Ladder filters aren't symmetrical, so they won't sound exactly the same on
both sidebands, even when the carrier point is optimized.
When carrier suppression is adjusted correctly at the DSB output (that is,
balanced between USB and LSB) there can still be a difference in carrier
presence USB vs, LSB owing to the carrier oscillator frequency position on
the shoulder of the filter bandpass slope. If the carrier oscillator is
closer to the center frequency of the filter on, say LSB, then it will pass
more low frequency audio and also allow more of whatever vestigal carrier
might exist to pass through the filter with less attenuation than on USB,
positioned further from center frequency.
73
Phil
k4dpk
VFO Stabilizers and PEP adapters for wattmeters...Check out
http://home.comcast.net/~k4dpk
----- Original Message -----
From: "Steve N4LQ" <n4lq@iglou.com>
To: <tentec@contesting.com>
Sent: Friday, January 07, 2005 10:28 PM
Subject: Re: [TenTec] 2.8khz and Carrier Balance??? (LONG!!!)
> Well I understand the need to adjust the carrier null but what I do not
> understand is the recent claims that somehow the 2.8khz filter has
something
> to do with getting the carrier null out of adjustment. Sure, it's a good
> idea to check it and tweak it up...er...down but I don't think changing
the
> filter affects anything.
>
> Steve N4LQ
> ----- Original Message -----
> From: "Mike Pagel" <k9uw@wi-net.com>
> To: <tentec@contesting.com>
> Sent: Friday, January 07, 2005 10:22 PM
> Subject: RE: [TenTec] 2.8khz and Carrier Balance??? (LONG!!!)
>
>
> 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
>
>
>
>
============================================================================
> ==
> 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 = 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.
>
============================================================================
> =
>
> 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.
>
> 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.
>
> 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@contesting.com [mailto:tentec-bounces@contesting.com]
> On Behalf Of Steve N4LQ
> Sent: Friday, January 07, 2005 6:42 PM
> To: tentec@contesting.com
> Subject: Re: [TenTec] 2.8khz and Carrier Balance???
>
>
> Would someone please explain to me what changing the filter from 2.4khz to
> 2.8khz has to do with carrier balance.
> I see no relation. The carrier is nulled by balancing the dc voltage on U4
> of the TX Audio board. No filter is anywhere near there. U4 feeds DSB into
> 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 -----
> From: "Harry Coates" <WA8HC@att.net>
> To: <tentec@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
> 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
> installed. Since I also have the Inrad 2.4 filters in both 2nd filter
> positions. Don just interchaged them and finished the job.
>
> Just a comment, for what it's worth.
> '73,
> Harry
>
> Harry N. Coates
> WA8HC@att.net
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