Tektronix MDO3034: no AC coupling with 50R termination?

[tron9000]

Got an issue and just wanted to make sure I've not changed a setting somewhere.

I have a 1:1 probe connected to a power supply rail to do some ripple tests and when I put the input to 50R termination, the AC coupling option greys out.

Any ideas why? I got an older scope that can do it, so why can't this one?

Cheers

[thinkfat]

I'm just guessing, but the 50 Ohm termination is likely meant to be "real" over the full bandwidth of the 'scope and a capacitive component would add something "imaginary". Hence, no AC coupling.

[tggzzz]

I have a 1:1 probe connected to a power supply rail to do some ripple tests and when I put the input to 50R termination, the AC coupling option greys out.

What's the benefit of using a 50ohm termination in that case? What's the cable length, lowest and highest frequency of interest?

Be aware of the highest voltage allowed across the 50ohm, and which "side" of an AC coupling capacitor it is.

[PartialDischarge]

In the TDS600/700  there was warning to reming you that the LF cutoff is very high in that case (around 200kHz). Maybe it was SW disabled in the MDO series

[TimFox]

The 50 ohm termination resistor has a finite power rating, probably no more than 2 W.  If AC coupling were allowed, you could easily have excess DC power dissipation in it.

[med6753]

Got an issue and just wanted to make sure I've not changed a setting somewhere.

I have a 1:1 probe connected to a power supply rail to do some ripple tests and when I put the input to 50R termination, the AC coupling option greys out.

Any ideas why? I got an older scope that can do it, so why can't this one?

Cheers

What scope is it that allows AC coupling with 50R termination? I have 2 scopes here (Tek 485 and Tek 2465) and a 7A24 plug-in for Tek 7000 series and all of them are DC coupled only. 

[MarkL]

Like the TDS600/700, the TDS3000 series can also do AC coupled with 50R termination.  It displays a warning that the lower BW cutoff is 100kHz.

If you want 50R termination, the DC level already has to be fairly low (5V RMS max).  You could try bringing the trace on-screen by using the offset.  Offset ranges are +/-0.5V, +/-1.0V, and +/-5V, depending on the V/div setting.

Or just use an external thru terminator with sufficient power rating.

[nctnico]

Another option is to use an external DC block. These are common to use on spectrum analysers; the higher end ones really don't like DC at their inputs.

[tggzzz]

Got an issue and just wanted to make sure I've not changed a setting somewhere.

I have a 1:1 probe connected to a power supply rail to do some ripple tests and when I put the input to 50R termination, the AC coupling option greys out.

Any ideas why? I got an older scope that can do it, so why can't this one?

Cheers

What scope is it that allows AC coupling with 50R termination? I have 2 scopes here (Tek 485 and Tek 2465) and a 7A24 plug-in for Tek 7000 series and all of them are DC coupled only.

Tek 485: look at C101, 2.2µF, diagram 1 top left. C101 is between the BNC socket and the 50ohm attenuator.