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OK1VKZ > TECHNICS 07.09.04 00:31l 45 Lines 2372 Bytes #999 (0) @ WW
BID : 69EOK0PPL057
Read: GUEST OE7FMI
Subj: Re:Lowpass filter 5MHz (G8MNY)
Path: DB0FHN<DB0FOR<DB0SIF<DB0FHK<DB0LJ<DB0RES<OK0PPL
Sent: 040906/2326z @:OK0PPL.#BOH.CZE.EU [Plzen JN69QR] openbcm1.04beta21
From: OK1VKZ @ OK0PPL.#BOH.CZE.EU (Zdenek)
To: TECHNICS @ WW
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Ad "Lowpass filter for 5MHz" from G8MNY et al. :
Supposing, both L´s in the first design are 2uH and the L´s in the second one
are 2uH and 1.5uH respectively, and R(in)=R)out)=50 Ohms, I have some remarks:
(1) To both schemes : Their VSWR is by 3.5 MHz and above hardly admissible
[>1.5].
(2) For lower decrement in permeable band (below their cut-off frequency) and
good suppression on higher frequencies, there are the much preferable "4K" or
Butterworth´s filters.
(3) There are good-class freeware programs enabling LC filter design (e.g.
ELSIE - Student edition - up to LC filters of order 7) or their analysis at
least (e.g. SIMetrix Intro). Both mentioned are in English and freely
available to download on internet (www.tonnesoftware.com ; www.catena.uk.com).
(4) Indisputably, they are other free programs for this reason too. But using
this two I´ve got indisputable experience, that their results differ quite
insignificant one another. And if by practical realization significant
deviations occured, the difference was always caused by a mistake or
inaccuracy in realization.
(5) To support my remarks mentioned above, I show most important results
obtained with ELSIE and verified with SIMetrix on both filters published by
G8MNY [(A) ; (B)].
Furthermore I add a filter "4K" [(C)], which was designed per ELSIE for
cut-off (-3dB) frequency 8 MHz. (The exactly computed results of L&C
components "4K" were rounded to tens of pF and to tenths of uH, and with this
rounded values the properties of this "4K" were computed again, then realized
and gauged. These rounded values are : C1=C3=360 pF ; C2=720 pF ; L1=L2=1.8
uH)
Cut-off (-3 dB) = (A)8.756 (B)7.388 (C)7.905 MHz
VSWR (3.5 MHz) = (A)1.78 (B)1.62 (C)1.19
VSWR (5 MHz) = (A)2.14 (B)2.18 (C)1.29
transmission (3.5 MHz) = (A)-0.440 (B)-0.339 (C)-0.123 dB
transmission (5 MHz) = (A)-0.737 (B)-0.800 (C)-0.212 dB
transmission (10 MHz) = (A)-9.4 (B)-50 (C)-14 dB
transmission (20 MHz) = (A)-43 (B)-37 (C)-47 dB
transmission (50 MHz) = (A)-84 (B)-59 (C)-88 dB
Moral of the story : Just a few hams have access to measuring instruments of
really good-class. But almost each of us can get some freeware computing
programs of adequate quality and learn to use them ...
73´s to all de Zdenek OK1VKZ
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