K3BU@aol.com wrote:
> rohre@arlut.utexas.edu writes:
>
> << The Omni VI is well documented in ARRL tests in QST magazine, and in the
> advertising over several years, as having a lower noise floor than Yaecomwood
> transceivers. It is therefore more sensitive to RF. It is also INCORRECT
> with modern radios, to run all the time with maximum RF gain setting, for as
> you correctly state, that would amplify noise as well as signal, and you may
> well introduce overloading of early stages and intermodulation products with
> excessive gain. >>
>
> <<and other "explanations">>
>
> I have discovered this "noise problem" on my second hand Omni VI. When I
> posted question to the reflector, I got similar answers (wishy-washy). One
> reflectoree described the source of the problem to me. Procedure to verify
> this is: turn your LP and DSP off, disconnect the antenna, trun your RF gain
> full up: you will hear high pitched hiss from audio. As he properly
> identified, the noise is caused by IF stages after the crystal filters and
> detected and amplified. On my Omni VI+ this happens when I turn my RF gain
> past 3 o'clock. This is what it is. Has nothing to do with sensitivity, front
> end, quieter radio, TenTec superiority. It looks like unsolved design problem
> in the IF stages. The remedy is to use LP or DSP on CW, on SSB use outboard
> filter or keep the RF gain just below the point where it starts hissing. If
> the signals and noise from the antenna is loud, this problem is masked. Looks
> like some type of filtering or adjustment of IF stages could solve the
> problem. I have not had the chance to look at it yet.
> One nice thing about Omni is that S meter's needle zero position doesn't go
> up with reduced RF gain.
>
> 73 Yuri, K3BU
Disconnect the coax between the bandpass filter bd and the IF audio bd. Does
the
noise drop? It should. Turn the "RF" gain all the way down. Turn the "Audio"
gain all the way up. Scrutinize the noise. Sometimes the logic bd. induces
noise into the audio chain.
With a resonant antenna, (50ohms) on 10 meters, connected to the Omni VI you
should show about S1-S3 of cosmic, galactic, urban, suburban, etc. noise in the
middle of the afternoon. Oh yes, tune to a clear spot (no signals or line
noise)
in the band for this test.
Having "IF hiss" is usually normal. A post IF (In front of the Product
Detector)
2.4kc filter would reduce this noise. Considering R&D design time,
bill-of-material costs, and other
business costs the retail price of the OMNI VI+ from the factory with just this
change would be around $2985.00. Since we are fixing an otherwise very good
"radio" why don't we add
a second receiver, 200 watt xmtr, IF DSP, UP conversion IF, RF speech
processing,
automatic ant tuner, and on and on! Before you know it the OMNI VI price will
be
up there
with the ICOM 781.
If the OMNI VII+ (yes seven) has all the features of the ICOM 781 and Yaesu
1000D
(no MP please) and KENWOOD 950SDX I am ready to cut a check tomorrow! Please
Ten
Tec no whimpy 16 or 24 bit A to D converters for the DSP IF. Lets have a DSP IF
at 30mcs and get away from this OLD archaic 16-24 bit technology. It's time to
get away from the PC
world type thinking and into robust heavy duty real number crunching.
Bottom line folks is "put your money where your mouth is!"
73 Bill N4WC
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