TenTec
[Top] [All Lists]

Re: [TenTec] Hearing disabled CW OP Seeks Special Omni Filter

To: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Subject: Re: [TenTec] Hearing disabled CW OP Seeks Special Omni Filter
From: Ken Brown <ken.d.brown@hawaiiantel.net>
Reply-to: ken.d.brown@hawaiiantel.net,Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Sun, 30 Apr 2006 10:44:40 -1000
List-post: <mailto:tentec@contesting.com>
>> Is it possible I would actually be happier with 500 Hz filters in the 6.3
>> and 9 mHz IFs, since one op said they work a little better with the passband 
>> tuning on the Omni VI+ than the 250 Hz filters do?
>>     
When using narrow filters in both the 9 MHz IF and the 6.3 MHz IF it is 
necessary to have the PBT adjusted so that the bandpasses of both IFs 
coincide. Otherwise you won't hear much at all. The adjustment can be a 
bit critical if both filters selected are really narrow. I think there 
may be some drift in the 15.3 MHz VCXO on the passband tuning board, 
requiring the PBT knob to not always be in exactly the same position to 
get the two IF bandpasses coincidental.

The Inrad roofing filter modification put the new roofing filter before 
the 15 kHz two pole filter,  Q1 and the noise blanker pulse detector, 
and the noise blanking switch diodes. Putting a narrower filter before 
all these stages, instead of where Ten-Tec put the narrow 9 MHz filters 
can help you copy weak signals amongst strong close in (in frequency) 
signals that would otherwise produce intermodulation distortion in those 
stages which the filter precedes. This is likely to be an advantage in 
contest situations more than any other.

There is a possible disadvantage to putting a narrow filter before the 
noise blanker circuit. The kind of noise that noise blankers work best 
on, is short, strong pulses. The strong pulses are detected by the noise 
detector, which then switches off the IF signal path using the noise 
switch diodes. With the pulses short, you barely notice that ALL signals 
are blanked for a short period. The pulse noise is blanked and the 
signals you want to hear are mostly still there. By putting a narrow 
filter in front of this, the pulses can be reduced in amplitude and 
stretched out in time duration. So the period of  IF signal path shut 
off can be longer, and perhaps more noticeable, possibly rendering the 
noise blanker less effective. This does not make the Inrad roofing 
filter modification undesirable, it just means there are some trade 
offs. And you can switch it in or out. Some people claim that noise 
blankers are completely useless anyway. I find that I do occasionally 
have noise that the blanker works very well on.
>> 3)  If I were able to find the special N4PY "lowered tone" CW filter, what
>> other set of CW filters should I buy that are compatible with it?
>>     
The "low note CW filter" for the Omni VI is the Ten-Tec model 221. It is 
specified as 250 Hz bandwidth, centered on 9.000500 MHz, which will 
result in a 500 Hz tone when the BFO is set to 9.00000 MHz as it should 
be in CW mode. Ten-Tec does not list these as an available item any 
more. I got mine through this reflector.
>> 4)  In addition to crystal filters, should I invest in the Timewave
>> DSP-599zx especially for SSB summertime noise reduction? (As you probably
>> know, for hearing disabled, getting rid of as much background noise as
>> possible is a priority for both CW and SSB.)
>   
With the kind of noise you are describing, you may not want to 
compromise the functionality of your Omni VI noise blanker with a narrow 
roofing filter before the NB circuit. I would suggest narrow 9 MHz 
filters in the locations that Ten-Tec provided in their design.

I have not yet tried an outboard DSP NR system with my Omni VI. I'd be 
interested to hear other peoples experience with this. Remember in the 
Omni VI, the DSP circuitry is ALWAYS in the audio path, whether your 
have NR turned on or off. So with an outboard DSP NR, the audio would be 
going through two A to D and D to A processes. I've thought about 
putting my JPS NIR-12 in line INSTEAD of the built in DSP rather than in 
additon to it. Haven't got around to doing it yet.

DE N6KB

_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec

<Prev in Thread] Current Thread [Next in Thread>