A question about impedance and function generators

Discussion in 'General Electronics Chat' started by Involute, Jun 16, 2011.

  1. Involute

    Thread Starter Active Member

    Mar 23, 2008
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    I'm looking at used function generators and most have at least one output that says 50 ohms, The manuals say the specs apply to a 50 ohm impedance (or load). Some have an additional output that provides TTL levels. What's the difference between the two (50 ohm and TTL)? I presume I can plug the TTL output directly into a TTL circuit, but not the 50 ohm. How am I supposed to use the latter? Thanks.
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
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    50 ohm output is pretty much standard for function generators, as it is a match for the 50 ohm coaxial cables that are often used to interconnect equipment. Whereas a TTL output is only appropriate for a digital signals of standardised levels, a 50 ohm output is generally useful for analogue or digital functions of arbitrary level.

    In some cases, the main output may be from 50 ohms impedance, with a range of available levels and waveforms, and an auxiliary fixed level TTL pulse output may be provided for such purposes as synchronising oscilloscopes.

    Normally the 50 ohm output will need to be coupled using an appropriately matched cable and terminated in a 50 ohm resistance in order to get the specified output level and wave shape. This is particularly important at higher frequencies.

    At frequencies from a few MHz downwards, it may also be possible to operate into higher impedance or even an open-circuit, when up to twice the rated output voltage may be obtained. Whether or not this is acceptable will depend on a number of factors, including the length of the interconnect and how much performance degradation can be tolerated.
     
  3. Involute

    Thread Starter Active Member

    Mar 23, 2008
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    Thanks for the thorough reply, Adjuster. If I wanted to take the 50 ohm output and drive a TTL-based circuit, how would I determine the proper termination value, and how do I implement it (e.g. a resistor in-line with the signal? Between the signal and ground?)? Thanks.
     
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
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    The standard termination is 50 ohms between signal and ground. The termination is never going to be too precise if you are putting a gate load in parallel so 51 ohms or maybe 56 ohms should do.

    That's a low enough resistance to pull down the TTL input, but whether or not you could get the levels right would depend on the output adjustments available from your generator - and setting them wrongly could blow things up. You might consider making an interface using something like a level comparator which could be made more tolerant.

    It's a long time since played with TTL though, and past my bedtime - perhaps someone else will come along with a recipe for doing this.
     
  5. Involute

    Thread Starter Active Member

    Mar 23, 2008
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    What levels would I be looking for? I've often seen terminators like this one on this kind of equipment, so I wonder if something like that's sufficient. The fact they make them at all suggests they're used for something, but they may be intended for analog work rather than digital.
     
  6. rdl2004

    New Member

    Aug 19, 2007
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  7. Involute

    Thread Starter Active Member

    Mar 23, 2008
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    Good resource. Thanks.
     
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