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Bandpass Filter for 1.6 MHz

H

Hans Dampf

Jan 1, 1970
0
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
 
J

Joerg

Jan 1, 1970
0
Hello Hans,
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

Sounds like a low pass plus a high pass.
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.

Come on, tuning the via slot of an inductor core isn't that hard. Sounds
like you are German, in that case get a Bernstein tuning kit.

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?

My clear choice would be LC. Unless cost is not an issue :)

The topology depends on how true your group delays have to be. I suggest
to buy a copy of William's "Electronic Filter Design Handbook". You
could also design via SW. I used to do it with Otto Mildenberger's DOS
filter tools. You can also design active filters with it if you really
want to. Basically you enter the desired stop band, pass band and slope
windows and it spits out filter order and values.

Regards, Joerg
 
J

John Jardine.

Jan 1, 1970
0
Hans Dampf said:
Hallo

To clean up the outputs of a DDS I am looking for a bandpass filter with the
[...]
Regards,
Hans
I'd definately go with Joerg on the LC's. The 1M6Hz filter (especially) with
it's Q of 16 would be useless in that single opamp bandpass form.
If you went L+C with a 10kohm source and load resistance then each filter
circuit has a sufficiently low capacitor value that a 50pF trimmer cap'
could provide the final tune. For inductors, I'd just use standard E12 Toko
items. -Then- use a times 2 opamp to return to unity gain.
john
 
Q

qrk

Jan 1, 1970
0
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
Usually, one uses a low-pass filter after a DDS. If you need a
bandpass, LC filters work well at your frequencies of interest.

If you really want to use active filters, consider using a gyrator
based filter. Gyrators are quieter than a MFB or state variable
filter. Plus, the component sensitivity of a gyrator is around 1. The
state variable types tend to have a component sensitivity near the Q
value. At your frequencies of interest, you will have GBW issues to
deal with. See Jim T's web page for some gyrator ideas and hint:
http://www.analog-innovations.com .
 
J

Joseph2k

Jan 1, 1970
0
Hans said:
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.
 
J

John_H

Jan 1, 1970
0
Joseph2k said:
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.

There are applications that would enjoy reduced level of out-of-band
spurs below the DDS noise floor. It's not necessarily ignorance that
makes an engineer look for standard design elements to achieve stringent
design goals.
 
R

Rene Tschaggelar

Jan 1, 1970
0
Hans said:
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. [snip]

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

Since the only noise source of interest is the master
clock and its harmonics, I'd suggest some (frequency-)
distance to the clock, and lowpass the master clock
out. So, have at least 16MHz Masterclock, this gives
you a decade and thus 20dB per order of the filter.

Rene
 
H

Hans Dampf

Jan 1, 1970
0
Hello Joerg
Thanks for your reply.
Sounds like a low pass plus a high pass.
Yes, for the bandpass having 200 kHz center frequency a LP/HP
combination would do the job. But for the 1600 kHz filter the Q of
16 (bandwidth 100 kHz) needs a true resonator type bandpass.
Come on, tuning the via slot of an inductor core isn't that hard. Sounds
like you are German, in that case get a Bernstein tuning kit.
I would rather electronically tune a varactor than use a trimmable inductor.
Since money and board space is not critical, the additional tuning DAC
per filter may be worth it.
My clear choice would be LC. Unless cost is not an issue :)
I agree. Cost is not an issue though. Harmonics produced by the OpAmp
would be a killer anyway. The goal is to obtain a REALLY low noise and
spurious free output tone.
The topology depends on how true your group delays have to be. I suggest
Another specification is that the delay be as short as possible. If the DDS
output amplitude is changed, the filtered signal should react as fast as
possible,
i.e. in less than 10 us. This conflicts with the bandwidth requirement.
Short
group delay has priority.
to buy a copy of William's "Electronic Filter Design Handbook". You could
also design via SW. I used to do it with Otto Mildenberger's DOS filter
tools.
Thanks for these suggestions.

Regards,
Hans
 
H

Hans Dampf

Jan 1, 1970
0
Sali Rene
Since the only noise source of interest is the master
clock and its harmonics, I'd suggest some (frequency-)
distance to the clock, and lowpass the master clock
out. So, have at least 16MHz Masterclock, this gives
you a decade and thus 20dB per order of the filter.
Besides the noise sources you mention, there is also quantization noise
which
sets the DDS noise floor, and some very weak spurious tones, no matter
how clean the DDS clock is. The signals will be used for AC-biasing of
some very sensitive sensors. Without filtering, the very broadband noise and
together with the spurious tones would sum up to a power level comparable
to the signals to detect. Filtering the DDS signal is therefore essential.

Regards,
Hans
 
H

Hans

Jan 1, 1970
0
My questions: What circuit should I use here? Active opamp or LC, which
Usually, one uses a low-pass filter after a DDS. If you need a
bandpass, LC filters work well at your frequencies of interest.

If you really want to use active filters, consider using a gyrator
based filter. Gyrators are quieter than a MFB or state variable
filter. Plus, the component sensitivity of a gyrator is around 1. The
state variable types tend to have a component sensitivity near the Q
value. At your frequencies of interest, you will have GBW issues to
deal with. See Jim T's web page for some gyrator ideas and hint:
http://www.analog-innovations.com .

Hi Mark
Thanks for the suggestion to use Gyrators. I will have a look at it.
Hans
 
J

Joerg

Jan 1, 1970
0
Hello Hans,

Yes, for the bandpass having 200 kHz center frequency a LP/HP
combination would do the job. But for the 1600 kHz filter the Q of
16 (bandwidth 100 kHz) needs a true resonator type bandpass.

You could still do it with LP/HP if keeping the same architecture is
key. Considering your group delay tracking requirements it might be key.

I would rather electronically tune a varactor than use a trimmable inductor.
Since money and board space is not critical, the additional tuning DAC
per filter may be worth it.

You may also need to generate low noise 10V-20V for the varicap unless
you already have that.

Another specification is that the delay be as short as possible. If the DDS
output amplitude is changed, the filtered signal should react as fast as
possible,
i.e. in less than 10 us. This conflicts with the bandwidth requirement.
Short
group delay has priority.

Can't you just replace all the DDS with some nice low noise PLLs and be
done with it? If you do that right you may not need any filtering.

Regards, Joerg
 
F

Frank Bemelman

Jan 1, 1970
0
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.

Oh, and what part would that be? There is not much you can 'screw up'
on a bare DDS itself, it's the DDS principle that is already screwed
up, unless you believe that when your signal shown on an oscilloscope
is a clean sinewave when it looks like that. There's all kinds of
rubbish in it.
 
T

Tim Shoppa

Jan 1, 1970
0
Hans said:
Sali Rene

Besides the noise sources you mention, there is also quantization noise
which
sets the DDS noise floor, and some very weak spurious tones, no matter
how clean the DDS clock is.

If quantization noise is the issue, then you need more bits on your
DDS.

There are test-equipment-grade synthesizers (some of them do more than
just sine waves) that promise 16 or 18 bits (or maybe more!) of
amplitude resolution. A lot more expensive than a $5 chip from AD, in
fact they are closer to $50000. I would think that seeing as how your
max frequency is 1.6MHz (and these synths are often rated to hundreds
of MHz) that there is a happy middle ground.

Tim.
 
T

Tim Shoppa

Jan 1, 1970
0
Hans said:
Besides the noise sources you mention, there is also quantization noise
which
sets the DDS noise floor, and some very weak spurious tones, no matter
how clean the DDS clock is. The signals will be used for AC-biasing of
some very sensitive sensors. Without filtering, the very broadband noise and
together with the spurious tones would sum up to a power level comparable
to the signals to detect. Filtering the DDS signal is therefore essential.

As an afterthought, rather than constructing one tunable filter, it is
almost certainly better to make a number of fixed-tuned filters and
select the output via a multiplexer etc. You mention sweeping etc.,
obviously you do not want to switch filters in the middle of a sweep.
Depending on switching timing, the multiplexer could be relays.

If you didn't have the 10usec sweep or slew time you mention elsewhere
in the thread, this would be the perfect opportunity to use slug-tuned
coils tuned by steppers or maybe a voice coil :). (Don't laugh, it's a
powerful technique.)

Tim.
 
J

Joerg

Jan 1, 1970
0
Hello Tim,
If you didn't have the 10usec sweep or slew time you mention elsewhere
in the thread, this would be the perfect opportunity to use slug-tuned
coils tuned by steppers or maybe a voice coil :). (Don't laugh, it's a
powerful technique.)

I had a slug tuned oscillator a long time ago. It needed regular oil
changes for the thread. Clean the old oil off until spotless. Then add
weapons grade lube. Not too much but also not too little. Maybe a tad,
or a smidgen, or a wee bit. This was almost an art.

Regards, Joerg
 
A

Ancient_Hacker

Jan 1, 1970
0
I'd go with your basic LC tuned circuit. It's hard to beat the dynamic
range and noise performance of one or two LC's.
 
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