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DTMF additional frequency (harmonics) — Parallax Forums

DTMF additional frequency (harmonics)

black_tag10black_tag10 Posts: 7
edited 2004-09-28 22:00 in BASIC Stamp
hi,

i'm trying to make a dialing through a basic stamp 2P-40 using DTMF command. however when the circuit is connected, and dialing is made... the DTMF command doesn't seems to be functionally properly...

when my group tried to troubleshoot through a spectrum analyser, an additional harmonic has appeared at 100Hz frequency, no matter what number dialed.

is this 100Hz frequency critical??

regards;
black_tag

Comments

  • Jon WilliamsJon Williams Posts: 6,491
    edited 2004-09-20 03:39
    Are you filtering the output as shown in our documentation? I've used DTMFOUT (with acoustic coupling) to dial phones many times without trouble.

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    Jon Williams
    Applications Engineer, Parallax
    Dallas Office
  • black_tag10black_tag10 Posts: 7
    edited 2004-09-20 12:22
    i did wired up the given circuit connection, on page 109, figure 5.3 of the given Basis Stamp programming manual Version 2.0b... except the Sidactor, coz our lecturer did mention that this component is optional, moreover this sidactor is extremely hard to be found in singapore...shakehead.gif

    regards
  • K de JongK de Jong Posts: 154
    edited 2004-09-20 16:44
    100 Hz could be some residu from a rectified 50 Hz AC power.

    You might check your powersupply first. Just to be sureit is not coming from there.

    Klaus
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2004-09-21 00:07
    I agree, power supply noise can really make projects do weird things...I've seen it make LCD displays go haywire and make them display all kinds of garbage, even when nothing is being sent!



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    Chris Savage

    Knight Designs
    324 West Main Street
    P.O. Box 97
    Montour Falls, NY 14865
    (607) 535-6777

    Business Page:·· http://www.knightdesigns.com
    Personal Page:··· http://www.lightlink.com/dream/chris
    Designs Page:··· http://www.lightlink.com/dream/designs
    ·
  • black_tag10black_tag10 Posts: 7
    edited 2004-09-28 15:35
    ... dude thanks for the suggestions...·however the·AC mains is 50Hz, not really 100Hz... and the DTMF dial tone had shifted by another 100Hz...

    E.g. when number 1 is dialed, the frequency harmonics·will be generated as 697Hz & 1209Hz (http://www.ece.utexas.edu/~mason/codesign/dtmf.html)·and 2 frequency will occurred at the output, however the frequencies occurred on my Spectrum Analyzer are : 100Hz, 797Hz & 1309Hz ...

    can any guru of BS2SX·tell me what had happen...mad.gifshakehead.gif

    regards
  • Jon WilliamsJon Williams Posts: 6,491
    edited 2004-09-28 15:40
    Make the DTMFOUT pin an input (Hi-Z) and check with your spectrum analyzer then. If you still see the 100 Hz noise then it is coming from an external source. Perhaps a non-filtered full-wave rectifier is generating the 100 Hz hum from your 50 Hz input.

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    Jon Williams
    Applications Engineer, Parallax
    Dallas Office
  • black_tag10black_tag10 Posts: 7
    edited 2004-09-28 22:00
    hmm... the noise seem to be occurred after the 1:1 transformer...

    i tried to break down the circuit and test the functionality of each component, and realized that the 1:1 transformer happened to be <100 Ohms...(78ohms to be exact tested with my analog Multimeter)·will there be any problem due to the improper value of·transformer??????? cry.gif

    the given figure for the hardware connection·is·value at 600 - 600 ohms....

    Regards
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