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Re: Weak Signal Banquet NF numbers..



Hi Fred,

When you measure the noise figure of a down converter that convertes both
sidebends to the IF frequency, you end up with a 3dB error. Let's say you
have a 15 dB ENR noise source like a HP346 that has really flat response
from DC to 18 GHz. As you power the source on and off, the converter
converts both sidebands to the if frequency. This results in twice the noise
showing up at the IF. You get the noise from 2400 MHz and you get the noise
from the image. Both are almost the same signal level due to the flat noise
source.
When you calibrate the noise meter, it turns the noise source on and off and
looks for the delta in noise between the two settings. It only looks at the
noise "once" not "twice" as in the double conversion explained above. The
noise figure meter when measuring noise figure looks for noise delta between
a resistor and a noise source. The bigger the delta the more sensitive the
preamp or converter is. Now, if you down convert both sidebands (twice the
noise) and the calibration is based on only one bandwidth of noise, the
meter thinks that you have a really good converter because it sees a really
big delta. This results in a error of around 3 dB. So when measuring a
converter with no image rejection in a noise figure meter that expects a
single sideband, you need to add a correction factor of almost 3dB to the
measured result.

I hope this makes sense. There are many different correction factors that I
have not accounted for the bulk is here.

73's

Pieter N4IP


----- Original Message -----
From: "Frederick M. Spinner" <fspinner@hotmail.com>
To: <amsat-bb@AMSAT.Org>
Sent: Wednesday, May 23, 2001 19:54
Subject: [amsat-bb] Weak Signal Banquet NF numbers..


> Here are the numbers that were measured at the Weak Signal Banquet at
> Dayton:
>
> New DEMI 2.4 GHz Preamp (assembled by W1BFN):
>    0.58 dBNF/16.95 dB gain
> Old DEMI 2.4 GHz Preamp (assembled by W0FMS, measured at Winnipeg CSVHF
> meeting 2000):
>    0.68 dbNF/11.78 dB gain
>
> Modified Drake 2880, lengthing bandpass filter strips and IF cap mods
(W0FMS
> modifications):
>    6.00 dBNF/23.0 dB gain  @ 123 MHz IF, 2401 MHz center freq at RF
>
> Same Modified Drake 2880, with the OLD DEMI Preamp above in front:
>    1.25 dBNF/37.5 dB gain  @ 123 MHz IF, 2401 MHz center freq at RF
>
> Transystem 3033 bought at the flea, unmodified.  Note the image freq
> concerns, though..
>    1.42 dBNF/28.8 dB gain  @ 123 MHz IF, 2401 MHz center freq at RF
>
> The NF debate about the Transystem applies-- but maybe someone can explain
> to me that when you pulse a broadband noise source such as the noise head
on
> a new Agilent NF Analyzer, how the meter can distinguish one noise
sideband
> from another if both are getting converted to the IF?  So I understand the
> problem, but why doesn't it affect the NF readings?
>
> Anyway-- the T/S converter can operate in that range with careful mods,
> maybe an ISM filter, etc.... cheaply...
>
> Interestingly enough, when I massaged these numbers through the
> AO-40 spreadsheet, the gentlemen on the board harping the SSB converters
> have a point.  The SSB converters with a 0.8 dB system noise figure have
> about a 3 dB lower minimum discernable signal (noise floor) than the Drake
> w/preamp and/or the Transsystem.  Now if the massive gain could somehow be
> handled, the Transystem with either DEMI preamps in front of it could have
> an edge on the SSB converter.  Also a Drake that had the 2 dBNF or so mods
> done to it that had a DEMI preamp in front of it could also outperform the
> SSB converter.
>
> But if not, for the $350 I've saved not getting overpriced SSB electronic
> equipment, I'm close enough with the Drake/DEMI preamp or the Transystems
> units with the 3'X2' BBQ's (hell, I spent it on a G-5500 Yaesu rotor that
> seems to be backordered from Japan at Dayton..)
>
> So, this in hand, maybe I'll come up with some suggestions in the next few
> weeks for cheap, simple Mode-S recvs and I also have some ideas for mods
to
> the Transystems.
>
> Fred W0FMS
>
>
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