[TenTec] 2.8khz and Carrier Balance??? (LONG!!!)

[Mike Pagel]

Hi Steve:

My 6+, also equipped with 2.8's, had its carrier balance adjusted when =
the
2.8's were installed.  Over time, however, I began to get reports of a =
weak
carrier.  When the rig went into Ten-Tec for service recently, I asked =
them
to adjust the null.  Unfortunately, the rig came back with the carrier
stronger than when it went in.  I was getting reports of a S7 carrier on =
a
20 over SSB signal, so I was rather unhappy.  Luckily, I was able to =
adjust
the pots myself by listening to another receiver in the shack.  No more
reports of a carrier.

The following posts (from several years ago) define the need to make the
carrier balance after swapping out the filters.  There is also the
suggestion of replacing the one turn pots with 25 turn pots to ease the
adjustment.  In my case, the adjustments were very difficult.  If I even
THOUGHT of turning the screwdriver, the carrier level changed.

Hope this explains the reasons for needing to null the carrier.

73, de Mike, K9UW
Amherst, WI



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=3D=3D
July 9, 2001

Hi, Everybody,

I have wide, 2.8 kHz Inrad filters in both IFs of my Omni VI now. My
transmitted SSB signal showed all the signs of an incorrect BFO =
insertion
frequency: poor carrier rejection and poor opposite sideband rejection.

A friend of mine across town told me he could hear me on the opposite
sideband and actually make out what I was saying from the very low =
frequency
audio getting through.

All this was confirmed by measurement. Even after re-nulling the =
balanced
modulator carefully, I measured the carrier rejection at only 28 dB. 20 =
dB
of that rejection comes from the balanced modulator, and only 8 dB from =
the
filter. Using an RF generator to inject a 9 MHz transmit signal into the =
9
MHz IF board, I confirmed that the filter was passing with little
attenuation both above and below the 9.000 and 9.003 MHz LSB and USB =
carrier
frequencies, respectively. (Remember that the mixing scheme inverts the
sidebands in the Omni VI.)

On purpose, since I happened to be testing in LSB mode, I moved the BFO
frequency to (9.00000 MHz - 100 Hz =3D 8.9999 MHz), in other words, an
additional 100 Hz away from the lower skirt of the filter, and that =
really
cleaned up the signal. So I did the same thing  to the USB BFO crystal. =
Same
good results. However, one undesirable side effect of doing this is that =
the
frequency readout is now off by 100 Hz. I guess I have no choice but to =
get
used to this.

The stock 2.4 kHz filters do not exhibit this behavior. I have measured =
them
and listened to them and they have sufficient rejection at the BFO
frequencies.

I know several folks on here have gone to these filters, so have you =
guys
really checked carefully the carrier suppression by listening on another
receiver or by measurement with test equipment? Has anyone found what I
found? Or do I just have a filter that's simply too wide to operate
correctly inside the Ten Tec 9.000 to 9.003 MHz IF window? I guess what =
I'm
asking is 1/ is the Inrad filter really exactly supposed to be 2.8 kHz =
wide
or 2/ in reality is it wider, giving rise to the problems I've =
described, or
3/ do I have a filter that's out-of-spec? Before you answer #3, I would
really appreciate hearing from you if you've actually measured this.

I want to be fair and clear that I haven't spoken to Inrad yet about =
this to
give them a chance to respond. I am looking for more of a sanity check =
right
now. Thanks.
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=3D
=3D

July 10, 2001
Al and all,

I have recently been through the same problem with my OMNI 6+ and the =
INRAD
2.8khz filters. I had installed both 2.8KHZ INRAD filters in my OMNI =
last
year and they were working well until this past spring. At which time, I
noticed that a small amount of carrier was present in my transmitted =
signal
on USB . This was a particular problem because I use the OMNI 6+ to =
drive a
6 meter transverter and then a six meter amp, which wound up giving me =
30
watts of carrier and 100 watts of SSB !!!! ( a horrible signal to say =
the
least).

Preliminary attempts to rebalance the carrier in the OMNI 6 resulted in =
the
ability to find a razor sharp null point, which unfortunately would not
stabilize for more than a few minutes, making it impossible to set the
carrier balance alignment. My initial discussions with Ten Tec suggested
that I replace the balanced modulator chip, which I accomplished. =
However,
that replacement resulted in no change in the circuit performance. A =
second
discussion with Ten Tec provided a very interesting comment when I =
mentioned
that I had replaced the stock Ten Tec 2.4 khz filters with the INRAD
filters.  Ten Tec stated that their filters have 20db of carrier =
attenuation
built into the filter design which INRAD 's filters do not have !!!!

A quick call to George, W2VJN, owner of INRAD (and expert RF engineer =
!!!)
confirmed what Ten Tec had told me. Sure enough, replacing the INRAD =
filters
with the stock Ten Tec filters put the rig back in tip top shape and no
carrier on the USB transmitted signal !

However, using the Ten Tec filters defeated the purpose of the INRAD
filters, which is to improve the quality of the receiver audio. George,
W2VJN, at INRAD, offered to check both 2.8khz INRAD filters for me and
provide an analysis. He did this very promptly and could find no =
significant
difference between my 1 year old filters and a "test standard" that he
maintains in his shop. However, he suggested that since the most =
significant
filter (audio wise) was the 6MHZ filter and that the carrier suppression =
is
driven in the 9mhz circuit, that I retain the 2.8Khz filter in the 6mhz =
slot
and replace the 2.8khz INRAD filter at 9mhz with a 2.4khz INRAD filter. =
This
was accomplished and the radio  is working beautifully. The carrier =
balance
adjustment has a reasonable null point which is not razor sharp and has =
been
stable for several weeks now.=20

The OMNI 6+ receiver audio sounds great as I have the INRAD filters plus =
the
INRAD audio mod installed....approaching my Collins gear more and more.=20

While this narrative does not answer what was wrong with either the old
2.8khz - 9mhz filter, or what shifted in my OMNI 6+ to no longer =
tolerate
the  INRAD 2.8khz filter, it did solve the problem very nicely.

Both INRAD and Ten Tec are to be commended on their technical support of
their customers. I hope that this information is helpful to all.

73,
Dave, K2DP
St. Louis, MO


Al  W6LX

-----Original Message-----
From: tentec-bounces at contesting.com =
[mailto:tentec-bounces at contesting.com]
On Behalf Of Steve N4LQ
Sent: Friday, January 07, 2005 6:42 PM
To: tentec at contesting.com
Subject: Re: [TenTec] 2.8khz and Carrier Balance???


Would someone please explain to me what changing the filter from 2.4khz =
to=20
2.8khz has to do with carrier balance.
I see no relation. The carrier is nulled by balancing the dc voltage on =
U4=20
of the TX Audio board. No filter is anywhere near there. U4 feeds DSB =
into=20
the 9mhz filter but the carrier must be nulled before it gets there so =
whats

going on with all this carrier balance talk I keep reading about????? =
Steve
N4LQ
----- Original Message -----=20
From: "Harry Coates" <WA8HC at att.net>
To: <tentec at contesting.com>
Sent: Friday, January 07, 2005 11:01 AM
Subject: [TenTec] Adding 2.8 filters in Omni VI opt 3


I have had the Inrad 2.8 filters in my Omni VI for several years, both =
IF's.

I think they're great. Plus the audio mod.

I just got my Omni VI back from S-ville having the TCXO unit installed =
and a

realignment done. During that work I recieved an e-mail from Don at TT=20
saying
they could not properly adjust the "carrier null" with the 2.8 filter in =

place, (9Mhz IF I believe), that I would have to have a 2.4 filter=20
installed. Since I also have the Inrad 2.4 filters in both 2nd filter=20
positions. Don just interchaged them and finished the job.

Just a comment, for what it's worth.
'73,
Harry

Harry N. Coates
WA8HC at att.net
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