[TenTec] Redesigning the filter section of the Orion IF

[Bill Tippett <btippett@alum.mit.edu>]

Hi Lee,

W9OY wrote:
>If it's just a matter of too much stage gain why not
just put a pad in the signal line with the 500hz
filter?

        In fact I tried putting a 6 dB pad on my 500 Hz
filter input.  That did compensate the gain correctly,
but I really don't know if it completely solved the IMD
degradation (since I have no way to test IMD here).  My
guess is that it should help, but not completely solve
the problem, because you are still dealing with 2 filters
and 2 amplifiers for the 500/250 positions versus 1 filter
and 1 amplifier for the other positions.  My gut feeling
that simpler is better.  My feeling is also supported by
YT1NT's measurements of the 250 Hz filter since there is
almost a perfect match between its insertion loss (-12 dB)
and the extra amplifier gain (+12 dB).  I'm duplicating
Sinisa's measurements on page 5 here (but the format may
become messed up):

********************************************************
Two-tone IM performance
Conditions: 7 MHz band, Main RX to ANT 1, Sub RX to ANT 2, PREAMP OFF, ATTN 
OFF,
LCW mode, 250 Hz DSP CW filter.
This table gives the maximum IM-free input level (not producing audible IM3 
products).
Actual level is given first, with S meter indication in parentheses
Both are in dB's over S 9 with S 9 = -73 dBm (50 uV into 50 Ohms).

roofing ------------- signal separation -------------
filter 0.7 2.0 4.0 10.0
6000 Hz +15 (+21) +16 (+22) +21 (+27) +33 (+38)
2400 Hz +4 ( +7) +11 (+16) +18 (+23) +27 (+31)
1800 Hz +15 (+18) +34 (+38) +37 (+39) +38 (+40)
1000 Hz +27 (+31) +35 (+39) +39 (+40) +41 (+40)
500 Hz +0 ( +8) +25 (+32) +28 (+35) +28 (+35)
250 Hz +23 (+30) +23 (+30) +28 (+36) +27 (+34)
****************************************************************

Note that the 250 is not as good as the stock 1000 Hz filter
at 2 kHz spacings (~9 dB worse).  It doesn't look bad at 700
Hz spacing because this is getting well within the 1000
Hz filter's bandwidth, but that is not really a fair
comparison.  My conclusion is that the problem is more
than simply gain compensation.

>I tried to look at the link you provided but
the page seems to have entered HTML heaven.

        Yahoo/Geocities is sometimes cranky!  Try again or go here:

http://www.geocities.com/va3ttn/

and then click on the very first link.  Sinisa's entire .pdf
document is well worth printing out and saving.

                                        73,  Bill  W4ZV

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