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home | newest check | boards | help index | log | ps | userlogin | send sysop | slog | status forward | bcm news | users | version | remove cookieG8MNY > TECH 09.05.24 18:11l 60 Lines 2598 Bytes #4 (0) @ WW BID : 7288_GB7CIP Subj: 6m VHF Filter Design Path: JH4XSY<N3HYM<W9GM<KC9UHI<KA1VSC<GB7CIP Sent: 240509/0906Z @:GB7CIP.#32.GBR.EURO #:7288 [Caterham Surrey GBR] $:7288_GB From: G8MNY@GB7CIP.#32.GBR.EURO To : TECH@WW By G8MNY (Apr 04) (8 Bit ASCII graphics use code page 437 or 850, Terminal Font) The other day I took my spectrum analyser to a club test night! I also took my RF pulse flat noise source (as published on packet) & put it through a standard 50MHz 2 pole filter, as in many text books. FILTER LAYOUT dB Wanted Spurious C Adj 0 エ .. .. レトトトツトトトトトツトトトソ エ :: RF SPECTRUM . . ウ === === ウ -20 エ :: . . ウ C C ウ エ .. . . レトエ___C C___テトソ -40 エ. . . . タトエ C x C テトル エ \____________/ \ タトトトチトトトトトチトトトル タトトツトトトツトトトツトトトツトトトツトトトツトト C = coils 50 100 150 200 250 300 MHz To my surprise on the analyser I noticed a strong broad peak at 280MHz that was about as strong as the 50MHz signal! This spurious response was unaffected by the 2 tuning Cs on the ends of the 2 coils! It looks to me as though the driving turns taps on the coils were coupling directly with each other because when we tried some earthed metal in the inter coil gap near the cold ends (x) the spurious reduced. But on later testing I measured only a few dB improvement with small plates at that point! But I found by experimenting, that an earthed 6mm wide copper strip between the coils 3/4 of the way up to the hot end & bent very near one of the coils did the trick! NEW LAYOUT dB Wanted C Adj 0 エ .. レトトトツトトトトトツトトトソ エ :: RF SPECTRUM ウ === === ウ -20 エ :: Reduced ウ C C ウ エ .. Spurious レトエ___C []C___テトソ -40 エ. . . . タトエ C C テトル エ \________ / \._./ \_ タトトトチトトトトトチトトトル タトトツトトトツトトトツトトトツトトトツトトトツトト [] Added Metal strip 50 100 150 200 250 300 MHz What this did was to upset the 280MHz overtone resonance of 1 of the coils to give 2 peaks, one @ 210MHz & the another @ 290MHz, but both peaks 40dB lower in level! This is much better than 1 big peak @ 280MHz @ 0dB Loss. Increasing the screening between the coils may have worked eventually, but will have had detrimental effect on the coupling factor, that is critical for best filter shape & minimum loss. I expect for 2 pole filters the ideal is to make sure that the 2 asymmetrical tuned circuits do not have identical spurious resonances! Anyone else tested there filters? Why Don't U send an interesting bul? 73 de John G8MNY @ GB7CIP
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