What’s the Best Scope for My Application?

Question: I’m looking to do some experimenting with digital chips. The clock frequencies I’ll be working with is 128 Mhz. I was thinking about purchasing a TDS2024B would this scope be adequate for this.

I would like to know some of the important differences between the TDS2000 and TDS3000 series oscilloscopes

Thank you for your help.

Answer: When choosing an oscilloscope, you typically want the oscilloscope’s bandwidth to be 3X to 5X the frequency of your signal, depending on the application. 

All oscilloscopes have a low-pass frequency response that rolls off at higher frequencies.  Traditionally, oscilloscope bandwidth has been specified as being the frequency at which a sinusoidal input signal is attenuated to 70.7% of the signal’s true amplitude. This is known as the “-3 dB point”, which is a term based on a logarithmic scale.  It means that a sine wave measured at the oscilloscope’s bandwidth rating will have an amplitude error of -3 dB, or almost 30%. 

Now, as you might recall, any complex signal – such as a digital signal – can be re-created by overlaying a series of sine waves of different amplitudes and frequencies.  These different sine waves are also known as spectral components.  To accurately represent a digital signal – like you might find running on a data bus – the fundamental, third harmonic and often the fifth harmonic spectral components must be captured during measurement.  If bandwidth limitations on the measurement system limit the captured bandwidth to less than these harmonics, the signal will lose key features.

Having said that, it’s also important to look at the rise time of your digital signal.  The edge speed (rise time) of a digital signal can carry much higher frequency components than its repetition rate might imply. You want your oscilloscope’s rise time to be at least 5 times faster than that of your digital signal.

If your clock is working at 128 MHz, you’ll probably want a scope with a bandwidth greater than 640 MHz.  I’d recommend that you take a look at our MSO/DPO4000 Series.

Now, to answer your other question:  One of the key differences between the  TDS2000 and TDS3000 is available bandwidth and performance.  The TDS3000C Series offers up to 500 MHz bandwidth and 5 GS/s sample rate compared to 200 MHz and 2 GS/s on the TDS2000B.  The TDS3000C also has deeper record length and more bells and whistles – more triggers, more automated measurements, DPO technology display, Wave Alert, and optional analysis packages like advanced waveform math and analysis, limit testing, telecommunications mask testing, and video debug.  I’d suggest going to www.tektronix.com/oscilloscopes.  We have a comparison tool there that will allow you to compare the TDS2000B and TDS3000C side-by-side to really see how they compare.

Good Luck!

Explore posts in the same categories: Ask Scope Guru Q/A, Oscilloscope Fundamentals


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