Relation between Sampling Frequency and Bandwidth

[deepak.sibi]

Hi,

1.What is the relation between Sampling Frequency and Bandwidth when you are using a Scope?

2.What is the maximum frequency you can measure on a 200 Mhz Oscilloscope?


TIA.,

Regards
Deepak

 

[FvM]

Hello,

1. I think, there is no exact relation. In single shot mode, it's obvious that you must have >2 points per cycle of waveform. If you equate bandwith with maximum frequency of sine wave, that could be displayed, this would be correct. But it also depends on how instrument software reconstructs waveforms from samples.

In repeated mode, when using equivalent-time sampling, there is no relation at all.

2. I assume, you're not measuring frequency but waveform. For signal frequencies near bandwith, no waveform information execpt amplitude and phase could be seen. You need at least threefold signal frequency to distinguish a sine wave from "not a sine wave", still without knowing the exact waveform.

Practically, if you are dealing with 100 MHz digital signals, you need 500 Mhz, better 1 Ghz oscilloscope to view signal quality parameters as overshoot and such.

Regards,
Frank

 

[echo47]

1. There is no relation, unless you limit your discussion to a particular type of scope. For example I have a scope with 500 MHz bandwidth and 5 GHz sample freq. I have another scope with 15 GHz bandwidth and about 200 kHz sample freq. My analog scopes don't sample at all.

2. Need more info about the particular scope, and which characteristic of the signal you are trying to measure. Scopes will usually display signals that are somewhat higher frequency than their specified bandwidth, but the displayed amplitude gradually decreases with frequency, and triggering becomes erratic or quits completely.

 

[deepak.sibi]

Hi All,

Thanks for the reply.

But for the last reply to my friend I am using a 200Mhz Tektronix Scope and I want to measure a Signal of 2.4Ghz.

Can it measure is my Question.Also with 200 Mhz Socpe upto what waveform can I measure.

TIA.,

Regards
Deepak

 

[echo47]

The frequency response of different model scopes rolls off at different rates, but most 200 MHz scopes won't show anything even close to 2.4 GHz.

Here's the approximate amplitude response of a classic Tektronix 475 analog scope. Even though it's advertised as 200 MHz, its -3dB bandwidth is a little over 250 MHz. Notice how the signal basically disappears above 700 MHz. The triggering stops working somewhere around 400 MHz.

    0 MHz -- 100%
  10 MHz -- 100%
100 MHz -- 100%
150 MHz -- 89%
200 MHz -- 85%
250 MHz -- 74%
300 MHz -- 49%
400 MHz -- 45%
450 MHz -- 32%
500 MHz -- 11%
600 MHz -- 3%
700 MHz -- 0%

When measuring amplitudes at 2.4 GHz, it's more common to use an RF power meter or spectrum analyzer.