Re: Bandpass Filter for 1.6 MHz

Hans Dampf wrote:

Hallo

To clean up the outputs of a DDS I am looking for a bandpass filter with
the following properties:
+ Center frequency 200 kHz to 1600 kHz, fixed
+ 3-dB bandwidth 100 kHz
+ Voltage gain 1
+ Max. signal level 2 V pp
+ Slopes 20 dB/dec

There will be multiple DDS, each producing a different frequency in the
range from 200 to 1600 kHz. Each DDS may sweep by +- 50 kHz around its
nominal frequency. The filters should reduce noise, harmonics, and aliases
from the DDS output signals.

Filters should be simple to construct and tune. The effects of component
value tolerances on the center frequency must be compensated somehow,
preferably by adjustment of one resistor.

I tried to use a multi-feedback (MFB) opamp bandpass filter topology. See
here for a schematic and dimensioning:
http://sound.westhost.com/project63.htm . I tried with R1 = 12 k, R2 = 62,
R3 = 24 k, C1 = C2 = 82 p. What I like about this circuit is that its
center frequency can be adjusted by just changing R2, without affecting
bandwidth or gain. What I don't like is that the voltage divider R1 - R2
heavily attenuates the input signal, resulting in an extremely noisy
output signal. It amplifies the opamp noise voltage by a factor of several
hundred!

My questions: What circuit should I use here? Active opamp or LC, which
topology? Which opamp?

Regards,
Hans


If you have screwed up your DDS so badly as to require (apparently serious)
cleanup you deserve to Flunk of be fired whichever is applicable. There
are obsolete parts (over 12 years old) that would not engender this level
of problems.
--
JosephKK
Gegen dummheit kampfen Die Gotter Selbst, vergebens.
--Shiller
.

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