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Re: What is the real Noise Figure of AIDC-3733????


It is my understanding (and I might be wrong) that when using an HP8970
noise figure meter, the meter assumes that you are measuring SSB noise
figure (unless you tell it) . This means that your measurement will only be
accurate if and only if a single sideband is converted to the IF. If you
have the meter in SSB and your device under test is DSB (has no image reject
filter) then there can be an error of 3dB (assuming equal bandwidth and gain
for both sidebands).

So let's assume that you have set the noise figure meter to measure SSB NF
like you're measuring a LNA and you have told the meter to do a frequency
conversion with a 2278 MHz LO. The meter turns the noise source on and off
and looks for a delta in noise at the IF. In this case not only will the
power from 2401 but also 2155 will end up at the IF. Since the detector sees
twice the power at the IF, it thinks that the NF is better than it really
is. As you know, the more power delta you see between noise source on and
off the better your noise figure is. To properly characterize the NF, the
appropriate correction factor has to be added.

Just because it has been measured, it does not mean it has been measured
right. And don't get me wrong, I am not saying that it was. I am only trying
to understand why the measurement does not make sense to me.

I have seen MANY incorrect NF measurements taken at VHF conferences by the
gurus in this hobby and everyone believes what the equipment reads. One
example is taking noise figure measurements on a cavity low noise amplifier
for two meters. Many of these amplifiers have bandwidths that are much
narrower than the measurement bandwidth of the Noise figure meter (3 MHz on
the 8790B) . These amplifiers sometimes also have return losses that are
POSITIVE ( as in return gain). In this case there are many correction
factors that need to be taken into account for mismatch and bandwidth.

What do you think?

Pieter Ibelings

----- Original Message -----
From: "Gary "Joe" Mayfield" <gary_mayfield@hotmail.com>
To: "Pieter Ibelings" <rfspace@yahoo.com>; <amsat-bb@AMSAT.Org>
Sent: Monday, May 07, 2001 21:11
Subject: Re: [amsat-bb] What is the real Noise Figure of AIDC-3733????

> Pieter,
>      Your logic is sound, but ounce of real data outweighs a pound of
> theory.  I am assuming (I know a dangerous word) that Ward, wc0y took good
> measurements with his HP8970A noise figure meter.
> http://communities.msn.com/wc0y&naventryid=112
> I do question his IF frequency when using an input of  2360 MHz.  This
> not agree with my measurements.
>      Here is my shot at an explanation.  The unit does not cover all that
> bandwidth without changing the LO.  This would explain the added expense
> (and reliability hit) of having a socket for the crystal.  If you look
> around TranSystem's web site most of the downconverters show not one
> continous, but two discrete bands.  They even brag about there ability to
> easily do "custom" LO frequencies.
> http://www.transystem.com.tw/chinese/products/waidc.htm
>      Does this make sense?  Disagree and maybe we can both learn.
> 73,
> Joe
> ka0yos@amsat.org
> ----- Original Message -----
> From: "Pieter Ibelings" <rfspace@yahoo.com>
> To: <amsat-bb@AMSAT.Org>
> Sent: Monday, May 07, 2001 5:26 PM
> Subject: [amsat-bb] What is the real Noise Figure of AIDC-3733????
> > Ok guys,
> >
> > I am a little confused here and maybe I am missing something. I have
> > reports of noise figure measurements on the transystems converter in the
> 1.2
> > dB range. I am a little troubled by these figures. I have not made the
> > measurements myself because I do not have the N connector adapter to do
> > right. Here is why I am a little confused:
> >
> > 1. The spec seems to indicate that this converter receives from 2150 to
> 2686
> > MHz so let's assume that the filtering inside is "Low Loss" at all of
> these
> > frequencies.
> > 2. The LO in a standard unit is 2278.
> >
> > When receiving 123 MHz at the IF (2401 RF), the image is at
> > MHz). From the converter specs this frequency is in the passband. We can
> > therefore assume that when receiving on 123 MHz, all of the noise power
> > present at 2155 will also be converted to 123 with little loss. This
> > result in a doubing of the noise floor at 123 MHz. The noise figure
> be
> > low in the preamp but since the image is also being converted, the
> resulting
> > conversion NF will be around 3 dB worse. This makes it impossible to
> an
> > effective NF of 1.2 dB since subtracting 3 dB from this figure would
> result
> > in -2.2dB NF if an image filter is used. Since the desired signal (at
> 2401)
> > is only contained in ONE sideband and the DUT is converting BOTH
> sidebands,
> > shouldn't the NF be 4.2 instead? The 3 dB increase assumes equal
> conversion
> > gain and bandwidth for both sidebands.
> >
> > Now the interesting part is:
> > If we change the LO to 2256 and increase the IF to 145 MHz, the Image
> moves
> > to (2256-145=2111 MHz). Based on the sticker on the case of the unit,
> > frequency is not supported. We can assume that the filter is lossy and
> > should get up to 3 dB better noise figure out of the converter.
> >
> > Does all of this make sense? Maybe my brain is shot after being up all
> night
> > and all weekend (like the bats) listening to ao-40.
> >
> >
> > Check out page 20 of this PDF:
> > http://www.stanford.edu/class/ee314/Handout/HO11.pdf
> >
> >
> > Pieter N4IP
> >
> >
> > ----
> > Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
> > To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
> >

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