Author Topic: Better 10:1 Probes for MSOX2024a ??  (Read 1040 times)

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Offline jstarr

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Better 10:1 Probes for MSOX2024a ??
« on: May 18, 2013, 08:46:53 pm »
I find the N2863B probes very awkward to attach and remove. The scopes BNC jacks are a bit too recessed and due to the odd rectangular shaped neck of the probe connector, when I remove a probe, my fat fingers push against the adjacent probe and more of a concern I find I am pushing
against the small delicate vertcal control knob.

I understand the basics of matching probe impedance to the scope but find the Agilent recommendations a little confusing so I appreciate any advice.

The MSOX2024A has a 1Meg ~11pF input.  The 10:1 N2863B is listed in the Agilent literature as a having 10Meg input resistance (when terminated into 1Meg input.  The probe connector has 10Meg//15pF printed on it.

Prior to buying the MSOX2024a, I picked up four new Agilent 10073D (500Mhz) 10:1 probes for the InfiniiVision 5000-6000 series at a hamfest for a great price.  These are constructed much better than the N2863B, having narrow rounded probe connectors that make attachment and removal from my MSOX2024a a breeze.  The InfiniiVision scopes also have 1 Meg input at 12pF and recommended probes are the N2863B or 10073D  BUT the 10073D has a input resistance of 2.2 Meg even though it is 10:1. 

My basic understanding is that at lower frequencies the 10:1 attenuation is driven by the scope's 1 Meg resistance input and probes input resistance to give a resistive 10:1 voltage divider while at higher frequencies it is mainly driven by capactive voltage dividing. 

How would the 10073D probes underperform the N2863B probes?

I did a fast test this by comparing the two probes in Ch1 and Ch3 with a constant RF voltage source (HP8920A at 0 dBm) stepping from 1khz to 300 Mhz and I compared the Vpp measured by the MSOX2024a.  Both probes behaved nearly identically except the 10073D was reading about 5% lower between 160 - 200Mhz compared to the N2863B.  However, between 200Mhz and 300 Mhz the attentuation of the signal was much steeper for the N2863B, I was surprised that for a 200Mhz scope, with the 10073D probe the sensitivity and measured rise time of the 300Mhz RF signal still looked pretty good with the 10073D.   Next I compared a 50Mhz square wave with both probe types and saw no difference in shape, rise time of Vpp.

 So I guess I am good to go with the 10073D?

j
 

alm

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Re: Better 10:1 Probes for MSOX2024a ??
« Reply #1 on: May 18, 2013, 09:03:39 pm »
The one potential problem that I could see is that 500 MHz probes are usually matched to a specific scope front-end, since the input impedance is usually not very well behaved at those frequencies (which is why wide bandwidth scopes tend to be limited to 50 Ohm). But the MSO-X is only 200 MHz anyhow, and the fact that the N2863B is recommended for both the DSO-X 2000 series and the InfiniiVission 5000-6000 scope suggests that these scopes have a very similar front-end design. So I don't see any reason why the 10073D would perform worse than the N2863B. Especially if the frequency response and transient response (was this 50 MHz square wave fast compared to the rise time of the scope?) is similar.

The only obvious disadvantage is that DC loading will be higher due to the 2.2 MOhm input impedance. Note that this is only relevant below 10 kHz or so, above this the capacitance dominates the input impedance.
 


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