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Another radio amateur about to buy a DSO ...
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G0HZU:

--- Quote from: bdunham7 on November 11, 2023, 09:09:16 pm ---
--- 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  :)

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--- 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.

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As I mentioned in that other thread, by deliberately adding some inductance in series with the 50R termination the circuit will mimic approx 50R in parallel with a negative capacitance and it will do this over a wide bandwidth. The negative parallel capacitance will cancel some of the existing 18pF over a fairly wide bandwidth.

So that simple trick alone will mask some of the input capacitance when the 50R terminator is selected.

The other way to do it is to arrange the 50R termination as a potential divider made up of a series 20R and then switch in a shunt 30R with a relay when in 50R mode. This will mask the 18pF input capacitance yet further and when combined with the inductance trick, the scope could achieve a decent VSWR over a large bandwidth. The penalty will be a higher noise floor due to the attenuation introduced by the 20R and 30R potential divider.
Mike99:
The 2430A service manual doesn't show the input arrangement in enough detail but I've just measured mine.

Sorry about the layout I can't get the hang of the attachments.

Also ignore the cal description, it is calibrated over the full measurement range.

R+jX for the 1M input.



VSWR for the 50 ohm input



Mike
Fungus:

--- Quote from: Mike99 on November 11, 2023, 03:50:59 pm ---Yes I know these can be hacked but I would be reluctant to try until it's out of warranty.

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All you're doing is entering a license key as if you'd bought it from Rigol. Inputting a few numbers can't void the warranty.

nb. It can easily be removed, too.
Performa01:
I don't have an SDS2000X-E, only SDS2000X Plus. I honestly cannot tell whether the little 2000X-E has a true 50 ohms path like the SDS2000X Plus (and higher models) have, but the following demonstration should still be universal to show how an external through termination is only an option at low frequencies. For higher frequencies, a proper internal 50 ohms path is mandatory to get a reasonable VSWR , hence reasonably accurate measurements.

Look at reply #237 here:

https://www.eevblog.com/forum/testgear/rigol-dho804-test-and-compare-thread/msg5114334/#msg5114334
Mike99:

--- Quote from: Performa01 on November 12, 2023, 11:26:03 am ---I don't have an SDS2000X-E, only SDS2000X Plus. I honestly cannot tell whether the little 2000X-E has a true 50 ohms path like the SDS2000X Plus (and higher models) have, but the following demonstration should still be universal to show how an external through termination is only an option at low frequencies. For higher frequencies, a proper internal 50 ohms path is mandatory to get a reasonable VSWR , hence reasonably accurate measurements.

Look at reply #237 here:

https://www.eevblog.com/forum/testgear/rigol-dho804-test-and-compare-thread/msg5114334/#msg5114334

--- End quote ---

That's very interesting, thank you. It's nudging me towards the 2202X-E which has a selectable 50 ohm input.

Mike
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