Re: Switching a Tek TDS 5204 to 50 Ohms input impedance - Strange!


Hi , sorry to but in but I think Tek have got it right . The scope has no idea if the voltage measured is from a hi-impedance current limited source or a low-impedance constant voltage source with little or no current limit . The 50R termination in the scope is expensive and difficult / costly to repair so they play safe to protect it . I would not be complaining about that .
regardsBrian Skilton

On Monday, 15 June 2020, 15:15:13 BST, Jokken Feldhaar <> wrote:

Hi Ed,

I think you missed my main point, the photo diode is quite high impedance.

The problem is, that at 1M impedance I get 5+ Volts on the input,
decreasing to 4 mV when switched to 50 Ohms. But the Scope measures the
voltage at 1M, and decides that this is too high to switch to 50 Ohms.
It completely ignores the impedance-induced voltage change.

So I have to manually disconnect the diode, switch to 50 Ohms, and then
reconnect the diode. Tek could have done better, IMHO.

Grgards, Jochen DH6FAZ

Am 15.06.2020 um 13:13 schrieb Ed:
When the photo diode output is connected to 50 ohms vs 1megohm, I would expect the voltage to drop unless there is an amplifier in line. And even if there is, it would have to have a low output impedance to drive 50 ohms with the same voltage as 1 megohm.


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