DSO Scopes

Discussion in 'General Electronics Chat' started by r010159, Mar 31, 2014.

  1. r010159

    Thread Starter New Member

    Mar 31, 2014
    I have purchased a HP/Agilent 54111D. I need some help to understand. I apologize in advance for my ignorance and this may be the wrong forum to post. Please point me to the right forum.

    Let me ask a basic question on DSO specs. I see oscilloscopes have come a long ways offering performance and features at a fraction of the price of 25 year old equipment.

    I am finding Agilent and Tektronics and BKPrecision scopes to offer 500 MHz capability, but at a sampling rate much higher than the 1Gs/s of the 54111D. Why the need for these much higher sampling rates to accomplish the 500 Mhz bandwidth? I find 200 Mhz scopes sampling at 2Gs/s. There are others with higher sampling rates. Also there is a difference between sing-shot and repetitive bandwidth. When a scope is specified to have 200 MHz, is this single shot or repetitive bandwidth?

    The effective resolution of my scope is 6-bits to 8-bits. This is frequency dependent. There are scopes that provide a claimed nominal resolution of 8-bits and even a higher effective resolution, perhaps up to 12-bits. This must also be frequency dependent, correct?

    Any help would be appreciated. And if you only can refer me to another document or forum on the Internet please do.
  2. crutschow


    Mar 14, 2008
    Lower sampling rate oscilloscopes are more subject to aliasing of higher frequency signals (those higher than 1/2 the sample rate). If you sub-sample a high frequency signal, it generates a bogus lower frequency (alias) signal that can appear on the screen. It's rather like undesired image frequencies in a superhet receiver. The higher the sample rate the higher the input frequency must be to generate this alias frequency.

    The stated frequency response may be repetitive and/or single-shot, it depends upon the design.

    The resolution may or may not reduce with frequency. Again it depends upon the design.
  3. r010159

    Thread Starter New Member

    Mar 31, 2014
    Thanks for your replay, crutshow! For one scope with 200 Mhz bandwidth, according to Nyquist, you only need a sampling rate of 400s/s. This one scope bass a sampling frequency of 2Gs/s.

    So let me think...I think I remember that in order to capture transient signals in single-shot mode, you need a sampling frequency of 4 to 5 times that of the frequency of the signal being captured. This means for 200 Mhz signals a 1Gs/s sampling rate would be sufficient. This assumes the 200 MHz is the single-shot bandwidth. So maybe it samples at 2Gs/s (which apparently is overkill) to insure enough performance to accurately capture these signals?

    And in continuous mode, this ability would represent a bandwidth of up to 1 Ghz (at the very least 500 Mhz). So why does this scope only have a bandwidth of 200 Mhz?

    Am I getting this wrong?


    PS: I am aware that there is also a "4 to 5 times" rule for the bandwidth needed to accurately capture a square wave of a given frequency. So a 200Mhz scope would be only able to accurately represent a square wave at 40 Mhz.
    Last edited: Mar 31, 2014