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



Pieter,

    All quite valid!  I did  not make the measurement, so I cannot claim one
way or another for sure.  His measurements did agree with the company specs
(There again we all know they wouldn't fudge the numbers hihi).  I will be
interested in what performance improvements you see with the filter.  If it
is noticable I will next in line to do it.

     After more thought I think you are probably right.  2401 is not in the
original passband, but notched out.  This could explain a lot.

73,
Joe
ka0yos@amsat.org

P.S. Are you intending to get the connector.  If not, and you have access to
the meter.  I would be happy to try to find one for you.

----- Original Message -----
From: "Pieter Ibelings" <rfspace@yahoo.com>
To: "Gary "Joe" Mayfield" <gary_mayfield@hotmail.com>; <amsat-bb@AMSAT.Org>
Sent: Monday, May 07, 2001 8:44 PM
Subject: Re: [amsat-bb] What is the real Noise Figure of AIDC-3733????


> Joe,
>
> 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
> N4IP
> N4IP
>
>
>
>
>
> ----- 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
> would
> > 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
> seen
> > > 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
> it
> > > 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
> (2278-123=2155
> > > 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
> will
> > > result in a doubing of the noise floor at 123 MHz. The noise figure
> might
> > 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
> have
> > 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,
> this
> > > frequency is not supported. We can assume that the filter is lossy and
> we
> > > 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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