Low frequency LFO with a DVM meter?

Discussion in 'General Electronics Chat' started by relicmarks, May 1, 2008.

  1. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    What is the cheapest brands for DVM meters that has frequency measurements and transistor, capacitors,inductors all in one meter?

    THey have Craftsman DVMs with frequency measurements for around $30


    My main questions is that most oscilloscopes can't measure low frequencys well on the oscilloscope display , the waveform is not static and stable its just to low of a frequency

    When i'm measuring a LFO circuit from 10hz to 20hz on my oscilloscope it just looks ilke a dot moving up and down and its hard to measure the frequency

    So What instead of buying a storage oscilloscope wouldn't a cheap DVM meter with frequency setting measure low frequencys and LFO frequencys better?

    I just put down my craftsman DVM meter , set it to frequency and measure the output frequency of the LFO speed/rate and it should display the 10hz frequency right?

    Or are the cheap meters not good with frequency counting?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Well, a cheap meter is just that - a cheap meter.

    Some of them are surprisingly accurate. I picked up a couple of cheap DVM's from Harbor Freight, $2.99 and $3.99 on specials. They even have hFE measurement built-in, but no L/C measurement nor frequency. They're fine for most hobbyist-type applications.

    How accurate the counter is depends on how well it was initially designed, how many digits are displayed, and how recently it's been calibrated. If it has not been calibrated, all bets are off.

    You can make a rudimentary check by measuring the frequency of the AC voltage supplied to your dwelling. In the US, it's very close to 60Hz. From moment to moment it may vary slightly - but over the long term, it's pretty stable.

    They need to keep it stable not only so that clocks keep the correct time, but also that the power grid stays synchronized.

    A $30 meter will work fine for most hobbyist purposes. But if you require real accuracy, you will have to pay quite a bit for an accurate laboratory-quality instrument - that will need annual calibration.
     
  3. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    The new craftsman DVM have frequency , Temperature and duty cycle

    I was thinking the DVM meter measuring frequency would be better to display the measurement that a oscilloscope at low frequency don't you think?

    Even do the duty cycle percentage too , on the DVM it reads out the duty cycle percentage i think the DVM meter would be better also for this

    For Temperature what degrees are in the RED or bad for transistors ,ICs, regulators, diodes? do i have to read the data sheets for the temperature operating degrees ?

    Measuring Temperature: with a DVM
    Because i don't want to put my finger on the power transistor or IC's cause it hurts when they get HOT so do i just put my black probe on ground and the red probe on the IC's body not the pins right? the chassie of the transistor?
     
  4. Norfindel

    Active Member

    Mar 6, 2008
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    The DVM uses a thermal probe to measure temperature, some include the probe when you buy them, but on some of them, you must buy it separately as an accesory. You cannot measure temperature with the standard probes.
     
  5. Caveman

    Active Member

    Apr 15, 2008
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    Yes, you ALWAYS have to read the datasheet!
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    I'm gonna play devil's advocate here, for home use calibration isn't as big an issue, especially for DVMs. Crystals and other time bases do vary over time, but it is in extremely small percentages. If the equipment was good to begin with the odds of it dying from other causes before it's accuracy drifts significantly is pretty good.

    When I was doing metrology work we had a 10Mhz time base calibrated from the atomic clock (the real deal, not what you buy at Walmarts, which is a radio reciever getting the time from an atomic clock far, far away). There are techniques to get it down to parts per billion, but for the most part a home user would never notice the difference.

    Still, I've wondered if the 10Mhz WWV signal is based off their clock...
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    They broadcast the WWV time signal at 2.5MHz, 5MHz, 10MHz, 15MHz and 20MHz too ;) There may be more frequencies, but that's just off a little "cheat sheet" I have kicking around here. There's also the WWVB that transmits on 60KHz.

    Oh, here's NIST's site:
    http://tf.nist.gov/stations/wwv.html

    There are IC's out that have decoders for the WWVB time signal, such as the Temic Semiconductor's U4226B.
    Datasheet available from here:
    http://www.alldatasheet.com/view.jsp?Searchword=U4226B
     
  8. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    NIST's tranmission won't get you too far in accuracy. For those parts per billion or lower, you'll need Loran-C or sattelite to synchronize your own Cesium standards. Not something for the hobbyist ... unless the HP-5071's have dropped in price in the used equipment stores.

    But back on topic ... what are you doing that requires you to "view" the 10 Hz or 20 Hz signals? You might be better off using a "recording oscillograph" or a storage scope with the persistence set high enough to trace the signals you require.
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    Just to answer the base question, any meter that does frequency for hobbyist use (read that 4 digits) is going to be accurate enough. We are talking a maximum accuracy of .1%, and truthfully, for most hobbyists dinking around .5% is fine. A counter with a 6 digit display is going to need occasional calibration, as the number of digits now means it should display to .001% accuracy, and that is approaching the accuracy of crystal time bases.

    I understand what you are talking about with Oscopes, they make many models that will overcome that, the latest being digital scopes, that use memory to make a static display. Their a bit expensive though. Older solutions involve scopes with special high persistence displays, that act as an analogue version of memory. They have some advantages over the newer digital scopes, but unless I'm mistaken are pretty much obsolete. Digital storage of waveforms can introduce a form of distortion called aliasing, where a signal looks clean and readable, but is actually way off. This has to do with the digital sampling of the scope strobing with the frequency being measured (and the frequency is usually on the high side). Given their expense this is not a problem for a hobby budget though.

    I wonder if a digital camera can be set up with a slow persistance, similar to older film cameras? This would be a really good solution to the problem.
     
  10. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    yes the storage o-scope haev a high persistence displays or a digital memory o-scope but both are very pricey

    I just got a $40 DVM with frequency setting to measure low frequencys it seems to work i think

    LFO output is 10hz i put my DVM on frequency setting the display on my DVM reading says 10hz

    The $40 DVM also has duty cycle reading in % percentage which is cool too because on the analog o-scope its hard to know where is 13% or 42% or 67% lets say and then you can fine tune it with trim pots using a DVM meter on duty cycle setting much better than a digital o-scope and analog o-scope
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    If the meter has the function to measure periods (which is doubtful) that is a better method to measure low frequency signals, as it gives you the inverse number but with a lot more digits of accuracy.
     
  12. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    Bill do you think that meters can't measure low frequencys good at all because they measure frequency bad? and not by the period?
     
  13. Wendy

    Moderator

    Mar 24, 2008
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    No, not what I said. Think of it this way, you have a freq between 10 and 11 Hz. That is approx 10ms of difference between them. If you have the period, say 95.635ms, then take the inverse and you have a more accurate number. It has to do with sampling technique. With a period counter, it is counting ms or us for every cycle, with the other it is counting how many cycles in 1 second. Either way you end up with a accurate count, but to get to .1 Hz resolution you have to set a freq counter for 10 seconds gate time. Sorry, didn't mean to disparage your new meter, this was meant to show a alternative technique for low frequency measurements. I use a DVM model myself, but I also have a new more expensive model in reserve (never been used matter of fact).
     
  14. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    No i didn't take it bad at all i was just asking

    So whats the difference between a "period counter" VS a frequency counter?

    Period counter counts in ms or us for every cycle

    Frequency counter counts how many cycles in 1 second

    Is every DVM meter only a frequency counter NOT a period counter? only the bench meters are period counters?
     
  15. Wendy

    Moderator

    Mar 24, 2008
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    More expensive freq counters have the ability to measure periods, just an added feature. They use the same time base, but instead of using the time base connected to the gate they tie the gate to the input frequency and count the time base, which ends up with an inverse number. Purely a matter of internal connections.
     
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