't have two noise source)
This should be Y-factor. I was going
to mention that to use the last method (which you
call Gain Method)
you have to make sure you're most definitely above
the Analyzer Noise Floor. But it seems that in your
application (PA) you may well be. With this
said, this is how I would go by doing it:
1) Set the analyzer to the proper settings (i.e.
ref level, minimum required attenuation, freq
span, RBW/VBW). Note that attenuation and frequency
span will affect the analyzer noise floor (RBW/VBW
will as well, but the Analyzer can take these
effects out). So the narrower the freq span
the better resolution you'll have and the lower
the internal attenuation (this will be set by
the incident power into the Analyzer) the better
resolution as well.
2) Look at the noise
floor before and after transmission. You should
see at least a 10dB jump while transmitting.
This will indicate that the noise generated
by the transmitter (i.e. noise you want to measure)
is at least 10dB above the analyzer noise floor.
3) Measure the noise power (Pn)
while transmitting. Now you can use your equation,
where the thermal noise power is -174dBm/Hz.
Which means that you have to either state the
measured noise power in a 1Hz BW, or state the
thermal noise in the measured BW. To make it
clearer, if your measuring BW (i.e. RBW) is
1kHz, the thermal noise will be 30dB higher
(i.e. -174dBm/Hz + 10*log(1kHz) = -144dBm).
Most Analyzers come with a noise marker which
will allow you to measure noise power in a 1Hz
BW directly, then you can apply your equation
directly.
Note:
1) A notch filter maybe useful to notch transmitter
power and reduce level into Analyzer. This will
allow to go with minimum attenuation (0dB?)
which will give the best Analyzer noise floor
(w/o the use of an external LNA). 2) Remember
when measuring noise to always list your measuring
bandwidth.
Hope this helps.
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Simon Liu |
Post subject:
Posted: Wed Sep 06, 2006 10:20 pm
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Captain |
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Joined: Wed Jan 05, 2005
2:20 am Posts: 13 Location: BeiJing,China
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Simon Liu |
Post subject:
Posted: Thu Sep 07, 2006 10:34 am
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Captain |
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Joined: Wed Jan 05, 2005
2:20 am Posts: 13 Location: BeiJing,China
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I tried today, it can works.
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Posted 11/12/2012
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