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

[Duane - N9DG]

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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