[Boatanchors] Calibrating S-Meters & Marconi Attenuator

[Glen Zook]

Most of those manufacturers that used 100 microvolts for S-9 in the alignment (i.e. some Collins) actually applied the 100 microvolts through a resistive network that represented a 6 dB matching pad.  This then dropped the actual voltage to 50 microvolts.  Such was necessary when using a number of the older signal generators that really did not have an impedance of 50 ohms.  Those included the Measurements Model 80, Model 82, and Model 180, the military TS-497 series and the URM-25 series, the Hewlett-Packard Model 606 and Model 608, and similar units.

Those generators came with 6 dB external pads that matched the output impedance to various figures of which the 50 ohm and 75 ohm versions were the most common.

With most "modern" signal generators that have an output impedance of 50 ohms using such a matching pad is not usually necessary.  

Glen, K9STH

Website:  http://k9sth.com


--- On Sun, 6/8/08, Al Klase <al at ar88.net> wrote:

S-meters are NOT precision instruments.  The difference between 50 and 75 ohms, for instance, is going to be completely lost in the sauce. Just come close to 50uV.  Some receivers claim 100uV for S9.