Products > Test Equipment
making an 20:1 coax probe
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G0HZU:
Yes thanks, the downward blip seen in the first spice simulation is what I'd expect to see from a poorly terminated cable. The input capacitance of a typical 100-200MHz scope will probably be enough to cause this.

Note that the input impedance of a typical 200MHz bench scope is best modelled with something other than 1Meg in parallel with about 17pF. A better model is shown below. Up to maybe 300MHz the network below would model the input impedance of such a scope a lot better than a simple model of 1Meg || 17pF. To model it better, there needs to be some series resistance at the input and this is often in the range of 25R to 50R. I'm not sure if a 200MHz Picoscope will be like this though. Pico might use a different input to this.

tggzzz:
What's the cause of the "extra" 25-50ohm resistor? If a length of lead, then why not an inductor?
G0HZU:
Most scope inputs deliberately have some series resistance fitted near or at the input connector. This can act as a damping resistor.
David Hess:
The input resistor also acts as a fusible link.  In the past, a 1/8 watt carbon composition resistor was commonly used directly at the BNC.  Since the input impedance is nominally very high, the series inductance of a leaded part has little effect and this makes a convenient way to connect a chassis mounted BNC to the board with the input circuits like attenuators.  Obviously this resistor is *not* present if an internal 50 ohm switchable termination is present and other arrangements are required.  There is also likely a resistor directly in series with the FET gate at the FET after the input protection network.

Higher frequency probes may implement a t-coil termination at the BNC which has all kinds of advantages when driving a capacitive load.  The oldest schematics of Tektronix high impedance *and* low-z probes show this but they were intended to be used with 1M inputs because internal terminations were not used then.  The low-z probe has the termination built in and the t-coil drives the capacitive load of the 1 megohm input.  The advantage is low capacitance just like a normal low-z probe, but it is of course also limited to lower voltages because of the low impedance termination and 1 megohm inputs have limited bandwidth.  The t-coil provides the necessary broadband impedance matching to minimize the effect of the lumped capacitive load.  I do not think anybody makes low-z probes like this anymore, but Tektronix still was in the 80s.

Nothing would prevent doing the same thing with a low-z probe intended to be used with an oscilloscope input that has a switched termination.  I suspect Tektronix made a probe like this at some point for their 250 to 500 MHz instruments but a quick search did not find it.  Details are scarce for early instruments which implemented switched termination, but they might have implemented the t-coil impedance matching internally.  If so then they went to some effort to keep it secret; the relevant schematics that I have found suspiciously lack detail in that one area.

My fastest oscilloscope with an internal switched termination is only 300 MHz and I have never seen any extra aberration from using a 50 ohm source or probe interacting with the input capacitance.  There is something there, but a fast high impedance probe shows the same thing.
G0HZU:
Here's a old plot comparing a basic model against the input of a Tek 465 scope when measured with a VNA. The plot shows parallel resistance and parallel capacitance. You can see the input is only 1Meg || 20pF below about 1MHz.

By 100MHz the parallel input resistance Rp has fallen to about 160 ohms. The model shows very good agreement with the real scope.
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