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unexpected noise

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unexpected noise

Postby denise » Wed Jul 05, 2017 7:32 pm

I have a brand new 6402C that I am trying to understand. 'started by connecting included probe to calibrator and observing greater trigger jitter than expected. I then noticed that the noise on channel B decreases dramatically when Channel A is enabled - even with no probe or anything else connected to Ch A input. Why is this??? While it is true that the calibrator frequency is low enough that I can improve capture characteristics by enhancing to 12b, or limiting the bandwidth, I am still curious. By the way, SW install and initial setup was easy - maybe even intuitive - good job!
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Re: unexpected noise

Postby Gerry » Tue Jul 11, 2017 10:05 am

Hi Denise,

You say the jitter was larger than expected. Could you tell us exactly what it was (or better still save and post the psdata file for it)?

The noise should not increase on channel B like that. Could you post the saved psdata files for the noise on Channel B with Channel A enabled and disabled, and also give us more detail of how everything is setup and connected up.

Regards,

Gerry
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Re: unexpected noise

Postby denise » Mon Jul 17, 2017 2:13 am

Gerry,

I have spent a little more time with the Picoscope and I do not believe the performance is acceptable. I tried to address some environmental matters and so I disconnected a second computer, I disconnected a docking station, I disconnected the second monitor, etc., etc. until I had nothing left but the Picoscope and my SurfaceBook laptop (running on it's battery!).

Then I tried using the built-in function generator, but it performed about the same as the front panel 'calibrator' test point. With nothing but a single probe connected to the test output using any channel (at least A, B, or C) I had so much noise that it would occasionally double trigger (that is it would satisfy the rising edge trough the threshold twice in a few percent of a cycle). Finally, I just connected the tip of the probe to it's ground lead and observed about 5% of full scale as noise. I then looked briefly at the spectrum and saw multiple peaks in the display - so I'm guessing I not receiving a broadcast station...

Also, as I mentioned, if I limit the input BW to 20MHz, or introduce the SW lowpass filter, or if I select 10/12b, the trace is pretty and looks ok to me. But, 20MHz 'scopes are quite a bit less valuable than 200MHz models. Anyway, unless you have some wisdom to share, I'd like to RMA this unit (and I'll enclose some printed images).

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Re: unexpected noise

Postby denise » Mon Jul 17, 2017 6:35 am

attached are a couple of images of the displayed square wave 'calibrator' signal - I have other images,including an FFT, but I guess they are too big to post here

anyway, this is the noise level that I think is way too high

I will try to call tomorrow
Attachments
20170717-0003.psdata
(9.95 KiB) Downloaded 13 times
20170717-0002.psdata
(27.43 KiB) Downloaded 12 times
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Re: unexpected noise

Postby bennog » Mon Jul 17, 2017 5:01 pm

Are you near a radio transmitter. ?
The trace shows a 99.4 Mhz signal AM modulated.

can you repeat the test on a location couple of km (more than 5km or 3.5miles) from where you did the first test.

Or try a different computer (maybe your laptop is leaking the 100Mhz signal over the USB cable to the scope)

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Re: unexpected noise

Postby Gerry » Tue Jul 18, 2017 1:44 pm

Hi denise,

As pointed out by bennog, your data file clearly shows that you have interference (not noise) due to an amplitude modulated carrier frequency typical of a radio transmission. The approximate transmission frequency is 100MHz, (although the modulating waveform is not at audio frequencies). The modulation is 100%, so the multiple peaks in the spectrum plot that you saw are due to the frequencies used for modulation, as well as the carrier).

So, could you answer the following:
1/ Do you get the modulated waveform, with no probe connected to the input channel (i.e. does it just modulate around 0V)?
2/ Does this also happen on channels A and C?
3/ What USB lead have you been using (were you using the Blue Pico USB 3.00 lead)?
4/ When you were using the function generator what frequency signal were you using?
5/ What do you have connected (if at all) that is 3/4 m long (e.g. USB lead/probe/power cable) This would be a 1/4 wavelength of 100 MHz (turning it into an antenna)?

Also, as we are now just diagnosing either a fault or setup issue. So, could you send your answers in to us at support@picotech.com. Could you also send a photo of your setup and it's surrounding area.

Regards,

Gerry
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