Ken Brown ken.d.brown at verizon.net
Sun Aug 7 15:02:47 EDT 2005

Hi Don,

The Inrad roofing filter modification inserts a narrower roofing filter 
BEFORE the 9 MHz Mixer IF Board. The coax  going to connector 25 (RF IN) 
is unplugged and plugged into the Inrad modification board. Another 
piece of coax from the Inrad board then plugs into connector 25. This 
arrangement allows the addition of the modification to be done without 
having to cut traces or unsolder any components from the original 
equipment. Some other wire(s) are connected to control the switching of 
the additional roofing filter in or out.

Since the modification can be switched in or out, you don't loose any 
capability, as long as you do not take away the function of some other 
control to perform the switching function. I believe the recommended 
method is to make the Inrad mod switch in whenever you are using the N-2 
filter selection on the Omni VI+ or Omni VI version 3. When used with 
other Omni's you have to come up with some other idea.

There is a problem with putting this narrower roofing filter in front of 
Y1 and the noise blanker circuit. A narrow filter will attenuate and 
stretch out (in the time domain) the noise pulses, making the noise 
blanker operation less effective. Generally speaking it is better to put 
narrow and sharp skirted filters AFTER the noise blanker. However this 
would have required a more difficult modification procedure to install, 
using a soldering iron. Some people claim that the noise blanker is 
useless anyway. I find that the noise blanker works well for me, and I 
would not want to give it up when using my N-2 filter. I guess it 
depends on the kind of RF noise environment you operate in. If you use a 
different control to do the switching, you could have it either way.

Replacing Y2, the standard 2.4 kHz filter with a 2.1 kHz filter would be 
a better way to go in my opinion. However that puts it after Q1 and the 
noise blanker switching diodes. It is possible that the IMD you are 
hoping to prevent by using a narrower roofing filter could occur in 
these devices, before the signals get to the filter. And you are right, 
this would also affect your SSB receive AND SSB TRANSMIT, and it would 
not be switchable. Even if it totally prevented SSB operation it would 
be no great loss to me. Again this is a matter of operating preferences.


>Dave - K1FK,
>You mean you have replaced the standard 2.4 kHz with an INRAD 2.1 kHz filter 
>for the roofing
>Guess it would do the same improvement for my OMNI 6+.   I work 99% CW so 
>the narrower
>bandwidth on SSB should not be detrimental.  What say?
>Don,  VE1BN at ns.sympatico.ca

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