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R: Weak Signal Banquet NF numbers..



Hallo Stan,

If the DBS noise figure measured on the 24 GHz converter is 8 dB and if i am
not able to prevent  the image noise to reach the input of the mixer  it is
like
to receive in 24 GHz having a 24 GHz single sideband noise figure of
11 dB
If i  add a passband filter that remove the image noise from the input of
the mixer it is like to receive 24 GHz with a single sideband noise figure
of 8 dB plus ONLY the insertion loss of the bandpass filter.
Adding a filter  that reject the image noise you dont want to add 3 dB to
the single sideband noise figure.
Since the filter  rejects the image noise from my ears leaving the 24 GHz
signal unchenged,it acts as a noise figure improvement of 3 dB or 11-3=8 dB
with the only penalty due of its insertion loss .
Otherwise why to add a passband filter ?

73 de i8CVS Domenico


----- Original Message -----
From: Stan <stan@capeonramp.com>
To: Jon Ogden <na9d@speakeasy.net>
Cc: Pieter Ibelings <elpieter@hotmail.com>; Frederick M. Spinner
<fspinner@hotmail.com>; <amsat-bb@AMSAT.Org>
Sent: Friday, May 25, 2001 2:58 PM
Subject: Re: [amsat-bb] Weak Signal Banquet NF numbers..


> Hello Jon and the net,
>
> What Peter is saying is that the noise figure indicated is about
> 3 dB better (lower) if the actual test is for double sideband, than
> if you were expecting a single sideband test result.
>
> The double sideband response can be determined by injecting a clean
> RF carrier at the devices mixer product frequencies. If the device
> is DSB, ie no or poor image rejection, there will be a response at
> the IF port when the injected RF carrier is at LO + and - the IF freq.
>
> Example: LO = 10,223.0 MHz, IF = 145.0 MHZ, high beat (USB) would be
> LO + IF = 10,368.0 MHz and the low beat (LSB) would be at
> LO - IF = 10,078.0 MHz.
>
> The savvy manufacturers' spec the noise figure as DSB, double
> sideband,
> if no image rejection is used on the RF side of the mixer.
>
> DB6NT specs his 24 GHz transverter RX noise figure as 8 dB DSB.
> When you add a bandpass filter, the NF jumps up by 3 dB to about 11 dB
> plus the insertion loss of the band pass filter.
>
> I hope this sheds some light.
>
> Stan, WA1ECF
>
>
> Jon Ogden wrote:
> >
> > 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."
> >
> > ----
> > Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
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