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G8MNY > TECH 14.01.16 11:03l 60 Lines 2600 Bytes #999 (0) @ WW
BID : 56506_GB7CIP
Read: GUEST
Subj: 6m VHF Filter Design
Path: IW8PGT<CX2SA<GB7CIP
Sent: 160114/0708Z @:GB7CIP.#32.GBR.EURO #:56506 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (April 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 ´ .. ..
ÚÄÄÄÂÄÄÄÄÄÂÄÄÄ¿ ´ :: RF SPECTRUM . .
³ === === ³ -20 ´ :: . .
³ C C ³ ´ .. . .
ÚÄ´___C C___ÃÄ¿ -40 ´. . . .
ÀÄ´ C x C ÃÄÙ ´ \____________/ \
ÀÄÄÄÁÄÄÄÄÄÁÄÄÄÙ ÀÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄ
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 ´ ..
ÚÄÄÄÂÄÄÄÄÄÂÄÄÄ¿ ´ :: RF SPECTRUM
³ === === ³ -20 ´ :: Reduced
³ C C ³ ´ .. Spurious
ÚÄ´___C []C___ÃÄ¿ -40 ´. . . .
ÀÄ´ C C ÃÄÙ ´ \________ / \._./ \_
ÀÄÄÄÁÄÄÄÄÄÁÄÄÄÙ ÀÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄ
[] 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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