[TenTec] OMNI VI/VI+ Roofing Filter Mod. vs NB Function and FM/FSK Mode Operation

[Dave Bowker]

An Inrad roofing filter mod. installed in the OMNI VI/VI+ renders the NB function inoperative.

The reason for this is that the noise pulse is stretched in width by more than 2000 times as it 
passes through the front end circuits and the narrow (either 600 Hz or 2.4 kHz) roofing filter. For
example, a 0.5 µS noise pulse appearing at the antenna input is stretched to several mS as it passes 
through a 500 Hz wide filter inserted in front of the OEM 15 kHz roofing filter. Since the signal
pickoff for the NB function remains after the OEM roofing filter, the NB circuitry sees this 
lengthened noise pulse, which it is not designed to process.

My own roofing filter (home brew 4-pole, 500 Hz BW, centered @ 9.000500 MHz) is selectable from the 
front panel using the PROC button. This facilitates selection of the Inrad #753 (400 Hz BW) or TT
#221 (250 Hz BW) filters with the N1/N2 buttons independent of the roofing filter and permits use of 
the NB (if needed) by deselecting the PROC.

Since I do not operate SSB, this is not a problem (for me) with the narrow filter. If one were to 
install the Inrad 2.4 kHz roofing filter mod, the Processor function would not be disturbed with the
PROC selected, however the 2.4 kHz roofing filter would not provide optimum performance in the CW 
mode, and the NB function remains inoperative with the roofing filter mod. selected.

BTW: The FM receive mode is also inoperative with the roofing filter mod. because of the narrow 
bandwidth preceding the FM receiver section;  FSK mode is inoperative because of the roofing
filter(s) incompatible offset(s).

Dave, K1FK
Fort Kent, ME