Re: Tek 468 and quantization?
From: Bob Stephens (stephensyomamadigital_at_earthlink.net)
Date: 10/14/04
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Date: Thu, 14 Oct 2004 20:22:39 GMT
On Thu, 14 Oct 2004 10:57:56 -0700, John Larkin wrote:
> On Thu, 14 Oct 2004 17:19:10 GMT, "John Smith"
> <kd5yikes@mindspring.com> wrote:
>
>>Hi -
>>
>>I bought a Tek 468 which I returned today. I thought it had a problem, but
>>the seller disagreed. I would like to know if I am wrong.
>>
>>The 468 is an analog/digital scope. In the digital mode, I noticed that the
>>trace had rectangular pulses on it of about .1 cm amplitude and apparently
>>random in frequency and width. This was with the inputs grounded using the
>>switch on the front panel. The amplitude of these pulses did not change when
>>I changed the attenuator setting except at the three most sensitive settings
>>of the attenuator (2 mV, 1 mV and .5 mV). Those three most sensitive
>>settings were active for digital storage operation only and were
>>non-functional in the analog mode.
>>
>>When I complained about it, the seller said that it was caused by
>>quantization, it met Tek's specifications, and that .1 cm was entirely
>>reasonable. I disagreed because all waveforms appeared noisy so one would
>>never be able to tell whether the observed waveform was at fault or the
>>scope was at fault. In addition, .1 cm is 1 part in 80 for the vertical
>>displacement (8 cm vertical grid). It didn't seem right that maximum
>>resolution would be 1 in 80 or even 1 in 100. Logically, I felt that the
>>resolution would be 1 in 256 so that quantization noise would be much less
>>than actually observed.
>>
>>Maybe I understand nothing about scopes and digital stuff. But, I could not
>>believe that Tek would sell such a product. Am I wrong?
>>
>>Thanks for any comments.
>>
>>John
>>
>
>
> I have a TDS2012 on my desk, displaying RS232 poll/reply sequences for
> a uP thing I'm testing. With no input, it has about 0.1 cm p-p noise
> at most gain settings, increasing to 0.5 cm p-p at 2 mv/cm. The noise
> appears, to me, to be a mixture of real front-end analog noise, adc
> quantization, and lcd pixellization. Welcome to the digital age!
>
> John
I had a TDS2012 on my desk and the quiescent noise was so bad I called up
the metrology lab we use for a repair quote. They said that this is a known
problem with this series of 'scopes and that no one including Tektronix
will fix them. He maintained that it is a condition that worsens with age
rather than an inherent design flaw.
Bob
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