Real analogue bandwidth of DSO068

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staceygeek
Posts: 11
Joined: Thu Feb 11, 2016 8:33 am

Real analogue bandwidth of DSO068

Post by staceygeek »

Hello everybody! I finally tested my DSO068 analogue bandwidth and ... I am disappointed. It has 5MHz analogue bandwidth claimed, according to the specification, but look at the picture, it shows what I got on the display when I fed the scope with 235kHz sine-wave signal. I thought the distortion occurs when the signal passes through the input circuit, but no way. I fed the scope with 10MHz sine-wave signal and used my 10MHz analogue scope to check the waveform in different test points of the circuit. So I can confirm, that up to 10MHz signal looks perfect all through its way to the pin 19 of A/D converter TLC5510. This IC directly communicates with ATmega64, so I cannot check anything else after A/D converter. Voltages at points VT, VS and VA have precisely the same values, specified in the circuit diagram (2.6V, 0,6V and +5V).

DSO068 displays reasonable waveform of the signal of maximum frequency of 400kHz. Trying to measure anything above it results in garbage on the display. And just in case, with 0,5uS resolution (the finest resolution of DSO068) you can observe a waveform of up to 1MHz. For 5MHz you need 1uS resolution.

To summarise the all above - The real analogue bandwidth of DSO068 scope where a waveform can be clearly displayed is 400kHz, far too narrow for the price it costs. Meanwhile, Frequency Counter part can correctly measure frequencies up to 8MHz.

Any comments from others are welcome. Has anybody achieved better measuring results?
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235kHz_sinewave_small.jpg
electrowiz
Posts: 1
Joined: Sat Jul 09, 2016 1:27 am

Re: Real analogue bandwidth of DSO068

Post by electrowiz »

Hi Staceygeek,

I found the same thing. Signal is good up to the A/D input. Should be no problem for the A/D, as it has a ~14MHz Analog input range. Just wondering if it is a software issue inside the Atmega MCU.

It really seems like it should be clean up past 200KHz which is where I start to see the problem. I loaded the latest firmware, still has the same behavior. It really looks like the A/D sample clock is getting mixed into the analog data stream.

Did you find any fix for this since your post?

electrowiz
jye1
Posts: 1221
Joined: Wed Feb 20, 2013 4:58 pm

Re: Real analogue bandwidth of DSO068

Post by jye1 »

The scope was specified 3M bandwidth only, not 5M. When timebase is set to 2us or faster equivalent-time sampling is used which relies on the moment of trigger to correctly construct waveform. When signal is noisy or its frequency is not stable equivlent-time sampling does not give good result. This is a weak point of the scope.
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