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G8MNY > TECH 16.03.04 09:11l 54 Lines 2537 Bytes #999 (0) @ WW
BID : 48516_GB7CIP
Read: DB0FHN GUEST OE7FMI
Subj: VHF Filter Design?
Path: DB0FHN<DB0FOR<DB0SIF<DB0EA<DB0RES<ON0AR<ON0AR<F6KMO<7M3TJZ<IK1ZNW<
GB7CRV<GB7CIP
Sent: 040316/0114Z @:GB7CIP.#32.GBR.EU #:48516 [Caterham] $:48516_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
The other day I took my spectrum analyser to a club test night! I also took my
RF pulse source (as on packet) & put it through a standard 50MHz 2 pole filter,
as in many text books.
dB Wanted Spurious
0 ´ .. .
ÚÄÄÂÄÄÄÄÂÄÄ¿ ´ :: . .
³ === === ³ -20 ´ :: . .
³ C C ³ ´ .. . .
Ú´__C C__ÿ -40 ´. . . .
À´ C C ÃÙ ´ \____________/ \
ÀÄÄÁÄÄÄÄÁÄÄÙ ÀÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄ
50 100 150 200 250 300 MHz
On the analyser we noticed a strong 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 as though the driving turns taps on the coils were coupling directly
because when we tried some earthed metal in the inter coil gap near the cold
ends the spurious reduced.
But testing @ home I measured only a few dB improvement with small plates at
that point! But I found that a copper strip between the coils 3/4 the way up to
the hot end & bent near one of the coils did the trick!
dB Wanted
0 ´ ..
ÚÄÄÂÄÄÄÄÂÄÄ¿ ´ ::
³ === === ³ -20 ´ ::
³ C C ³ ´ .. Reduced
Ú´__C ³C__ÿ -40 ´. . . .
À´ C C ÃÙ ´ \________ / \._./ \_
ÀÄÄÁÄÄÄÄÁÄÄÙ ÀÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄ
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 peak on @ 210MHz & 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 filter the ideal is to use 2 asymmetrical tuned circuits
that do not have identical spurious resonances!
Anyone else tested there filters?
/QSL
73 de John G8MNY @ GB7CIP
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