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
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|