Ultrasonic Sinewaves

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Comments

  • CoronaKidCoronaKid Posts: 25
    edited 2010-04-22 - 05:39:34
    tdlivings said...
    Thank's for the update.
    I have been following Drone's DDS and your project.

    As far as your wonderment what comes to mind is what type
    of power supply bypassing do you have on your op amp. For
    the higher frequencys you will need a type of capacitor that
    still looks like a capacitor at the hundreds of kilohertz.

    Could also be the reason your filter is running out of gas.

    Tom

    Hi Tom,
    I do have a standard 0.1uf bypass cap·soldered·between V+ (pin 7) and Ground (pin 4) on the underside of the ProtoCard board·supplied by uController.com, but it does not have very large lands for the ground foil.
    See: http://ucontroller.com/documentation/protocard.html

    I never bothered to compute the XC as I will be reboarding with a LMF100 and a 1% 10pin network dac as soon as parts arrive.· The LMF100 lowpass knee·tops·at 100k and I'm looking forward to see how the input clock freq adjusts the bandpass of the LP filter.· Not sure how to program prop to output adjustable clock on one pin and track the DDS freq on another.· I'm assuming a NCO counter would be the easy way but need some direction with spin code.

    CK


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  • VIRANDVIRAND Posts: 655
    edited 2010-04-22 - 08:36:07
    How about outputting frequency N and N+50000 through two resistors connected together and then
    through a capacitor to a diode in parallel with a capacitor to VSS.
    The resistors may need to be different but I'm not sure.

    The basic idea is to make a SSB/AM tone signal and demodulate it,
    and hopefully filter out harmonics easily. Sorry I don't have the font to make the schematic.

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    Coming soon. My open Propeller Project Pages and favorite links index.
  • tdlivingstdlivings Posts: 437
    edited 2010-04-23 - 00:39:32
    Corona

    ·Thinking you are seeing high freq noise coupled in from the supply or ground

    I ask if you are using a single ended power supply you bias the non inverting

    input of the amp up to say half way. If you did do you have a high freq style

    cap bypassing the voltage divider you used for biasing.

    What do pin 7 and 4 look like with your scope. AC couple and look at the noise.

    You say it looks clean at the input to the amp but thinking in another direction

    could it actually be a bit fuzzy there and the filtered output of the amp now allows

    you to see the chunky DDS signal. As you go faster and faster with DDS the steps

    added to the accumulator get bigger and bigger and you start to see the stair casing

    in the waveform. I have been playing with the AD9833 and with 3khz out the sine wave

    looks smooth but at 500khz you start to see the steps in the trace. I am not talking the

    whole trace looks chunky just in the width of the trace drawing the sine wave you see

    the steps.

    Tom



  • CoronaKidCoronaKid Posts: 25
    edited 2010-04-28 - 20:54:33
    Thanks for the hints guys....

    VIRAND -·You have an interesting concept.··Works like a mixer with the difference being 50Khz.· Can you explain·the·frequency N and N+50000 in more detail, and·suggest part values?· Diagram & sample code would be nice too.

    tdlivings·(Tom) -·My op-amp·(OPA350) is configured as a·voltage follower with a gain of one.··The inverting input (pin 2)·is tied to·the output (pin 6).· I understand about bypassing the voltage divider on the non-inverting input, so may try that later when I get time.

    Now that my Switched Capacitor Analog Filter chips came in, I have set up another dac R2R board with Linear's LTC1065 5th order Bessel Lowpass Filter.· I played with the self-cocking feature of the 1065 and found that·the max·stable clock was only 2.25 Mhz, which means a filter cutoff of 22.5Khz which is too low for my application.· However this chip works excellent as an audio filter.· I was very impressed!

    I will be expermenting·later with the MAX295 and the LMF100 after I find a·easy way to breadboard these non-dip units.· Suggestions anyone?

    Ultimately·I want to clock the SCAF from a prop pin, but have found another·issue.
    The voltage threshold·to the 1065's·Clock input is 4/3 volts, while prop is 3.3/0 volts.
    I suspect that a transistor input swithch biased a 5v may do the job??

    Also below are DAC output waveforms for Drone's V1.03 DDS code.

    Still learning...
    KD



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    Post Edited (CoronaKid) : 4/28/2010 9:05:23 PM GMT
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  • tdlivingstdlivings Posts: 437
    edited 2010-04-28 - 23:15:50
    Hi

    I have had success using these protoboards

    http://www.capitaladvanced.com/9000ser.htm·DigiKey has them

    I posted a pic of my AD9833 experiment using one on the Pro Dev Board

    http://forums.parallax.com/showthread.php?p=891245

    I followed you and ordered some max295 etc chips and some protoboards to

    play with. As you say learning.

    Nice pics.

    Tom
  • CoronaKidCoronaKid Posts: 25
    edited 2010-05-07 - 04:22:06
    Thanks Tom,
    As per your suggestion, I picked up some Surfboards for my·MAX295 and LMF100 switched capacitor filter chips.
    The boards·work excellent for prototyping with the demo board.· A good test of my soldering skills though!!

    I·want to·test the LMF100 first, because of·its higher frequency cutoff (100khz).
    After·reading·Nationals data sheet,·I'm a little confused about computing the resistor values for 60khz cutoff.

    I want to use a NCO prop pin to track the ultrasound from 25 to 60khz, so the clock needs to be 2.5Mhz to 6Mhz range.· Should the resistor values for the chip·be computed at the highest Khz·frequency?·

    The first page of the data sheet has an example of a 4th order Butterworth Lowpass Filter at 100Khz using a bi-polar supply.· I'm using a single 10v supply, so I used the example on page 22 (See attached photo).
    http://www.national.com/ds/LM/LMF100.pdf

    Can anyone give me some insight on this?
    confused.gif·Fred

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  • tdlivingstdlivings Posts: 437
    edited 2010-05-09 - 15:20:25
    Fred
    ·
    A quick read of the data sheet looked like some of the modes the resistors only effect Q
    so it would not matter which frequency.

    Tom


    Post Edited (tdlivings) : 5/9/2010 4:19:05 PM GMT
  • CoronaKidCoronaKid Posts: 25
    edited 2010-05-10 - 03:18:37
    OK·Tom,
    Yes, I see what you mean about the Q.· However, I do not have the tables from Nationals Filter Handbook which specifies the Q for each stage depending on the frequency.· They also point out that the first stage should be a lower Q than the first.· In their example the Q of the 2nd stage is 4.5 times higher that the first.
    I don't know why they are using 529Hz and 993Hz for a 1000Hz filter when to clock is supposed to determine the cutoff Fo??

    Since the gain (HoA & HoB) of each stage could be unity (1), I managed figured that R1A/B, R4A/B could all be 20K.· We want to use mode 3 (data sheet page 15) but still have no idea what values to·compute for R2A/B and R3A/B.

    Guess I will just try their 100khz Butterworth example on page one, even though their 3.5Mhz clock will only do 70khz at 50/1.· Not sure how they figured 100Khz?· Also R4A/B resistors are gone.

    Also, noticed they do not specify·a maximum·Clock frequency, only 3.5Mhz as a typical example.· There are major gains to be had by using 100/1 clock/freq ratio, but would need 6Mhz clock to get 60khz.· Guess I'll just have to try it!

    I'm want do use a single supply at 10v with divider for 5v.

    Confused in filter land.
    CK

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  • tdlivingstdlivings Posts: 437
    edited 2010-05-10 - 13:57:47
    Fred

    Found this· http://www.national.com/an/OA/OA-26.pdf

    There are some tables in it .

    Tom
  • CoronaKidCoronaKid Posts: 25
    edited 2010-05-11 - 03:44:16
    Tom,
    I looked through that pdf before and picked the 2 Qs for the Bessel Low Pass Filter.
    Q1 = 0.522 and Q2 = 0.806, but that did not help me figure out the R's.

    I need to use a Bessel filter as·I've read there·is minimal phase shift across this type of filter.· I may want to phase (FM) modulate the signal later, so bessel may be more flexible for my application down the road.

    The examples in the LMF100 datasheet are for either Butterworth or Chebyshev types, so they are not much help.· Also, all the chips ELectrical Characteristics are for Mode 1 and I need Mode 3.

    After much reading, I figured out what they were doing with 529Hz and 993Hz for a 1000Hz filter.
    They are stagger tuning the filters bandpass on each stage.· I don't think I need to do this if I'm tracking the clock with the input frequency from my dac (fo/100).

    The data sheet page 15 list mode 3, and fo is computed as (fCLK/100) x sq root of R2/R4.· I figure R2/R4 should equal 1 in order for the clock to determine the fo, so I picked 47k for both. I'm not sure about values for R3.· See photo enclosed.

    CK





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