DSO features checkup

Products & Technology DSO Quad
#1

Hi there.
Just today I have managed a small checkup of my brand-new DSO Quad, testing it with some signals and a pro scope.
Well, I have connected a HP signal generator 0 dbm (sine wave approx 0.5Vpp) on a 50 Ohms load, together with the Quad (ch-A) and a LeCroy scope (5GHz bandwidth).
So far so well, the first test is with frequency 1.0 MHz: clearly quite far from the 72 MHz analog bandwidth declared for the Quad.
The wave is well reproduced, having almost the same amplitude (no noticeable failures).
The first problem: by setting the trigger to AUTO (ch-A), the waveform displayed is totally wrong. It is not a sine wave but somewhat cut randomly: it is a clear firmware problem, I guess.
OK, never mind, let’s use the trigger set to NORM (rising slope).

Second problem, using a time/div under 1uS often the waveform freezes. I must turn off and on again to restore the activity.
However, during this situation the menus/switches are working fine (I can navigate, but the wave does not care about what I do).
I never noticed by using time/divs above 1 ms. I don’t know using from 1us to 1ms.

The second test wanted to measure the frequency at 10 MHz, even far from the 72 MHz.
One the LeCroy scope the signal is 10 MHz still showing the same amplitude of 0.5 Vpp (0dbm).
On the Quad the waveform is almost a straight line.
So I have tried to start from 1 MHz back, then step 1 MHz at time…just at 2 MHz the sine amplitude showed on the Quad is about 75% of the actual, that is -3 dB of bandwidth.
Over the problems above, my question is: how did you measured that 72 MHz of bandwidth?..As my test, I have detected just 2 MHz!

Thanks
Mario

#2

I do not think the analog bandwidth is said to be 72 MHz. The sample rate is 72 Mega samples / s though.

#3

I have the same suspect, but the product description clearly states:
DSO Quad is a pocket size 4 channel digital oscilloscope for common electronic engineering tasks. It’s based on ARM cortex M3 (STM32VCT6), providing 72Mhz analog bandwidth with integrated FPGA and high speed ADC. Internal 2MB USB disk could be used to store waveform, user application and upgrade firmware.

A little further a misleading:
Two 72Mhz analog channels and two digital channels
that can be interpreted in several ways…

Probably by adding somewhat like:
2 MHz analog bandwidth
few people are interested in.

Anyway, the 72Ms/s are real. What I find really strange is why the analog section is so limited.
I don’t expect to measure a 72MHz wave, but -for example- a square wave of 10 MHz with several harmonics!
I will test some square wave also.

#4

Yes, please do continue to explore and report here. This will be my first oscilloscope and I also read this statements and tried to understand the relation between sample rate and how high in freq I can use it. Someone guessed that 10 MHz would be the limit. I got notice today that I can pick it up, so tomorrow I will have the Quad in my hands. :slight_smile:

#5

You didn’t specify if channel “B” was disabled or not, and you didn’t specify what T/Div you were using. As you will see below, these both affect the results.

Capturing a waveform and displaying a waveform are two separate actions. I believe the display only has 8-bits of resolution, whereas the capture should have 14-bits resolution. When we get an XML file export capability for the Quad, then the actual capture data can be evaluated. Those steps in the low resolution display will affect the waveform appearance.

The Seeed Studio specifications do not specify analog bandwidth as a separate entity. They specify “36MS/s analog channels, up to 72MS/s if configured to single channel.” They have used “sample-rate” as a unit of channel bandwidth. I do see that the user manual does specify the analog bandwidth as you have stated … very peculiar.

With the DSO Quad, the square wave input will deteriorate above about 1/10 of the sample rate as is the case for most DSO scopes. This is why the more costly scopes have Gbyte sample rates. Combine this with the stepped 8-bit display resolution and the screen can turn pretty bad fairly quickly.

Another issue involved here is that when you increase the T/Div, then the sample rate decreases so as not to over-run the fixed sample buffer. So unless the T/Div is set at the faster T/Div, then the sample rate will decrease. Look at the sample-rate chart in BenF’s v3.61 User Guide and you will see how quickly the sample rate falls off as the T/Div is increased. His S/s numbers are different but the fall off rate will be similar. This is why it is recommended to use the fastest T/Div possible in conjunction with single-sweep to keep the sample-rate as high as possible. Then scroll across the stopped buffer to see all of the waveform. Of course when you do that you will discover some latent single-sweep triggering issues that have not yet been fixed.

The specs at :http://garden.seeedstudio.com/index.php?title=DSO_Quad#Specifications

state that prior to sys v1.31 each analog input would be 32MS/s which means about 3.2Mhz viewable, and 72MS/s for a single channel which means 7.2Mhz viewable. Sys v1.31 and later would double these figures. This seams to be the upper limits for square waves. Sine waves and triangle waves appearance are severely constrained by the Quad display resolution.

This is just like any other limited low-cost tool. The dumber the tool (or in this case, bandwidth challenged), then the smarter the user has to be to maximize the usefulness of said tool. To use these low cost DSO,s you have to acquire more knowledge than required to use a more expensive tool; to understand their limitations, and then to use them accordingly.

I am not justifying all of these issues, I am just attempting to explain your results. Maybe this will help to clarify your findings.