# Why am I calculating different sampling rate than the scope shows for a given memory depth?

I have this scope with the following real-time sampling rate and memory depth:

I know the relation between the memory depth, sampling rate and time duration as:

LM = SR x TD

Where LM is the long memory depth, SR is the sampling rate and TD is the total time on the scope screen(time per division x total number of horizontal divisions).

I also know/learned that the scope adjusts its sampling rate when we change the times per division setting.

So I first set the memory depth of the scope as LongMem which is given as 2Mpts:

Then I set the times per division as 1ms.

This means that the total duration for the plot is 18x1ms = 18ms. It is because in this scope the total number of horizontal divisons is 18.

So from the equation I should get the follwing sampling rate:

SR = LM/TD

SR = 2 x 10^6 / (18 x 10^-3 sec)

SR = 111MSa

But here is what the scope shows as its sampling rate which is 25MSa:

What can be the reason I am getting different result for the sampling rate?

• The scope might save more data than is shown on the screen (i.e. TD might be bigger than the screen width) If you stop the acquisition, then adjust the timebase position, do you see additional acquired data? – The Photon Jun 15 '18 at 15:49
• What scope is this, I guess it has 4 channels – PlasmaHH Jun 15 '18 at 20:19
• @PlasmaHH No it is two channel scope model name Siglent SDS1102CML – user16307 Jun 16 '18 at 14:46
• @ThePhoton I will try that when I have access to the scope on Monday – user16307 Jun 16 '18 at 14:47
• @ThePhoton You were right. The scope was not showing all data. I had to stop it by Run/Stop button and could see all the acquisition and made calculation with the new time duration. Thanks! You can write an answer if you fancy. – user16307 Jun 18 '18 at 13:57

## 1 Answer

Probably because at 1GSa/s you would only get about 2ms of storable data, and just your screen area needs an order of magnitude more. Scopes adjust their sample rate based on the sweep speed, based on the assumption that if you're sweeping more slowly, it's probably because you're not looking for high speed signals.

• When I set time div to 1u sec the sampling rate the scope shows is 250 MSa. I dont think it is because 2ms thing. – user16307 Jun 15 '18 at 12:30