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noise when use x100 probe with picoscope 4227

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noise when use x100 probe with picoscope 4227

Postby digbabo » Mon Nov 28, 2011 2:52 pm

Hi,
I am using picoscope 4227 to measure lightning impulse voltage up to +/-1MV. The input to the scope will be up to 1200V, I have to use x100 probe to reduce the input to range of +/-20V. When I use one of my x100 old probe (pb17 from GOULD maybe) with picoscope, it picked up alot of noise in the rise front region,sometime the noise even larger than the signal, and also the front time becomes longer. However, with the same probe and a tektronix DPO7254, there is not much noise in the signal. The TA132 works fine but not enough for high voltage. Do you know the reason for this? Do I have to do any screening or adjustment before I use the passive x100 probe (I cant see any from picoscope)? Any sugestion if I want to measure lightning impulse over-voltage?
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Re: noise when use x100 probe with picoscope 4227

Postby Hitesh » Tue Nov 29, 2011 1:39 pm

Hi digbabo,

Have you ensured that the probe is grounded correctly and are you using a laptop or desktop PC? If you are using a laptop try disconnecting the mains power supply.

Also, have you tried applying a low-pass filter on the Channel?

Please send in a .psdata file of your capture to support@picotech.com and we can take a look at it.

Regards,
Hitesh

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Re: noise when use x100 probe with picoscope 4227

Postby PeterF » Wed Nov 30, 2011 8:51 am

Hi,
It is impossible for us to duplicate your set-up but my guess is that the problem is caused by the rather old Gould probe being designed for a scope with much higher input capacitance (30pF) whereas the 4227 has about 15pF input capacitance.
Is there a trim available on the x100 probe?.
Apart from that, please be very careful, +/-1MV is a lot of voltage, both for you and the scope!
Have you considered using our range of isolated, differential probes:-
http://www.picotech.com/picoscope4227-a ... tml#active
Please contact us again if you have any other questions.
Regards,
PeterF.
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Re: noise when use x100 probe with picoscope 4227

Postby digbabo » Wed Nov 30, 2011 6:05 pm

Thanks for your replies,
I was ground my probe by both HV earth and measurement earth, use both desktop and laptop (with battery), it doesn't help. I don't want to use the lowpass filter as it will distort the waveform.
rather old Gould probe being designed for a scope with much higher input capacitance (30pF) whereas the 4227 has about 15pF input capacitance
I hope this is true, however this probe work perfectly for the big DPO7254 where its input capacitance is 13pF. There is no strim on the x100 probe.
Apart from that, please be very careful, +/-1MV is a lot of voltage, both for you and the scope!
Thank you for this, the generator is isolated, I only work with the low voltage arm (but still high for me and the scope!). I tried one active HV differential probe and also a 20db attenuator but what I recieved is not the impulse.
One thing I realize is the picoscope pick up a lot of noise through the air even when nothing is connected to the scope and peak imulse generated is few tens kV, and this noise seems to be amplified when the probe is connected. I will send you some of the data
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Re: noise when use x100 probe with picoscope 4227

Postby dat » Wed Dec 07, 2011 4:43 am

Measuring lightning impulses with a normal oscilloscope isd notoriously unreliably. There is a reason why the impulse scopes are so expensive. Not only are stray currents affecting the measurement, and voltage drop over the ground leads is creating noise in the attenuator; the strong electro-magnetic fields probably even create interferences within the PicoScope circuit board. You need to shield, shield and shield everything again and consider the current flow. Any ground currents flowing through the scope will create noise. The current can be capacitively or inductively induced. The whole test field will also need a proper high frequency ground system.

Getting this right needs an understanding of high frequency problems and vivid imagination of where the problems might be. I've seen setups that had 30%-50% measuring error before we fixed it.

The difference in the noise performance between the PicoScope and the Tektronix is mostly caused because the shielding of the PicoScope is pretty poor compared to the much much more expensive Tek or Gould.
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Re: noise when use x100 probe with picoscope 4227

Postby digbabo » Thu Dec 08, 2011 11:02 am

You are right Dat, most of the noise is gone when the whole system is shielded. The problem is all for screening, not the probe as the similar amount of noise can be seen by the x10 and x100 probes.
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