[TenTec] Omni VI plus - noise & INRAD filters

[Jim Reid]

Aloha,  

If you read to near the bottom of this long post you will find:

"Bottom line:  the performance of the Omni VI+,  on our
quietest HF band,  10 meters,  is NOT being limited,
nor compromised by noise contributions from circuitry,
logic stuff,  etc., within the rig itself.",

and,  I am keeping mine!  Now:

As to Omni VI+ noise,  Bill wrote,  in part:

> Disconnect the coax between the bandpass filter bd and the 
>IF audio bd.  Does the noise drop?  It should.  

Didn't get inside to do that disconnect,  but see on below,

>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.

Not sure if this was to be done with the above disconnect or not,
but I did it,  as I said,  without the disconnect.  With the RF gain
all down, and the audio full up,  I at last heard a light "crackling"
randomly intermittent.  Took that to be "shot noise"  or whatever
it is called with transistors in the audio amp gain stages.  Without
that,  the added "white" noise was just barely audible.  Could
hear nothing that sounded as if it would be "logic" circuit
generated noise.

> 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.

Ok,  so I live in a very RF "quiet" environment,  in a rural area of 
a small island out near mid-Pacific.  So my S meter this afternoon
on 28495 read S=0 when connected to my C4 (C3 with a 40 meter
dipole aboard the boom);  vswr of the C3 measured 1.1:1 at
28495 (no antenna tuner,  just the "barefoot" coax and transmission
line.  Oh  well,  I have a Force 12 B-1 balun up at the C3 attachment
terminals, and a choke bead balun also right at the antenna terminal
on the back of the Omni VI+,  so not quite barefoot,  I guess.

Anyway,  found that switching the antenna on/off with the in-line
remote coax switch (Ameritron RCS-8V)  that there were some
8 to 10 dB of actual noise coming into the back of the Omni
from the C3,  however.  This was determined by turning the
audio way loud,  and listening carefully to the "white" noise
output as an MFJ-762 Step Attenumator was actuated in
line with the Omni input terminals. 10 dB of inserted loss
caused no discernable difference in the audio noise
when either the RCS-8V was turned to OFF,  or an added
60 dB of loss was added with the Step Attenuator,  Seemed
that there was just a slight discernable delta when 8 dB of
loss was used,  and the same switching performed. (BTW,
this  MFJ attenuator has recently been re-enigeered and
improved by Tom Rausch,  W8JI,  that is the model I have,
not the original which was patterned after the attenuator
resistor list given in the ARRL handbook,  which is a bit
off.) BTW,  the AGC was OFF,  and the RF gain full
up for the above tests.  Was ok to turn OFF the AGC,
once I found that no S meter deflection was occuring
on either my antenna noise input or internal Omni noise.

My particular VI+ has essentially all INRAD filters,  including
the 2.8kHz unit in the front of the 9 MHz  IF section ( installed
to correct a possible CW keying "click" problem), and another
replacing the so called "standard" 2.4 kHz position at the front
of the 6.3 MHz IF strip.  

Also at 6.3 MHz, where TT puts the optional  1.8 kHz SSB filter,  
I have another INRAD, a 10 pole, #757 2.4 kHz filter.  

I have moved the TT 1.8 kHz filter to the slot
at 6.3,  where TT would have the 500 Hz CW filter,  and
then I have the INRAD 400 kHz CW filter in the slot
where the 250 Hz TT filter would be placed.

Also in the 9 MHz strip, at the N1 slot,  is the INRAD 10
pole # 756 2.4 kHz unit,  and at the N2 slot is an INRAD #753, 
400 Hz BW CW filter.

I could detect no delta in noise performance when any one,
or cascaded pairs, of the above SSB bandwidth filters
were switched in or out of the IF strips.  However,  with the
400 kHz bandwidth filters,  there was a 5dB increase in
noise when the C3 antenna was switched ON by the RCS-8V.
And this seems about right,  as,  were the BW shrunk 50%
from the SSB width around 2 kHz,  you would expect the
antenna noise to drop 3 dB,  and another 3 dB,  or a 6 dB
total improvement if you were dropping from 2 kHz to 500Hz
bandwidth;  so the about 5 dB I found seems correct as
far as intercepted "galaxy,  natural RF noise" at 28495 with
the bandwidth decreases used -- and of course there are
tolerances of which I have no idea in all these filters and
in the MFJ attenuate.  So seem to be in the ball park.

Bottom line:  the performance of the Omni VI+,  on our
quietest HF band,  10 meters,  is NOT being limited,
nor compromised by noise contributions from circuitry,
logic stuff,  etc., within the rig itself.  Therefore, there is
NO way the radio's performance could be compromised
by RF or IF strip noise,  nor audio stage noise,  as all are well
around 10 dB or more below the "natural" noise brought
in from the antenna on all  of our HF bands.

I plan to keep mine for a long time.

And no,  the bands have been so poor that I cannot now
reliably comment about the INRAD filters deltas from
the TT units originally  inside.  But what I have heard
and used,  I really like!  Especially the 400 Hz CW
filters with the NR (DSP + audio peak) ON.

Also,  just like the audio sound of the 2.8kHz BW set
up during SSB ragchews,  my most common mode.

As usual,  I write too much.

73,  Jim,  KH7M
On the Garden Island of Kauai




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