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Re: 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
>
>
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