To: | tentec@contesting.com |
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Subject: | [TenTec] Redesigning the filter section of the Orion IF |
From: | Bill Tippett <btippett@alum.mit.edu> |
Reply-to: | tentec@contesting.com |
Date: | Sat, 17 Jul 2004 15:11:28 -0400 |
List-post: | <mailto:tentec@contesting.com> |
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 _______________________________________________ TenTec mailing list TenTec@contesting.com http://lists.contesting.com/mailman/listinfo/tentec |
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