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



I got out the HP8970A operating manual today and put some more thought into
the method of measuring noise figure of the Transystem converter.

The 8970 measures NF by applying a wide-band noise signal to the device
under test.  This means that a nearly equal noise signal is applied to the
converter at the AO-40 frequency and its image.  The 8970 measures noise
from the device under test in a 4 MHz bandwidth at the IF frequency on its
display.  It does not do any filtering of this signal in the upper, lower,
or double sideband mode settings.  Instead, it calculates what input RF
frequency produced the IF output according to the external LO frequency and
the noise sideband mode selected.  The calibration factor for the noise head
is then applied.  Since the HP noise source is constant to within a few
tenths of a dB from 10 MHz to 18 GHz, the calibration factor that is applied
will change by only a few tenths of a dB between different noise sideband
modes.

I therefore believe that the NF measurements posted on my web page at
http://communities.msn.com/wc0y of around 1.2 dB are accurate, and could not
be influenced greatly by the noise sideband selection.

Although some image rejection is built into the converter already, Pieter's
method of using a dielectric filter may be more effective because of its
narrower bandwidth.  This, and removing the losses from the original filter
in front of the first stage amplifier probably do improve the NF.

I also want to calm anyone who removed the stub and is using the converter
with a 123 MHz IF output.  This stub was more important for image rejection
in the original MMDS application of the converter, and I doubt it will
affect NF much while receiving 2401 MHz.

Peiter, please let us know your NF results on the modified converter when
you get them.  Sounds like an interesting mod to get more performance.

73,
Ward - WC0Y

----- Original Message -----
From: "wc0y" <wc0y@msn.com>
To: "Pieter Ibelings" <rfspace@yahoo.com>; "Gary "Joe" Mayfield"
<gary_mayfield@hotmail.com>; <amsat-bb@AMSAT.Org>
Sent: Tuesday, May 08, 2001 1:33 AM
Subject: Re: [amsat-bb] What is the real Noise Figure of AIDC-3733????


> Let me give my understanding of how the AIDC-3733 is designed to work, and
> why it is consistent with low noise figures (1.2 dB) even when tested in
> double sideband noise mode.  The key to the low noise figure is the stub
> that many people are removing to increase converter gain when using a 145
> MHz IF.
>
> The 122 to 222 MHz IF output band is below the LO frequency.  Its image
> input is in the 2400 to 2500 MHz band where we hams want it to operate.
The
> designers could not allow the 2400 to 2500 MHz span to add noise and
> interference, so they added the tuned stub after the LNA to filter it out.
> That stub is what removes the image noise and keeps a good noise figure
when
> using a 123 MHz IF.
>
> The other case to consider is when the converter is modified for a 145 MHz
> IF.  Then, with a LO frequency of 2256 MHz, the input image frequency is
> 2111 MHz, and the input RF filters have rolled off to attenuate the image
> noise as you suggested.  The tuned stub does not degrade noise figure
> appreciably since it is after the LNA.
>
> So, one image noise span is suppressed for each IF frequency case; for the
> 123 MHz IF it is the stub, and for the 145 MHz IF it is the RF filter
> roll-off.
>
> Removing the stub in a 123 MHz IF converter would be a mistake.  Then, the
> image noise would come through and the noise figure would be degraded.
> Removal of the stub for a 145 MHz converter is a good thing to do.
>
> BTW, the default mode of the 8970 is to measure double sideband.  It has
to
> be programmed into a single noise band mode.
>
> I hope this makes sense when I read it in the morning.  My TranSystem
> converter is on the tower waiting for another AO-40 pass or I would take
> some more measurements to test my theory of how it operates.  Anyone want
to
> send me a spare 3317 for these measurements?
>
> 73,
> Ward - WC0Y
>
>
>
> ----- 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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> >
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