Oscilloskope 50Ohm Input

[iMo]

Updated above for your 50ohm sampling head.

Btw, your calibrator output is most probably 500mV (not 250mV), such one can get your step's voltage numbers..

[rhb]

"The calibrator output is a 250 mV square wave with a rise time of approximately 20 ps and a period of approximately 10 microseconds."

Tek 11801 User Reference Manual p 42
part # 070-7037-00
Revised May 1989

[iMo]

Ok, 250mVpeak at the 50ohm output when loaded with 50ohm (internally 500mV).

[rhb]

FWIW I received four 50 ohm thrus today from:

https://www.amazon.com/gp/product/B07G566JC7

measured two wire DC resistances using a 34401A and a banana to BNC adapter:

49.900, 49.937, 50.011 & 50.029

TDR with my 11801 showed results on par with my other 50 ohm thrus.

I don't currently have a 50 ohm splitter so there are reflections because of the mismatches,  I have ordered 50 ohm resistive splitters.  Once those arrive from China I will test them and post results both for the splitters and for the thrus. 

These are not Pomona or Pasternack quality, but they are better than I can measure at this point. I plan to improve my measurement abilities.  At $5 each, very satisfactory.  And probably indistinguishable from the high priced thrus at 200 MHz.

Have Fun!
Reg

[NDani]

Some outputs are defined to operate into a 50ohm load. If they don't have the specified load then:

• for low frequency signals the voltage will be higher than indicated
• the output could theoretically oscillate, but that shouldn't happen with any well designed circuit
• for high frequency signals (think RF), connecting cables act as transmission lines. If transmission lines are not terminated in the characteristic impedance, then the amplitude will vary with frequency. Search terms: VSWR, transmission line.

The last is the most important case.

Some scopes have an internal 50ohm termination. In most but not all low-end scopes, a 50ohm resistor is simply connected in parallel with the 1Mohm//20pF input. That is imperfect and the source of a specification "VSWR<1.3".

If there is no internal 50ohm termination, then it can be simulated by either a BNC t-piece plus 50ohm termination, or a through termination. That would still be in parallel with the 20pF, with VSWR consequences.

Hi all,

Sorry if this has already been asked, it's quite a long thread to check all posts.

The quoted post points out that on lower-end scopes the 50 ohm termination is achieved by switching a 50 ohm resistor in parallel with the high impedance input and that this can cause high VSWR at higher frequencies because of the scope's input capacitance. If I understand correctly, the situation would be similar when using a feedthrough BNC 50 ohm termination (but worse because of the unterminated stub after the feedthrough adapter).

How is the situation different on higher-end scopes? I use a Keysight 350 MHz scope in our lab which has switchable internal 50 ohm termination and 14 pF input capacitance. Now at, say, 200 MHz that input capacitance would cause a problem since a 50 ohm resistor in parallel with a 14 pF capacitor would create a VSWR of about 0.75.

My questions are the following:
- Is the input capacitance problem eliminated in higher-end scopes in 50 ohm mode? If so, how does the input capacitance disappear (or decrease)?
- Is it possible to eliminate the input capacitance problem with scopes that don't have an internal 50 ohm termination (and hence an external termination must be used)?

Thanks,
Daniel