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Bandpass Filter for 1.6 MHz

Discussion in 'Electronic Design' started by Hans Dampf, Feb 6, 2006.

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  1. Hans Dampf

    Hans Dampf Guest

    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
     
  2. Joerg

    Joerg Guest

    Hello Hans,
    Sounds like a low pass plus a high pass.
    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.

    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
     
  3. 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
     
  4. qrk

    qrk Guest

    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 .
     
  5. Joseph2k

    Joseph2k Guest

    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.
     
  6. John_H

    John_H Guest

    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.
     
  7. 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
     
  8. Hans Dampf

    Hans Dampf Guest

    Hello Joerg
    Thanks for your reply.
    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.
    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.
    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.
    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.
    Thanks for these suggestions.

    Regards,
    Hans
     
  9. Hans Dampf

    Hans Dampf Guest

    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. 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
     
  10. Hans

    Hans Guest

    My questions: What circuit should I use here? Active opamp or LC, which
    Hi Mark
    Thanks for the suggestion to use Gyrators. I will have a look at it.
    Hans
     
  11. Joerg

    Joerg Guest

    Hello Hans,

    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.

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

    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
     
  12. 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.
     
  13. Tim Shoppa

    Tim Shoppa Guest

    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.
     
  14. Tim Shoppa

    Tim Shoppa Guest

    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.
     
  15. Joerg

    Joerg Guest

    Hello Tim,
    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
     
  16. 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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