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I'm working on a piece of test equipment that is intended to be hooked up to an oscilloscope to measure the waveforms it produces. Since that's its reason for being, I figured it made sense to provide some form of dedicated connection, rather than hooking up probes to test points, and so forth.

The obvious solution is BNC jacks on the test instrument, and a BNC cable to the scope, but I've been told there are subtleties embodied in a scope probe that simple BNC cables won't encompass.

Is this true? If so, what would I need to with my test instrument to ensure that it can be hooked up cleanly to a scope and give accurate measurements?

The signals to be measured will be pulses on the order of 1us long, with p-p voltages of between 100mV and 5v.

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3 Answers 3

up vote 5 down vote accepted

In your case, you don't need to worry about he subtleties of scope probes. If you are going to feed high frequencies through coax into a scope, and 1 usec pulses count as such, what you want to do is set the scope input to 50 ohms, use 50 ohm coax, and make sure your test box output amplifiers can drive 100 ohms. I say 100 rather than 50, because you will want to series terminate your output with 50 ohms. That is, put a 50 ohm resistor in series with your test output amplifier and your BNC connector. This will, of course, reduce the amplitude of the signal at the scope by 1/2, but you know this and can compensate. Either that, or use a x2 amplifier in your test box, so that a nominal 5 volt output is actually driven to 10 volts, and gets divided down to 5 volts at the scope. Either way will work.

Doing this will let you drive a scope to much higher frequencies than you are interested in for this application, so it will do you just fine.

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What if my scope (A DS1052E, in this case) lacks a 50 ohm input mode? –  Nick Johnson Jun 3 at 4:32
Put a second 50 ohm resistor at the far end of the cable, right at the connector that plugs into your scope, in parallel (from center to ground). So to summarize: inside your box you have RT1 = 50 ohm in series with the 50 ohm cable, and at the far end (scope end) of the cable, at the far connector, another RT2 = 50 ohm to ground. At high frequency the driver sees the entire cable as a 50 ohm load, and the scope connector sees the entire cable as a 50 ohm source. Matching the source impedance prevents reflections and maintains signal integrity. Gain is still 1/2 so compensate accordingly. –  MarkU Jun 3 at 7:18
There are also BNC thru terminators that do the same thing (see POMONA 4119-50), BNC plug on one side and BNC socket on the other side, with a 50 ohm resistor from center to ground inside. –  MarkU Jun 3 at 7:27
Yes, they are, although eBay is your friend here. Part of the reason they're so dear is that they work into the GHz range. A cheaper way to do it is to get a simple 50 ohm BNC terminator, such as a Pomona PE6156, and a BNC T (1 female, 2 male), and use this to attach the cable and the terminator to your scope. It's not quite as good as the dedicated unit, but it will do the job. –  WhatRoughBeast Jun 3 at 11:45
As a convenience, you should be aware that BNC Ts come in 2 geometries. For a 1F/2M type, the female can be either on the stem of the T, or the arm. You want the version with the female on the arm. An (expensive) example is the Pasternak PE9174, but even the cheapest version will do. If you use the other variety, the cable has to connect parallel to the screen, and either the cable or the terminator will partially obstruct the screen. Yet another reason the original terminator is a winner. –  WhatRoughBeast Jun 3 at 12:01

Some scope probes use special pogo-pin contacts that engage onto fingers in a ring around the probes BNC jack. These contacts usage model varies by scope. In some of the simplest types a single pin communicates the probe's 1X or 10X characteristic to the scope to auto adjust the vertical volts/cm appropriately.

Some other scopes use this to communicate the type of probe connected including input impedance, single ended versus differential inputs and other characteristics.

When you connect a straight BNC cable you give up these "features" which of course may be important or not.

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Perhaps they are referring to the scope probe Capacitance? This is usually on the order of a 10pF and can sometimes cause or stabilize oscillations. Or, maybe the probe ground line acting like an antenna sometimes? Both issues can be mitigated through good connection practices, assuming this was the concern.

Another question would be whether you need some impedance matching network or just using the scope's HiZ mode.

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