Inrad Omni VI / VI+ Roofing Filter Mod and Noise Blanker

From the Inrad Reflector February 27, 2005

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

Ten-Tec Reflector Comments

August 7, 2005

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.


August 7, 2005

This is to clarify my rationale for using the PROC switch to activate the Inrad roofing filter modification. For those not familiar with the history of this, I performed this modification to my Omni VI+ (opt 3) over 2-1/2 years ago while testing various home-designed roofing filters, and needed a front panel selection capability so that I could record A/B test data while still keeping the N1 (500 Hz) and N2 (250 Hz) filters independently selectable.

The primary requirements (for the modification of the Inrad roofing filter implementation) were:

  1. the implementation needed to simply take the roofing filter selection off of the N1/N2 buttons,
  2. without incurring any additional components, with a
  3. minimum of effort, and
  4. with minimal impact to other operating modes and functions. These requirements narrowed the design to only one choice of front panel switch . the PROC switch, which is a "hard" switch with push-push states.

The question has been raised on the reflector as to why not use the FM or VOICE or other main keypad switches to select the Inrad roofing filter, switches that (appear to) have less of an impact on operational modes than the PROC switch.

The first reason is that ALL of the main keypad switches are "soft" switches, which are scanned by the microprocessor, and do not provide a permanent status of the selected state. Use of any of the soft switches for this modification would require an additional PC board with buffer and latch circuits, contrary to requirements 1, 2 and 3, and would require removal of the Logic Board in order to implement the interface in hardware. Further, use of the FM button would require a software change, as once the button is depressed; the radio is in the FM mode!

Use of the VOICE button on the main keypad would, in addition to the above four requirements, require the optional voice board to be installed, a further complication of requirements 1-4.

Dave, K1FK

August 29, 2008

It's also simple to control the Inrad filter with the ATTN switch if you don't use that function. Here's what you do: Skip Inrad's instructions numbered 18 through 24. Set aside the cable/connector provided for J4 on the Inrad board, you won't need it. Take the cable/connector provided for J3 and tie the lines together, then add an extension so the wire will reach up to the front of the Omni, plus 4 or 5". Locate the 3 pin white connector labeled 18, at the left front side of the Front End Board, as viewed from the front of the Omni. Cut the middle wire going to the connector, leaving a few inches behind on the connector in case you ever want to reconnect it. This is the line that goes to the Attn switch. Connect the lead from Inrad J3 to the end of the line going up to the front panel Attn switch. Tape up the end still attached to the connector. Set S1-S4 on the Inrad board to ON.

That's all. Inrad now switches in and out with the Attn switch and the led shows when Inrad is switched in. All Omni filter switches are unaffected.

73, Wayne K4ELO

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