Didn't TT have a MOD for the OMNI V's PBT board - to reduce hiss. Been a
while.
----- Original Message -----
From: "Duane - N9DG" <n9dg@yahoo.com>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Sunday, May 15, 2005 03:13
Subject: Re: [TenTec] hiss hiding - Some Corsair II Experiments - Revisited
> Well I did some more experiments tonight and made a few
> relative measurements. One thing that I was able to determine
> is that the PBT board in the CII provides about 10-12dB gain.
>
> I also found that the relative broadband noise from the CII
> varies by about that same 10-12 dB depending on whether the
> PBT is in line or is by-passed completely as described below.
>
> Test conditions for both configurations (PBT board in line
> and out):
>
> 1. Fed -120dBm signal from a signal generator on 28.2 MHz.
> Picked -120 dBm because it did not cause any AGC action (no
> S-meter deflection) in the RX with the RF gain fully
> clockwise.
> 2. AGC off, 2.4 kHz filter setting for the 6.3 MHz IF, PBT
> zeroed, RF gain control at max.
> 3. Fed PC sound card from headphone output jack.
> 4. Ran SR5 Spectrum Analyzer software.
> 5. With PBT board in line set CII's volume control to get the
> carrier tone to an even 10dB grid line of the SR5 program's
> display. Was also careful to not overload sound card or to
> allow the CII's audio amp be driven to compression. The CII
> volume control under these conditions about 4.5.
> 6. Observed the level of broadband noise on the SR5 display.
> 7. Moved the IF jumper cables in the CII to bypass the PBT
> board. All other CII settings and SR5 settings remained
> constant.
> 8. Observed the change in levels on the SR5 display. The
> carrier tone dropped by the 10-12dB, as did the broadband
> noise.
>
> As an interesting side note the broadband audio noise from
> the CII extends from near 0 Hz to about 7 kHz. The IF filter
> passband shape is readily observable but the broadband audio
> noise rolls off above ~2.5 kHz in a rather gradual taper out
> to the 7 kHz point where it essentially meets the noise floor
> of my audio test configuration. The SR5 program was able to
> display signals out to 11kHz.
>
> I chose 28 MHz and a weak signal to best simulate the
> conditions that I want to be using the CII RX for in my
> shack. I intend to use the Corsair II as a second RX for my
> VHF+ weak signal bands. The CII being fed signals from the
> DEMI transverter.
>
> Duane
> N9DG
>
> --- Duane - N9DG <n9dg@yahoo.com> wrote:
> > --- Steve N4LQ <n4lq@iglou.com> wrote:
> >
> > > Well if you turn down the "RF Gain" on a Corsair,
> > > virtually all the hiss
> > > disappears leading me to point blame at the IF chain.
> >
> > Actually there are two sources of hiss, one is indeed the
> > RF/IF stages, and the other is clearly the audio chain.
> >
> > As an experiment turn the RF gain all the way down (with
> > 50-ohm load or no antenna) and then the audio all the way
> > up
> > there's plenty of audio hiss. Can be further demonstrated
> > by
> > lifting connector 45 (RX IF in) to the IF/AF board and
> > crank
> > the audio full up again, about the same level of noise -
> > though the RF gain setting does have some effect. Also if
> > you
> > lift connector 76 (BFO in) you get a similar result, this
> > time the RF gain has no effect.
> >
> > This set me to wondering if I could actually characterize
> > the
> > amount of noise contributed by the PBT board in the Corsair
> > II a bit better. After studying the schematics some I saw
> > that it is quite easy to do. At least for a quick test. All
> > that I needed to do to bypass the PBT board was to take
> > plug
> > 75 from the PBT board and plug it into the position 73 of
> > the
> > XTAL filter board. That simple change bypassed the PBT
> > board
> > completely. As it turns out the PBT board adds a good share
> > of the overall RX noise, - more than I had originally
> > thought.
> >
> > Needless to say this jumper cable reconfiguration removes 8
> > poles of IF filtering at the 6.3 MHz IF and PBT goes bye
> > bye
> > too. And there is only the 2.4 kHz bandwidth IF filter at
> > 9MHz left, essentially makes the Corsair II into an Omni
> > A/D/C. Didn't study it any further to see what the
> > ramifications are to the TX side. So DO NOT necessarily
> > consider this to be a mod that you want to make.
> >
> > DISCLAIMER: EXPERIMENT WITH THIS AT YOUR OWN DISCRETION. I
> > take no responsibility for any problems that you may cause
> > by trying this yourself.
> >
> > Duane
> > N9DG
>
>
>
>
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