[TenTec] INRAD Omni VI roofing filter kit - success

[Chuck Guenther]

W4ZV quoted George at Inrad as follows: "Note also that a 4
pole, 600 Hz filter centered on 9000.7 will pass just about all of the
energy getting through your special 250 Hz filter."


This is true; however if you set the radio for a 500 Hz sidetone (and offset), you will get a frequency response that is asymmetrical (skewed towards the high side) due to the mismatch in center frequencies of the filters.  In order to center your listening frequency in the passband of the cascaded combination, you will have to increase your offset frequency (by adjusting the sidetone in the radio).  I used the nominal filter cutoff frequencies and bandwidths to calculate an "optimum" offset frequency of 544 Hertz for the 600 Hz roofing filter and the Ten Tec 221 filter combination.  

Here's how I did the calculations:

    Roofing Filter 600 Hz BW centered at 700 Hz  Nominal cutoff frequencies (-6 dB) are 462 and 1062 Hz

    Ten Tec 221 Filter 250 Hz BW centered at 500 Hz  Nominal cutoff frequencies are 390 and 640 Hz

    For the combo: the lower cutoff is approx. 462 Hz and the upper cutoff is approx. 640 Hz , resulting in a bandwidth of 178 Hz and     a (geometric mean) center frequency of 544 Hz.


A similar analysis substituting the Inrad 753 for the Ten Tec 221 gives a center frequency of 620 Hz and a bandwidth of 370 Hz in combination with the CW roofing filter.


I was interested in the latter combo, because that's what I'm currently using in my radio.  I've been using a CW offset of 550 Hz (which appears to be 70 Hz lower than optimum), and have noticed the asymetrical response skewed toward the higher frequencies in my operations.  Of course this can be compensated to a large extent by adjusting the passband tuning in the 6.3 MHz IF.

73
Chuck  NI0C