[TenTec] hiss hiding - Some Corsair II Experiments - Revisited

[Steve]

There is a mod to add a 9001.5 crystal after the last IF amp Q3, just before 
the BFO Mixer Q4 which claims to clean up the noise.  I have not tried it 
yet but looking for the crystal now.

Steve


----- Original Message ----- 
From: "Steve Baron - KB3MM" <SteveBaron at StarLinX.com>
To: "Discussion of Ten-Tec Equipment" <tentec at contesting.com>
Sent: Saturday, May 14, 2005 7:34 PM
Subject: Re: [TenTec] hiss hiding - Some Corsair II Experiments - Revisited


> 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 at yahoo.com>
> To: "Discussion of Ten-Tec Equipment" <tentec at 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 at yahoo.com> wrote:
>> > --- Steve N4LQ <n4lq at 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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