[TenTec] Confusion re Inrad filters

[Charlie Chisholm]

Hello -

I was perusing the Ten Tec reflector and came across your question about Inrad CW filters.

For the 6.3MHz IF, the two filters Inrad has are fine.  The 6.3 MHz IF has the passband tuning control, so you can place the filter passband anywhere you want. Just adjust the passband tuning control to center the filter on the incoming signal. If there is interference nearby, just move the passband tuning control one way of the other until the interference is reduced.

For the 9MHz IF, the situation is different.  There is no passband tuning control for that IF. The center of the passband is fixed at whatever the offset of the crystal filter is - 600 Hz for the #753 or 700 Hz for the #759 and #760. In older rigs such as the Omni V, the offset is not adjustable - it is fixed at 600 Hz. You would normally set your sidetone to 600 Hz and match the tone of the person you are working.  Your signal would then be right on top of his signal. But when using the #759 and #760 filters, which are fixed at 700 Hz, the station you are listening to will be out of the passband.  So most people set their sidetone at 700 Hz so they will be in the center of the 9MHz IF filter passband.  But that creates another problem -  since the radio has a fixed transmit offset of 600 Hz, you will NOT be right on top of the other station - you will be 100Hz off.  Not normally a big deal, but most people would prefer to be right on top of the station they are working.  That is why Inrad came up with the new 9MHz filters that have the 600Hz offset - so on the older rigs, everything lines up right. You just set your sidetone to 600Hz, match the tone of the incoming station to the sidetone, and you will be in the middle of the passband in both IF's - as well as right on top of the station you are working.

With the Omni VI series, the transmit offset is variable along with the sidetone, so if you are using the #759 and #760 Inrad filters (700 Hz offset), you just set your sidetone to 700 Hz, match the incoming signal to the sidetone, and you will be right on top of the station you are working, and right in the middle of the passband. If you haven't purchased filters yet, it just becomes a matter of preference - if you like 600Hz better than 700 Hz (I do), you would choose the 600Hz offset 9MHz filter.  If you prefer a 700Hz tone, get the 700 Hz offset filters. If you want both 400HZ and 250Hz filters for the N1 and N-2, you are pretty much stuck with the 9000.7 filters - nobody makes a 9000.6 250 Hz filter. Set your sidetone to match the filter (600Hz or 700Hz), then always tune the tone of the incoming signal to match the sidetone, and you will always be in the middle of the 9MHz passband. Adjust the passband tuning control to the middle of the matched incoming signal, and the filters in BOTH IF's will be centered on the signal.  Your transmit signal will also be right on top of the station you are working. That kind of performance is hard to beat.
Note - I just set my passband tuning control once and leave it alone unless I have interference really close in. I set my sidetone with a frequency counter, but you can also use your narrow filters to set the sidetone.  Here's how.  Select the narrowest filter in the 9MHz IF.  Tune in a clean CW signal until the signal is centered in the passband as good as you can get it.  Then simply set your sidetone to match the incoming signal. Your sidetone (and transmit offset) are now set to match the offset of the 9MHz filter - so if you have a 9000.7 filter, your sidetone is 700HZ.  If you have a 9000.6 filter, your sidetone is 600 Hz.

Hope that helps!

-Charlie NO7UP