Another radio amateur about to buy a DSO ...

[ebastler]

Was this not demonstration enough?
https://www.eevblog.com/forum/testgear/another-radio-amateur-about-to-buy-a-dso/msg5164407/#msg5164407

Oh, right. For some reason I only remembered the pulse responses from your tests, not the return loss plots. Thanks for pointing them out again. Yes, those will do! 

So the 2000X+ performs significantly better still than the 2000X-E -- which makes sense, given that it is specified up to 500 MHz. I am curious what it takes to improve the high-frequency matching. Presumably some of the "ingredients" cost money, and were hence not justifiable for the entry-level scope? So, besides properly chosen R and C as well as careful routing, what is required to get this right?

[bdunham7]

Anyways, it shows 50Ω path is not made by simply putting 50Ω terminator in parallel with 1MΩ/20pF input...

From the earlier discussions that I linked above, I'm not sure that is an entirely accurate explanation.  The external 50R across the existing 1M/20pF input also does not behave as you'd expect from a simple model using the stated values and depending on the exact terminator and setup, can perform much better than predicted--even if it isn't as good as the internal termination.

My theory was that the 20pF isn't just 20pF directly across the input but rather that it has some significant series resistance, not enough to matter in a 1M circuit but enough to take it out of the picture in the 50R case. 

[G0HZU]

The deed is done. I bought the Siglent SDS2202X-E and they gave me a discount 

Mike

Sounds good! If you plan to use it for ham stuff (including transmitter testing) make sure you don't exceed the AC voltage limits (vs frequency) for a typical digital scope.

For the internal 50R load this is usually about 5Vrms across all frequencies within the bandwidth of the scope.

For the 1Meg input, I'd recommend you keep all RF signals below about 10Vrms, especially at mid to upper HF and into VHF. The scope will only be rated for high AC voltages at low frequencies. So you can't expect it to cope with (say) 400Vpeak above a few kHz. There may be a derating curve for AC voltage versus frequency somewhere in the manual.

Even a 12W PEP 27MHz CB radio can pose a risk of damage to some digital scopes unless you use some form of inline attenuation.

[Mike99]

The deed is done. I bought the Siglent SDS2202X-E and they gave me a discount 

Mike

Sounds good! If you plan to use it for ham stuff (including transmitter testing) make sure you don't exceed the AC voltage limits (vs frequency) for a typical digital scope.

For the internal 50R load this is usually about 5Vrms across all frequencies within the bandwidth of the scope.

For the 1Meg input, I'd recommend you keep all RF signals below about 10Vrms, especially at mid to upper HF and into VHF. The scope will only be rated for high AC voltages at low frequencies. So you can't expect it to cope with (say) 400Vpeak above a few kHz. There may be a derating curve for AC voltage versus frequency somewhere in the manual.

Even a 12W PEP 27MHz CB radio can pose a risk of damage to some digital scopes unless you use some form of inline attenuation.

Thanks for the advice! I'm having fun playing with low level stuff at the moment. Love the FFT function!

Mike

[Mike99]

Anyways, it shows 50Ω path is not made by simply putting 50Ω terminator in parallel with 1MΩ/20pF input...

This would be a nice opportunity to demonstrate the difference.
@Mike99, any chance you could repeat the measurement with the scope's input set to 1 MOhm and an external 50 Ohm pass-through terminator? Thanks!

I don't have a pass through terminator, only a 6dB BNC attenuator.

Mike

[tggzzz]

Anyways, it shows 50Ω path is not made by simply putting 50Ω terminator in parallel with 1MΩ/20pF input...

This would be a nice opportunity to demonstrate the difference.
@Mike99, any chance you could repeat the measurement with the scope's input set to 1 MOhm and an external 50 Ohm pass-through terminator? Thanks!

I don't have a pass through terminator, only a 6dB BNC attenuator.

Mike

A 6dB passthrough attenuator followed by the imperfect internal attenuation will be better at reducing VSWR than an external terminator. That technique is frequently used inside equipment where components/subsystems have poor impedance matching.

Basic principle: gain is easy and cheap, wideband matching is difficult and expensive. But don't stick an attenuator in front of a low-noise front end, of course

[Mike99]

I have made some more measurements.

First was the 50 ohm input bandwidth, measured using an RF millivoltmeter:



Next was input VSWR. The graph compares the 50 ohm input with the 1Mohm input, the latter using a T piece with a 50 ohm load.



Finally, the impedance of the 1Mohm input.



Mike

[Orange]

Hi Mike,

An impedance plot for 50 Ohms would make this information complete :-)

[Mike99]

Hi Mike,

An impedance plot for 50 Ohms would make this information complete :-)

OK I'll do that later today.

Mike

[dshorthill]

I use this device to monitor the RF output of  a 1 kw ham amplfier on my 1204xe Siglent scope.  I have a bnc t with a 50 ohm termination on the scope end.  Works great.

https://www.ebay.com/itm/193686855709

Dave
k1xd

[Mike99]

I use this device to monitor the RF output of  a 1 kw ham amplfier on my 1204xe Siglent scope.  I have a bnc t with a 50 ohm termination on the scope end.  Works great.

https://www.ebay.com/itm/193686855709

Dave
k1xd

Hi Dave

Yes I have a similar device, not as high power but it gives -50dB coupling up to 500MHz. Very useful.

Mike G8GYW

[Mike99]

Hi Mike,

An impedance plot for 50 Ohms would make this information complete :-)

Here it is ...

Mike