[Test-Equipment] tube input impedance

Fri, 13 Dec 2002 00:53:19 +0000

> This antique plug-in (for 560-series scopes) has an input impedance of 10
> Megohm, and a maximum sensitivity of 10 microvolts per division.
 
> The input circuit is a pair of 5842 triodes (one for + and one for - input
> lines). Each has a 10 Megohm resistor from the input grid to ground which
> sets the overall input impedance.
 
> What I want to do is increase the input impedance to the maximum I can get 
> away with. A gigaohm is not too much, if this can be realized.

   You have the answer, we don't.  Replace the two resistors with resistors
half way between what they are now and where you want to go. 1/2 gig and
see if it works, if it does, double the values, if not half them. Keep going.
   It is thought that the resistors are necessary.
If you will be looking exclusively at RF with not too wide of frequency range
you can add a inductance (choke coil) in series with both resistors
increasing the input impedance greatly.
-- 
        73 (= Best Regards) de: Ron [email protected]
     100% Slack. since July, 1997 (still free!) SENT D&T are UTC
        Visit my HAM Web SITE at: http://www.qsl.net/ka4inm