[Microwave] Silly Rig Modification and Noise Temperature Ramble/Question

[Dave Brown]

What was the purpose of the two mods you mention- don't know the details but
they don't sound like mods intended to improve ultimate sensitivity- more
like improved strong signal performance- can't fault your logic so maybe
you're right!
73
ZL3FJ
----- Original Message -----
From: "Scott Townley" <nx7u@arrl.net>
To: <amsat-bb@amsat.org>; <microwave@mailman.qth.net>
Sent: Tuesday, October 15, 2002 5:21 PM
Subject: [Microwave] Silly Rig Modification and Noise Temperature
Ramble/Question


> My apologies up front, as this rambles a bit:
>
> I have been looking at improvements/modifications to my FT-726 for AO-40
> and MS work.  The two main published mods appear to be a front-end DGFET
> replacement, and an IF amp boost.
>
> As a starting point, I have measured my RX sensitivity (on 2m) as follows:
> any comments as to why this is right/wrong/different would be appreciated.
> Setup is siggen->HP VHF variable attenuator->rig->laptop.
>
> I used the fixed audio output of the FT-726 to feed my laptop soundcard
> running a program called Analyzer 2000 (ver. 5).  There are several slick
> audio analysis programs available, but this is the only one I have seen
> that measures SINAD directly (and you're not even restricted to a 1kHz
> tone!).
> I measured 12dB SINAD at -124dBm input...so 0dB SINAD (or signal=total
> noise power) at -124-12=-136dBm.
> **NOTE 1** AGC cannot be defeated in FT-726R, but it takes about -100dBm
> for any S-meter deflection, so it would appear that AGC is effectively
> disabled at these signal levels
> **NOTE 2** and to prove that to myself, a +/-3dB change in attenuator
value
> changed measured SINAD by +/-3dB, showing linearity of response.
>
> So, total noise power here is going to be 290K from the siggen plus the
> "unknown" input equivalent noise power of the rig-under-test (RUT?).
> -136dBm=Ttotal=827K (2.2kHz noise bandwidth assumed)
> Ttotal=Tsiggen+Trut so Trut=827-290=537K (4.55dB NF)
>
> This seems to be pretty good performance...
>
> Now I have to wonder to myself, why modify?  I always use a mast-mount
> preamp or some other low-noise gain element (e.g., downconverter for
AO-40)
> at the antenna, so with that noise temperture, even a mere 17dB of gain
> knocks the rig noise contribution down to not quite 11K...for satellite
> that's a lot less than you'll get from the downconverter and antenna, and
> for MS as well (right?) so it seems pointless to mod the rig at all.
>
> So...what have I screwed up here?
>
> Thanks for your expertise!
>
> ------------
> Scott Townley
> nx7u@arrl.net
> Gilbert, AZ  DM43di
>
> HF/VHF/UHF/Satellite MOBILE station
> --> yet NEWER (August 2002) address http://members.cox.net/nx7u
>
> ------------
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