Scope without 50 Ohm Input, Does 50 ohm Feedthru allow accurate Voltage reading

[jstarr]

I understood that one accurate method of analyzing signals coming from a DUT with 50 ohm impedance output on a scope with only a 1meg input was to use a 50 ohm feed through adapter at the scope input, in essence putting 50 ohm termination in parallel with the scope 1meg input so that the DUT sees almost 50 ohm input (49.997 ohms).  If the input signal is a pure 10Mhz sine wave, would the scope still give you an accurate Vp-p reading? I tried this on my Rigol 11002D using an accurate Tektronix 10x (20dBm) feed through attenuators with the scope set to 10x input and using a direct 50 ohm to 50 ohm BNC cable.  With an verified, calibrated Signal generator at 10Mhz precisely at 20.0 mW (+13 dBm) I verified the output on a HP 438A powermeter.  Then I verified then 10x attenuator when inserted correctly dropped 20 dBm on the power meter to -7dBm. 

However on the Rigol I found the -7dBm signal was reading 2x higher V-p than expected.  I thought it was a problem with the Rigol so I repeated the experiment on my old vintage Tek 7904A also using a high impedance vertical amp with the 50 ohm 10x feedthru and I also found the 50 ohm signal at twice the Vp-p of what it should be.

Is the principle of using a 50 ohm feed-through on a high impedance scope input just unreliable for measuring signal voltage levels within any degree of accuracy?    Is the problem that the scope still sees an impedance mismatch?

tnx
john 

[c4757p]

Double? You sure you haven't confused Vpeak and Vp-p? You shouldn't expect accurate voltage readings on an oscilloscope - that isn't their purpose - but double is a bit silly. You've definitely mixed them up in your description!

[Rufus]

I tried this on my Rigol 11002D using an accurate Tektronix 10x (20dBm) feed through attenuators

The 20db attenuator will be 20db in power which is 10db in voltage.

Your scope displays voltage the power meter displays power.

[alm]

in essence putting 50 ohm termination in parallel with the scope 1meg input so that the DUT sees almost 50 ohm input (49.997 ohms).  If the input signal is a pure 10Mhz sine wave, would the scope still give you an accurate Vp-p reading?

Not that this is your problem (my money is on confusion between Vp and Vp-p or power vs. voltage, as the people above me mentioned), but this calculation is not correct. The input impedance of your scope is something like 1 Mohm in parallel with 15 pF. The impedance of that 15 pF at 10 MHz is only 1 kohm, so the impedance as seen from the source would be off by about 2.5 ohm.

[jstarr]

Thanks for the good start to the discussion.  First a few comment to clarify:

The 20db tek attenuator is labeled also as 10x for voltage.  So I am not confusing power with voltage.  The scope measurement (readout and by my counting boxes) is Vp-p on the sine wave.  Whatever the Vp-p input is, the 10x attenuator would decrease the the voltage seen by the scope input 10 fold lower.

In your the comment: "something like 1 Mohm in parallel with 15 pF. The impedance of that 15 pF at 10 MHz is only 1 kohm, so the impedance as seen from the source would be off by about 2.5 ohm"  - so if I were to reduce the input signal say to 60 hz and make most of the input impedance purely resistance only then I should be able to assume a simple voltage drop across the scope 1meg resistor input?

tnx
j

[Rufus]

The 20db tek attenuator is labeled also as 10x for voltage.

Sorry didn't read your post carefully enough.

A 50 ohm attenuator is designed to be terminated with 50 ohms and would not be called feed through because there is no other kind.

To accurately measure 50 ohm signals on a scope without 50 ohm inputs you need a 50 ohm feed through terminator.

You could stick 50 ohm attenuators in front of it if desired.

[jstarr]

Hi Rufus,

Is there a difference between a 50ohm feedthu terminator versus 50 ohm attenuator?  I think both have a 50 ohm resistor between center pin and shield/ground.  In fact I checked all of my tektronix and narda... both the feedthru terminators and feedthrough attentuators measure ~51 ohms between center and shield.  I was told that without a nice expensive feedthrough terminator you can take a BNC Tee and just put a 50ohm resistor across one of the Tee outputs.   I have compared both the handmade tee terminator vs the tektronix 50 ohm feedthru term and both give same results.

Can we look at this as an impedance mismatch?  If I see a 2 fold increase in voltage at the scope then that implies an SWR of 2/1 so maybe despite my 50 ohm feedthrough there isnt a 50 ohm match but it would be more of a 100 ohm impedance at the scope input if the DUT is 50 ohm.  That would lead to my measuring 2x the voltage of the signal gererator output.  Or am I getting everything out of whack?

j

[c4757p]

A 50 ohm terminator and 50 ohm attenuator are completely different things. A 50 ohm terminator is a good quality 50 ohm resistor to ground. A 50 ohm attenuator is a voltage divider with its input and output impedances equal to 50 ohms; that doesn't mean it's 50 ohms to ground, and it is not meant to be used as a terminator.

[Rufus]

50 ohms :-

20db T attenuator
20db Pi attenuator
Feed through terminator

[jstarr]

Thanks Rufus - you are 100% . With a pure zero attenuation 50 ohm terminator, i do get correct Vpp readings on the scope matching the SigGen output at 50 ohm output.  I then put the 10x (20dB) attenuator between the 50 ohm feedthru and the cable and now the 10x reads correctly. 

best
j