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Another radio amateur about to buy a DSO ...

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RoGeorge:
18pF at 350MHz is only 25\$\Omega\$, while in the specs the impedance is given as 50\$\Omega\$.  No imaginary part and no || 18pF or other value is specified, which makes me guess the 18pF are compensated such that the 50\$\Omega\$ impedance is resistive only.

tggzzz:

--- Quote from: RoGeorge on November 11, 2023, 08:34:28 pm ---18pF at 350MHz is only 25\$\Omega\$, while in the specs the impedance is given as 50\$\Omega\$.  No imaginary part and no || 18pF or other value is specified, which makes me guess the 18pF are compensated such that the 50\$\Omega\$ impedance is resistive only.

--- End quote ---

Possibly, but many scopes - including from reputable manufacturers like HP and Tek - don't bother. Hence the desire to have it measured.

One scope that gets it right has two attenuated (one 50ohm, one 1Mohm), and an RF relay to switch between them. That has the advantage that an overload pops the relay back to 1Mohm before the 50ohm circuits are damaged.

bdunham7:

--- Quote from: Mike99 on November 11, 2023, 07:14:03 pm ---Well now I'm leaning towards the 2202X-E! I'm going to the dealer's on Tuesday for a demo of various models and planning to walk out with one, so if I take my NanoVNA with me I'm sure they'll let me look at the VSWR  :)

--- End quote ---


--- Quote from: RoGeorge on November 11, 2023, 08:34:28 pm ---18pF at 350MHz is only 25\$\Omega\$, while in the specs the impedance is given as 50\$\Omega\$.  No imaginary part and no || 18pF or other value is specified, which makes me guess the 18pF are compensated such that the 50\$\Omega\$ impedance is resistive only.

--- End quote ---

There were two previous discussions on this:

https://www.eevblog.com/forum/beginners/whats-the-input-capacitance-of-an-oscilloscopes-50-ohm-input/

https://www.eevblog.com/forum/testgear/need-50-ohm-input-on-an-(old)-oscilloscope-that-has-only-1m-ohm-inputs-bnc-tee/msg4484371/#msg4484371

They're not long, and you'll find that I tested a Tek 485, Tek 2465B and a Siglent SDS2354X+ with a NanoVNA.  The 485 is best of all, but the Siglent is essentially perfect to 350MHz, as good as the 2465B and more than likely at least as good as the 2430A since the attenuator design is similar to the 2465B.  The Siglent had a VSWR of 1.43 @ 500MHz, but it only curved up significantly over the 400-500MHz span.  Below that it was low enough to not worry about, IMHO.

Also in that discussion I measured the Siglent with an LCR meter and it seemed to indicate that the input capacitance of 18pF had some 10k ESR.  Some other theories and diagrams were presented, but what seems clear is that the input capacitance specified for the 1M input is not simply the equivalent of an 18pF directly across the input.  Thus statements observing that an 18pF capacitor would have a very low impedance at the upper end of the scopes BW aren't really relevant because the input has additional complexity.  An 18pF capacitor with a 10k resistor in series still looks a lot like 18pF if the rest of the circuit is 1M.  But even you just slap a 50R resistor across all that, the 18pF now looks like about 10k, not 25R, and the phase angle will be nearly zero. 

Ultimately I don't  have accurate enough stuff to try and model these inputs with any real precision, but it seems pretty clear to me that it isn't just 50R with 18pF across it and then they simply fail to mention the capacitance.  The results from that would be really bad, people would notice. 

Mike99:

--- Quote from: tggzzz on November 11, 2023, 09:01:51 pm ---
--- Quote from: RoGeorge on November 11, 2023, 08:34:28 pm ---18pF at 350MHz is only 25\$\Omega\$, while in the specs the impedance is given as 50\$\Omega\$.  No imaginary part and no || 18pF or other value is specified, which makes me guess the 18pF are compensated such that the 50\$\Omega\$ impedance is resistive only.

--- End quote ---

Possibly, but many scopes - including from reputable manufacturers like HP and Tek - don't bother. Hence the desire to have it measured.

One scope that gets it right has two attenuated (one 50ohm, one 1Mohm), and an RF relay to switch between them. That has the advantage that an overload pops the relay back to 1Mohm before the 50ohm circuits are damaged.

--- End quote ---

That's what the 2430A does. It has a 50 ohm termination resistor and thermal sensor. The manual states 1.3:1 VSWR to 150MHz. I'll measure it shortly.

Mike

tggzzz:

--- Quote from: Mike99 on November 11, 2023, 09:17:30 pm ---
--- Quote from: tggzzz on November 11, 2023, 09:01:51 pm ---
--- Quote from: RoGeorge on November 11, 2023, 08:34:28 pm ---18pF at 350MHz is only 25\$\Omega\$, while in the specs the impedance is given as 50\$\Omega\$.  No imaginary part and no || 18pF or other value is specified, which makes me guess the 18pF are compensated such that the 50\$\Omega\$ impedance is resistive only.

--- End quote ---

Possibly, but many scopes - including from reputable manufacturers like HP and Tek - don't bother. Hence the desire to have it measured.

One scope that gets it right has two attenuated (one 50ohm, one 1Mohm), and an RF relay to switch between them. That has the advantage that an overload pops the relay back to 1Mohm before the 50ohm circuits are damaged.

--- End quote ---

That's what the 2430A does. It has a 50 ohm termination resistor and thermal sensor. The manual states 1.3:1 VSWR to 150MHz. I'll measure it shortly.

Mike

--- End quote ---

Sounds like the earlier HP1740, except that doesn't have the relay. You can guess a common cause of failure!

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