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R: Weak Signal Banquet NF numbers..



Hello Jon

If no selectivity is provided in front of a mixer in this case no image
rejection do exist and the mixer will exhibit
the same passband curve both for the required and image frequency.
This means two equal power are taken from the white noise spectrum,and are
added one to the other in to the mixer.
Since both  noise power have  the same value,the noise that belong to the
image band deteriorate by 3 dB the noise figure for the required band
If  we made a Noise Figure measurement  using a conventional Noise Figure
Meter (PANFI) ,when the noise
source is OFF it send a Tcold power both in to the image band and into the
required band
The same happens when the noise source is ON and send at the same time a
Thot power in to the
image band and into the required band
To measure the noise figure the instrument make the ratio
(Thot -Tcold)/Tcold  or Y factor and ask "how many time the Thot temperature
of the head minus the Tcold temperature of the head is greater than the
Tcold temperature of the head ? "
In doing this ratio or interrogation the instrument is not able to
discriminate
if  (Thot -Tcold)/Tcold belong only
to the required band or both to the image band plus the required band.
In this case if the measured Noise Figure is 1 dB it happens that in
practice work  the
noise figure for the required band in wich we receive is in reality 1+3=4 dB
In conclusion,by measuring a system that converts both sidebands with no
image rejection the actual measured noise figure is 3 dB better than that in
real work.
Modern Noise Figure Meters have now the provision to measure the single
sideband
noise figure taking into account systems that converts the noise in both
sidebands.

73 de i8CVS Domenico

----- Original Message -----
From: Jon Ogden <na9d@speakeasy.net>
To: Pieter Ibelings <elpieter@hotmail.com>; Frederick M. Spinner
<fspinner@hotmail.com>; <amsat-bb@AMSAT.Org>
Sent: Friday, May 25, 2001 4:53 AM
Subject: Re: [amsat-bb] Weak Signal Banquet NF numbers..


> Peter,
>
> While I follow and agree with what you are saying, I am not sure I
> understand your final conclusion.  I need some clarification.
>
> Are you saying that by measuring a system that converts both sidebands
with
> no image rejection that the actual noise figure is 3 dB worse or 3 dB
> better?
>
> I would think that by converting both sidebands, you are actually
generating
> more noise and if you aren't taking that into account that your noise
figure
> number will be worse than it really is.  Perhaps I have it backwards.
>
> Help?
>
> 73,
>
> Jon
> NA9D
>
> on 5/23/01 7:56 PM, Pieter Ibelings at elpieter@hotmail.com wrote:
>
> > 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
> >>
> >>
> >> _________________________________________________________________
> >> Get your FREE download of MSN Explorer at http://explorer.msn.com
> >>
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> >>
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> >
>
>
> -------------------------------------
> Jon Ogden
> NA9D (ex: KE9NA)
>
> Member:  ARRL, AMSAT, DXCC, NRA
>
> http://www.qsl.net/ke9na
>
> "A life lived in fear is a life half lived."
>
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>





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