Am 2022-05-16 15:16, schrieb Robert LaJeunesse:
FYI there are some rather flat video filter ICs that have been made in
the past. The 6th order HMC1023LP5E is tunable, at its 28MHz setting
its flat then down 0.1dB in the teens, down 0.35dB at 20 MHz. That
same setting is 60dB down at about 90 MHz. It is also a dual part,
designed for matched I-Q filtering.
Declared dead at DigiKey.
Sent: Sunday, May 15, 2022 at 5:29 PM
From: "ed breya" <[email protected]>
The actual filter I've been using does a good job on the higher
frequencies, but is poor on flatness. It has about 2-3 dB p-p passband
ripple, with periodicity around 5-7 MHz. I've tried various padding
arrangements at both ends, all of which tend to flatten it only a
little
bit at best. Looking at it with the TG/SA setup, the character is
intrinsic to filter, and not due to just its reaction to the mixer and
cabling and such.
I hate building filters. Designing them in principle is easy, with all
sorts of available tools online, but actually rounding up the real
parts
(and their parasitics) and physical implementation is a PITA. But, I
suppose I'll have to do it eventually for this project. I know how
nice
it can be, with the right filter, but for now, I'll have to go with
what
I have.
Did you choose a Chebyscheff design to start with? These accept some
ripple
in the pass band, maybe some dB, to buy a steep rise of attenuation
above f-3dB.
best regards, Gerhard
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